Simple Interest Calculator
Savings & BankingInterest on the principal alone — no compounding.
Principal, rate & term
1,095 days · 36 months · 3 years
- Principal$10,000
- Interest$1,500
A planning estimate — banks and lenders may round, use their own day-count convention or revise the rate, so treat these figures as a guide rather than a quote.
Simple vs compound over time
The straight line is flat simple interest; the dashed curve compounds once a year. The shaded band between them is the difference.
How the compounding rhythm changes it
Interest earned on the same deposit and rate — flat, versus compounding yearly, monthly and daily.
- Simple$1,500
- Compound · yearly$1,576
- Compound · monthly$1,615
- Compound · daily$1,618
Period-by-period schedule
| Year | Interest | Simple balance | Compound balance | Difference |
|---|---|---|---|---|
| 1 | $500 | $10,500 | $10,500 | +$0 |
| 2 | $1,000 | $11,000 | $11,025 | +$25 |
| 3 | $1,500 | $11,500 | $11,576 | +$76 |
How this is worked out
- Put the term in years: 3 yrs is 3 years.
- Apply I = principal × rate × time — $10,000 at 5% for 3 years — which works out to $1,500 of interest.
- Add it to the principal: $10,000 + $1,500 = $11,500.
- Spread evenly that is $500 a year, $42 a month or $1 a day.
- Compounding the same rate once a year would add $76 more, reaching $11,576.
Know what this estimate is based on
- Jurisdiction
- General mathematical model
- Scope and limitations
- Projection only. Actual APY, posting dates, compounding, taxes, withdrawal rules, deposit protection, and fees depend on the financial institution and country.
- Source links checked
- Jul 30, 2026
Built and regression-tested by Smart Tools Lab. It has not been individually reviewed by a licensed financial, tax, or legal professional.
How to use
- 01
Type in your principal — the original amount you're saving, lending, or borrowing — because simple interest is figured on this starting sum and nothing else.
- 02
Set the annual interest rate as a yearly percentage; the tool holds that rate flat and never lets it build on itself.
- 03
Enter the term and pick whether the number counts as days, months, or years, and the calculator converts it into a fraction of a year for you.
- 04
Open the optional tax and inflation fields if you want to trim tax off the interest and see what the ending amount is really worth in today's money.
- 05
Read the headline outputs — total interest, final balance, and the earnings broken out per day, per month, and per year.
- 06
Turn on the simple-versus-compound view and scroll the period-by-period schedule to watch the straight line separate from the compounding curve.
Formula
Simple interest rests on one flat calculation: I = P x r x t P = principal, your original amount r = annual rate written as a decimal (5% becomes 0.05) t = the term measured in YEARS Convert the term to years before you multiply: entered in days -> t = days / 365 entered in months -> t = months / 12 entered in years -> t = years, used as is The rate only ever touches P, so every year adds the exact same dollars and the balance rises in a straight line instead of a curve. Final balance = P + I Fold in tax on the interest: after-tax interest = I x (1 - tax rate) after-tax balance = P + after-tax interest Fold in inflation to see real value: real value = after-tax balance / (1 + inflation rate) ^ t which is what your ending balance would buy at today's prices.
Example
Put in $10,000 at 5% for a 3-year term and the arithmetic stays refreshingly plain: I = 10,000 x 0.05 x 3 lands on $1,500.00 of interest, so the balance finishes at $11,500.00 — a flat 15% total return that arrives as $500.00 each year, $41.67 each month, or $1.37 a day, identical from the first period to the last because nothing is ever added to the base that earns. Hand that same 5% to an account that compounds once a year and it ends at $11,576.25, or $1,576.25 of interest — just $76.25 more across the full three years, the slim head start compounding builds only once a term runs past a single year.
Definitions
- Principal
- The starting amount you deposit or borrow. In simple interest it never changes: every dollar of interest is figured on this original figure alone, never on interest you've already earned.
- Simple interest
- Interest paid only on the original principal, so the same dollar amount accrues each period. It follows I = P x r x t and grows your balance in a straight line rather than a curve.
- Interest rate (annual)
- The yearly percentage applied to your principal. You enter it as an annual figure, and the tool scales it to whatever term you pick, whether that term is counted in days, months, or years.
