Simple interest calculator

Simple interest is interest worked out only on the amount you start with. It never earns interest on itself. Enter any three of these: the amount, the rate, the time and the interest. The calculator finds the fourth. It can also count the exact days between two dates.

For loans, notes and short-term deposits · Free, no sign-up · How it works

I want to find

Principal

The amount lent, borrowed or saved.

Rate

Use the rate on your loan papers or account terms. For scale, 3-month Treasury bills averaged 3.4% a year from 1928 to 2025.

Time

Simple interest

$816.00

Total at the end: $6,816.00

$204.00Interest a year

$17.00A month

$0.56A day (÷ 365)

Principal 88.0%Interest 12.0%

Simple interest on $6,000 at 3.4% a year for 4 years is $816.00. The total is $6,816.00.

The work: 6,000 × 0.034 × 4 = $816.00

In Excel or Google Sheets: =6000*0.034*4

Added back once a year at the same rate, the interest would be $858.57, which is $42.57 more. See compound interest

StartHalfway4 years

Your balanceWhat you put in

Simple interest adds the same amount each period, so the total rises in a straight line.

Hypothetical illustration based on your inputs. Not a prediction, a recommendation, or the performance of any Amaltash strategy.

How it works: interest is the amount times the yearly rate times the time in years. It is worked out on the starting amount only and never added back.

Results change each time you change an input.

Real rates change. Fees, taxes and penalties are not included, and a rate you are offered can differ from this one.

See every year

Period	Interest that period	Interest so far	Total
Year 1	$204.00	$204.00	$6,204.00
Year 2	$204.00	$408.00	$6,408.00
Year 3	$204.00	$612.00	$6,612.00
Year 4	$204.00	$816.00	$6,816.00

What is simple interest?

Simple interest is a fee for using money, worked out only on the starting amount. That starting amount is called the principal. Past interest never earns more interest, so the same number of dollars is added each year. On $6,000 at 5% a year, that is $300 every year.

Year	Interest that year	Total
1	$300	$6,300
2	$300	$6,600
3	$300	$6,900
4	$300	$7,200

The total goes up by the same $300 each year. On a chart, that is a straight line. That is why simple interest is called linear.

Here is where simple interest is used:

Car loans. A US agency says simple interest is far more common on them. Its name is the Consumer Financial Protection Bureau, or CFPB for short.
US Treasury bills and notes. A note pays a fixed rate every six months.
Late bills paid by US agencies. A federal rule shows how to count it by the day.

Savings accounts and CDs work another way. They state an APY, a yearly rate that counts interest earned on interest.

See how the same money grows with the compound interest calculator →

Check what cash earns with the high-yield savings calculator →

How to use this simple interest calculator, step by step

Pick what you want to find: the interest, the principal, the rate or the time. Then fill in the other three boxes. The answer shows at once, with the math written out. Choose years, months, days or two dates for the time. A table and a chart show the total growing.

Choose what to find. That box goes away, since it is the answer.
Type the principal. It is the amount lent, borrowed or saved.
Type the rate. Mark it as a yearly rate or a monthly rate.
Set the time in years, months, weeks or days. Or pick a start date and an end date.
To find the principal, the rate or the time, type the interest. You can type the total at the end instead.
Read the answer, the steps and the table. Share the link to keep your numbers.

The Loan payments tab is for a loan you pay back each month. It builds interest by the day and lists each payment.

The rate box starts with a past average for Treasury bills. The note under the box gives its years. It is not a rate to expect, so type the rate from your own loan or account.

The simple interest formula: I = Prt

The simple interest formula is I = Prt. Multiply the principal by the yearly rate, then by the time in years. Write the rate as a decimal, so 5% is 0.05. For $6,000 at 5% for 4 years, that is 6,000 × 0.05 × 4 = $1,200. Add the principal to get the total, $7,200.

I = P \times r \times t

In words: Multiply the amount by the yearly rate, then by the number of years.

$I$: the interest, in dollars
$P$: the principal, the amount you start with
$r$: the yearly rate, as a decimal (5% is 0.05)
$t$: the time, in years

A = P + I = P\,(1 + r\,t)

In words: Add the interest to the amount you started with.

$A$: the total at the end, principal plus interest
$P$: the principal, the amount you start with
$I$: the interest, in dollars
$r$: the yearly rate, as a decimal (5% is 0.05)
$t$: the time, in years

Write the rate as a decimal. 5% becomes 0.05.
Put the time in years. 18 months becomes 1.5 years.
Multiply the principal, the rate and the time.
Add the principal to get the total.