- Term
- How long the money stays invested or borrowed, which you can enter in days, months, or years. Because t in the formula is expressed in years, a 6-month term counts as 0.5 and a 90-day term as 90/365.
- Final balance (maturity value)
- Your principal plus all the simple interest accrued over the term — what you would walk away with at the end. For $10,000 at 5% over 3 years, that comes to $11,500.00.
- Accrued interest
- Interest that has built up so far but has not yet been paid out or folded into the principal. Under simple interest it accumulates at a steady, predictable pace: the same amount every single day.
- Per diem interest
- The interest earned or owed for one single day, found by dividing the annual interest by the day-count. Payoff quotes on many auto loans price the exact settlement date this way; here it works out to $1.37 a day on $10,000 at 5%.
- Compound interest (for contrast)
- Interest figured on your principal plus previously earned interest, so the balance bends upward over time rather than holding to a straight line. It ties simple interest at exactly one year of annual compounding, sits slightly behind below a year, and pulls decisively ahead only past that point.
- Effective/total return
- The full percentage your money grows across the whole term, not the yearly rate. Three years of 5% simple interest delivers a 15% total return — $1,500 of interest on $10,000.
- Day-count (365)
- The number of days used to convert an annual rate into a daily one. This tool divides by 365, so a term you enter in days is measured as that many 365ths of a year.
- Tax on interest
- The portion of your earned interest owed to the government, which this tool can subtract to show what you actually keep. Interest is generally treated as ordinary income, though the rate that applies depends on your own situation.
- Real (inflation-adjusted) value
- What your final balance is worth in today's purchasing power once rising prices are accounted for. A larger number on paper can still buy less, so this view tells you whether you genuinely gained ground.
Good to know
What simple interest is — and the one formula behind it
Simple interest is the plainest way money can grow. It pays you a flat amount on the sum you started with — your principal — and nothing more. The interest never joins the pile that earns, so the base stays fixed for the whole term. That single decision, keeping interest off the earning base, is what separates simple interest from every compounding cousin. One formula captures all of it: I = P · r · t. Here P is your principal, r is the annual rate written as a decimal, and t is the term measured in years. Multiply the three and you have I, the total interest — not per year, but for the entire stretch. Your final balance is simply P + I. Because the rate always bites the same original principal, the interest you earn each year is identical. Year one pays exactly what year five pays. Plot the balance and you get a straight line, not a curve — a constant slope climbing at the same pace from start to finish. There is no acceleration, no snowball, no interest-on-interest. A quick example makes it concrete. Put $10,000 to work at 5% a year for 3 years. Run the numbers: 10,000 × 0.05 × 3 = $1,500.00 in interest, for a final balance of $11,500.00. That $1,500 arrives in three equal $500.00 slices, one per year, and it lands at the same rhythm whether you check monthly or daily. The whole appeal is predictability. You can see the endpoint the moment you know P, r, and t — no schedule to grind through, no surprises hiding in later periods. Simple interest trades the extra growth of compounding for arithmetic you can do on the back of an envelope, and that clarity is exactly why it still anchors so many everyday loans and notes.
Simple versus compound: where the paths split
Set simple and compound interest side by side at the same rate and something surprising happens: for a while, they are nearly twins. Compound interest adds each period's interest back to the base, so the base keeps swelling and future interest is figured on a larger number. Simple interest refuses that step. The gap between them starts small and widens the longer you stay invested. The pivot point with annual compounding is exactly one year. At the twelve-month mark the two are identical — compounding hasn't had a chance to fold anything back in yet, so both just pay one year's rate on the principal. Cross past a year and compounding pulls decisively ahead; the curve lifts off the straight line and never looks back. Watch it over three years. Your $10,000 at 5% earns $1,500.00 of simple interest, ending at $11,500.00. Let that same rate compound once a year and the ending balance is $11,576.25 — interest of $1,576.25. Compounding's head start is worth $76.25 here, and that edge would grow to hundreds and then thousands over a decade or two. Over long horizons compound wins, and it wins by more every year. But the popular claim that compounding always beats simple is false below one year. A compounding curve sits under its own straight chord for sub-year terms, which means simple interest is briefly ahead. Take the same $10,000 at 5% for just 6 months: simple pays $250.00, while the annual-compound equivalent is about $246.95. For that half-year, simple actually earns roughly $3.05 more. So the honest picture has three zones: below a year simple nudges ahead, at exactly a year they tie, and beyond a year compounding takes over for good. The chart in this tool draws both lines so you can see precisely where your term lands on that story.
Where you actually meet simple interest
Simple interest is not just a classroom exercise — it sits quietly inside a lot of real borrowing. Many auto loans accrue it on the outstanding balance, charging you interest for each day you hold the money rather than folding unpaid interest into a growing base. Plenty of personal loans and promissory notes work the same way, spelling out a flat rate on the amount still owed. That daily accrual is why a payoff quote often comes with a per-diem figure — the exact dollars of interest that pile on each day until your check clears. Ask for a ten-day payoff and the lender adds ten of those per-diem amounts to the balance. Understanding simple interest lets you check that a quote is fair and time your payment to shave off a few days of charges. Short-term and bridge financing lean on simple interest too, because the term runs in weeks or months and nobody expects interest-on-interest over such a brief window. You'll also see the idea in parts of the bond world: some instruments quote simple interest, and Treasury bills are sold at a discount to face value, so the return is the gap between what you pay and what you collect at maturity rather than a compounding yield. Margin balances at a brokerage are another place daily simple interest shows up. Now the flip side, and it matters: most savings accounts and CDs compound. They add earned interest back to your balance so it starts earning too. That means if you use this calculator to project a real deposit, simple interest will usually understate what the account actually pays. Treat that as a feature — the tool doubles as a teaching and comparison instrument, showing you the floor of straight-line growth and letting you weigh it against the compounding a real bank product would deliver.
Getting the term right: days, months and years
The formula wants t in years, but real agreements rarely speak in tidy whole years. So this calculator lets you enter the term in days, months, or years and handles the conversion for you. Pick the unit that matches your paperwork and you sidestep the most common source of a wrong answer. The conversions are straightforward. A term in months becomes years by dividing by 12, so 6 months is 0.5 years and 18 months is 1.5. A term in days divides by 365, so 90 days is roughly 0.2466 of a year. Once the term is expressed in years, I = P · r · t runs exactly as before — the unit selector is just doing that division quietly in the background. Days matter more than people expect, because simple interest on loans is frequently a per-diem game. The per-day interest is your principal times the annual rate, divided by 365. On $10,000 at 5% that comes to about $1.37 a day. Multiply by the number of days you actually hold the balance and you have the interest — which is exactly how a lender builds a payoff figure. Here's the catch worth knowing: the day count is a choice, and different conventions give slightly different numbers. Dividing by 365 versus 360, or counting the first day but not the last, nudges the total. Over a short term that difference is pennies; on a large balance held for months it can turn into real money. If a quote surprises you, the day-count convention is often the reason. The practical habit is simple. Enter the term in whatever unit your contract uses instead of converting in your head, where rounding creeps in. Let the tool translate to years, and check that the per-day figure it reports lines up with any per-diem number your lender gave you.
Reading your results: total interest, final balance and the per-period breakdown
Your results panel turns three inputs into a full picture, and each number answers a specific question. Total interest is the whole I from the formula — every dollar the principal earns across the entire term, not per year. Final balance is that interest added to your principal, the amount you'd walk away with when the term ends. For $10,000 at 5% over 3 years, that's $1,500.00 of interest and an $11,500.00 balance. The per-period figures break the same total into a rhythm you can feel. Interest per year is $500.00, per month is $41.67, and per day is $1.37. These aren't three independent sums layered on top of one another — they're one flat stream sliced three ways. Because the base never changes, every slice is equal: this month earns what next month earns, and this year matches every other year. That's the straight-line accrual that defines simple interest. The tool also shows total return over the term as a percentage — here 15%, which is just the 5% rate multiplied by the 3-year term. It's a quick way to see how far your money moved without reading the dollar figures. Two optional adjustments refine the view. A tax input trims the interest by whatever share goes to tax, showing what you actually keep. An inflation adjustment re-expresses the ending balance in the spending power of today's dollars, since a dollar three years out buys a little less than a dollar today. Finally, the period-by-period schedule lays out the accrual row by row, year and month, so you can watch the balance climb in equal steps. Nothing accelerates; each line adds the same amount as the one before. Reading down that column is the clearest way to internalize what simple interest really is — the same dollars, the same pace, every single period.
Tax on the interest you earn
The dollars simple interest hands you are, in most situations, treated as ordinary income — the money you receive is generally viewed the same way a paycheck is, not as some lightly taxed windfall. That one fact reshapes what a rate really means to you. A quoted return describes what your money earns before anyone else takes a cut; what actually stays with you is smaller, and the gap tends to widen as your earnings grow. This tool lets you fold that reality into the projection. Switch on the tax option and enter the rate that applies to you, and the calculator subtracts that share from the interest — never from your original principal, which was already your own money — then reports an after-tax total beside the headline figure. The interest line you read becomes the interest you keep. A few things are worth holding in mind. Interest is generally owed as income in the period it is credited to you, whether or not you withdraw it, so a long simple-interest note can create an obligation before you ever touch the cash. Only the interest is taxed; getting your principal back is not an income event. And treatment differs by the kind of account and by where you live — some interest sits inside tax-advantaged wrappers, some carries state-level rules on top of federal ones, and definitions shift over time. The calculator uses the single rate you supply as a clean, honest stand-in, not a substitute for the particulars of your own return. Treat the after-tax number as the one that answers the question you actually care about: not what did this earn, but what did this leave me with. When you weigh two offers, comparing their after-tax interest keeps the contest fair, because a higher headline rate in a heavily taxed account can quietly finish behind a lower one that is sheltered. For anything binding, confirm the specifics with a qualified tax professional.
Inflation and what your money is really worth
A balance can grow and still leave you worse off. Simple interest adds the same fixed dollars each period, but the worth of a dollar is not fixed — as prices drift upward, each future dollar buys a little less than the one before it. So the final balance the calculator shows is a nominal figure: correct in dollars, yet not the whole story about what those dollars can do. That is why the tool offers an inflation adjustment. Enter the rate at which you expect prices to rise, and it recasts your ending balance into today's dollars — the buying power your future total would carry if you could spend it right now. The distance between the plain final balance and this real figure is the ground quietly lost to rising prices while your interest was accruing. Here the non-compounding nature of simple interest matters in an uncomfortable way. Prices tend to climb in a compounding fashion, each year building on the last, while your simple interest marches in a straight line at a flat dollar amount. Over a long enough term those two paths can cross: a modest simple rate that felt safe may deliver a real value below what you started with, meaning you technically earned interest yet lost buying power. The nominal number went up; the today's-money number went down. This is not a reason to fear the figure, only to read it honestly. Use the real value to judge whether a given rate and term actually carry you forward. A low simple rate parked for years is especially exposed, because it never accelerates to keep pace. When the real figure disappoints, the remedy is usually a higher rate, a shorter horizon, or a compounding vehicle — the point of showing it is to let you see the erosion before you commit, rather than discover it afterward.
A full walkthrough with real numbers
Let's put a real deposit through the whole calculation so nothing stays abstract. Start with $10,000 of principal, a rate of 5% a year, and a term of 3 years. Simple interest asks only what the original amount earns, so you multiply the three together: 10,000 times 0.05 times 3. Five percent of $10,000 is $500 of interest each year, and because simple interest never lets those earnings start earning, three years simply stacks three identical $500 layers. That comes to $1,500.00 of total interest. Add it back to the untouched principal and your final balance is $11,500.00. Across the full term you have earned 15% on what you put in. Now slice that same result by period, which is where the straight line becomes tangible. The interest is $500.00 per year, every year — not rising, not fading. Divide by twelve and it is $41.67 per month. Spread the yearly figure across the days in a year and it lands near $1.37 per day. Each of those numbers holds steady for the entire term, because the base that earns never changes. That steadiness is the whole personality of simple interest. For contrast, picture the same $10,000 at the same 5% but compounding once a year. After year one the $500 of interest joins the balance and itself begins to earn; the same happens after year two. Run it out and the final balance is $11,576.25, an interest total of $1,576.25 — about $76.25 more than the simple version over these three years. That $76.25 is the entire prize compounding wins here, and it is worth seeing plainly: real, but modest at this rate and over this span, and earned only because interest was allowed to build on interest. Raise the rate or stretch the years and that gap widens; keep both small and simple interest stays remarkably close.
Making simple interest work for you
Because simple interest is charged only on the original balance and only for the time that balance stays outstanding, you hold two honest levers — and they point in opposite directions depending on which side of the loan you sit. If you are borrowing on a simple-interest note, time is the cost. Every day the balance sits there adds another flat slice of interest, so a shorter term and any early payment work directly in your favor — pay ahead of schedule and you stop buying days you no longer need. On a true simple-interest loan there is generally nothing in the math that penalizes this, because the interest was never front-loaded or compounded; you are simply charged for fewer days. Ask for a per-diem payoff figure and you can see exactly what one more day of waiting costs. If you are saving, the lesson runs the other way. Simple interest is the floor, not the ceiling. Most deposit accounts and certificates compound, and over anything past a year that compounding pulls ahead of a flat rate. So treat a simple-interest projection as a baseline and, where you can, choose a vehicle that lets your interest earn interest of its own. On either side, shop the rate itself — it is the single input with the most leverage, and a fraction of a point, held across a long term, moves the total more than most people expect. Read the day-count assumptions too. A quote built on a 360-day convention behaves differently from one built on 365, and the unit a rate is quoted in can hide or reveal cost. Enter the real numbers here in days, months, or years, compare the after-tax and real-value outputs, and let the figures — not the headline rate alone — decide which offer genuinely serves you.
Common misunderstandings about simple interest
A few tidy assumptions about simple interest turn out to be wrong, and each one can cost you. The first is treating it as the same thing as APR. An annual percentage rate is a disclosure figure meant to fold a loan's rate together with certain fees into one comparable number; simple interest is a method for calculating the interest itself. A loan can quote an APR and still accrue day to day on the outstanding balance, or not — the label and the mechanics are separate questions, and assuming they line up can mislead you about what you will really pay. The second is expecting interest on interest. Simple interest never does this. The $500 a flat 5% pays on $10,000 does not itself begin earning next period; the base stays put. If a projection keeps accelerating on its own, something is compounding, and it is not simple interest. The third is believing a bigger headline rate always wins. Rate and time are multiplied together, so a high rate over a short stretch can hand you less total interest than a lower rate left in place for years. Nine percent for a few months and four percent for several years are not comparable until you actually run each one through the calculation — the sticker rate alone settles nothing. The last, and the one that catches savers, is assuming simple always means smaller or cheaper. As a savings method it usually understates what a real, compounding account would pay, so simple is not a synonym for less. As a borrowing method it is often the friendlier arrangement, precisely because paying early genuinely shrinks what you owe rather than unwinding a stack of pre-computed interest. The word describes how the interest is figured, not whether the deal is good for you. Put your own principal, rate, and term into the tool and let the numbers, not the vocabulary, tell you where you stand.
Frequently asked questions
What is simple interest?
Simple interest is a flat charge figured only on the money you started with — your original principal — and never on interest that has already piled up. Because the earning base never grows, you gain or owe the exact same dollar amount each period, so the balance rises in a straight line instead of curving upward. That one trait is what sets it apart from every compounding method.
What is the I = P · r · t formula?
The formula is I = P · r · t: principal times the annual rate times the term measured in years. For $10,000 at 5% over 3 years that's 10,000 × 0.05 × 3 = $1,500.00 of interest, giving a final balance of $11,500.00. The detail people trip on is that t is always in years, so a term you enter in days or months gets converted to a fraction of a year first.
How does simple interest compare to compound, and is there really a point where simple wins?
Yes, and it catches most people off guard. With annual compounding the two are identical at exactly one year, and for any term shorter than a year simple interest actually edges ahead, because a compounding curve dips just below the straight line before it catches up. On $10,000 at 5% for six months, simple pays $250.00 while annual compounding lands near $246.95 — so simple earns roughly $3.05 more. Only past the one-year mark does compounding pull decisively ahead.
Does choosing days, months, or years as the unit change my answer?
The unit is just for convenience — the tool converts whatever you type into years (days ÷ 365, months ÷ 12) before applying the formula, so genuinely equal terms give equal interest. One subtlety is worth knowing, though: because days are counted as 365ths of a year, 90 days works out to 0.2466 of a year while 3 months is 0.25, so on $10,000 at 5% they come to $123.29 and $125.00 — close, but not identical. Pick the unit that matches how your loan or deposit is quoted.
What does the "interest per day" figure mean?
It's the total interest spread evenly across every day of the term, which works because simple interest accrues at a steady rate. In the $10,000-at-5% example the tool shows $500.00 per year, $41.67 per month, and $1.37 per day — each one just the annual amount divided down. Lenders often call that daily number a "per diem," and it's precisely what a payoff quote uses to tack on interest for each extra day you carry the loan.
Do real bank savings accounts actually use simple interest?
Usually not — most savings accounts, money market accounts, and CDs compound, often daily or monthly, so they earn slightly more than a flat simple figure would suggest. That means leaning on this tool as a deposit projector tends to understate what a real account pays. Treat the simple result as a clean baseline, then use the built-in comparison to see how much compounding would add on top.
Where does simple interest genuinely apply in the real world?
It shows up far more in borrowing than in saving. Plenty of auto loans, personal loans, and promissory notes accrue simple interest on the outstanding balance, and short-term or bridge financing, margin borrowing, and Treasury-bill discounting frequently work this way too. The per-diem payoff quote on a car loan is a textbook case of simple interest doing its job.
How does the tax option work here?
When you turn on the tax setting, the tool applies your rate to the interest you earned — not to your principal, which was already yours. So on $1,500.00 of interest, the tax is drawn from that $1,500.00, and the after-tax figure shows what actually stays in your pocket. It's meant as an estimate for comparing scenarios, not a stand-in for your real tax situation.
What is the inflation or real-value number telling me?
That figure restates your final balance in today's buying power, since a dollar a few years out generally purchases less than a dollar right now. The nominal balance still shows the actual amount you'll hold, while the real value shows what that sum would feel like at current prices. It's a gut check on whether your interest is truly building wealth or just treading water against rising costs.
Can the term be a fraction of a year?
Absolutely — fractional terms are where simple interest earns its keep. Because I = P · r · t scales smoothly, half a year simply uses t = 0.5, and a 90-day note uses t of about 0.2466. Entering the term in days or months lets the tool handle the fraction for you, which is exactly how short-term notes and payoff quotes are calculated.
Is simple interest better for a borrower or a saver?
For a saver keeping money in place longer than a year, simple interest is the weaker deal, because a compounding account would pay more at the same rate. For a borrower it's generally kinder, since interest is charged only on the original balance and never snowballs on top of itself. That's why simple interest is common on consumer loans yet rare on long-term deposits.
How is simple interest different from an APR or a "flat-rate" loan quote?
Simple interest is a raw calculation on a balance, whereas APR is a standardized annual figure that folds in certain fees so loan offers can be compared fairly. A "flat-rate" quote is different again: it charges interest on the full original amount for the whole term even as you pay the balance down, so its true cost can run well above the stated percentage. This tool computes plain simple interest, so always confirm how a specific lender defines its rate.
Why doesn't my final balance match a compound-interest calculator?
Because the two are running different math. A compound calculator keeps folding earned interest back into the base, so each period's interest is figured on a growing balance, while this tool always applies your original principal. Over three years at 5% that gap is $76.25 — $11,576.25 with annual compounding versus $11,500.00 here — and it only widens the longer the term stretches.
How much will I actually earn or owe?
Enter your principal, your annual rate, and your term, and the tool hands back the total interest along with the final balance in one step. For $10,000 at 5% over 3 years you'd see $1,500.00 of interest and an $11,500.00 balance — a 15% total return across the full term. Flip on the tax or inflation options to see how those figures shift once real-world drag is accounted for.
What counts as a realistic simple interest rate?
It depends entirely on whether you're saving or borrowing. Deposit-style rates usually sit low, in the low single digits, while consumer loans and short-term notes can run much higher, so a rate north of about 20% is more typical of borrowing than of any account paying you. If the tool flags your rate as unusually high for a deposit, double-check whether you've entered a loan rate by mistake.