Here is one year of simple interest on common amounts:

Principal	4%	5%	6%	8%	10%
$1,000	$40	$50	$60	$80	$100
$5,000	$200	$250	$300	$400	$500
$10,000	$400	$500	$600	$800	$1,000
$25,000	$1,000	$1,250	$1,500	$2,000	$2,500
$50,000	$2,000	$2,500	$3,000	$4,000	$5,000
$100,000	$4,000	$5,000	$6,000	$8,000	$10,000
$1,000,000	$40,000	$50,000	$60,000	$80,000	$100,000

For two or three years, multiply the cell by 2 or 3. So 5% on $10,000 is $500 a year, or $1,500 over three years.

See what money you add each month grows to with the future value calculator →

Find a doubling time with the rule of 72 calculator →

How to find the rate, the time or the principal

Turn the formula around and divide. To find the rate, divide the interest by the principal times the time. To find the time, divide the interest by the principal times the rate. To find the principal, divide the interest by the rate times the time. Keep the time in years.

r = \dfrac{I}{P\,t}

In words: Divide the interest by the principal times the years.

$r$: the yearly rate, as a decimal (5% is 0.05)
$I$: the interest, in dollars
$P$: the principal, the amount you start with
$t$: the time, in years

t = \dfrac{I}{P\,r}

In words: Divide the interest by the principal times the yearly rate.

$t$: the time, in years
$I$: the interest, in dollars
$P$: the principal, the amount you start with
$r$: the yearly rate, as a decimal (5% is 0.05)

P = \dfrac{I}{r\,t}

In words: Divide the interest by the yearly rate times the years.

$P$: the principal, the amount you start with
$I$: the interest, in dollars
$r$: the yearly rate, as a decimal (5% is 0.05)
$t$: the time, in years

You want	You know	Work	Answer
Principal	$600 interest, 4%, 2 years	600 ÷ (0.04 × 2)	$7,500
Rate	$2,000 earns $270 in 18 months	270 ÷ (2,000 × 1.5)	9%
Time	$4,000 at 5% earns $500	500 ÷ (4,000 × 0.05)	2.5 years (30 months)
Rate, from a total	$8,000 grows to $9,200 in 3 years	(9,200 − 8,000) ÷ (8,000 × 3)	5%

If you know the total at the end, take away the principal first. What is left is the interest.

Simple interest does not build on itself. For growth that does, use a yearly growth rate.

Find the yearly growth rate between two values with the CAGR calculator →

Simple interest for months and days

The rate is for a full year, so turn months or days into a part of a year first. Divide months by 12. Divide days by 365. Nine months is 9 ÷ 12, or 0.75 of a year. On $6,000 at 5%, nine months of interest is $225, and one day is 82 cents.

t = \dfrac{m}{12}

In words: Divide the number of months by 12 to get years.

$t$: the time, in years
$m$: the number of months

I_{\text{day}} = \dfrac{P \times r}{365}

In words: Multiply the balance by the yearly rate. Then divide by 365.

$I_{\text{day}}$: the interest for one day
$P$: the principal, the amount you start with
$r$: the yearly rate, as a decimal (5% is 0.05)

One day of interest has a name: per diem. That is Latin for "by the day".

Here is $6,000 at 5% a year for three lengths of time:

Time	Part of a year	Interest
9 months	9 ÷ 12	$225.00
26 weeks	26 ÷ 52	$150.00
90 days	90 ÷ 365	$73.97

Some loans state a rate for each month. Multiply it by 12 to get the yearly rate. So 1.5% a month is 18% a year. On $2,000 for 3 months, that is $90 of interest.

Interest for one day on a loan balance, on a 365-day year:

Balance	4%	6%	8%	10%
$5,000	$0.55	$0.82	$1.10	$1.37
$10,000	$1.10	$1.64	$2.19	$2.74
$20,000	$2.19	$3.29	$4.38	$5.48
$30,000	$3.29	$4.93	$6.58	$8.22
$40,000	$4.38	$6.58	$8.77	$10.96

Credit cards call their per-day rate the daily periodic rate.

See a card or loan rate by the day with the APR calculator →

Simple interest between two dates

Count the days from the start date to the end date, then divide by the days in a year. Leave out the first day and count the last one. From January 15 to July 15, 2027 is 181 days. On $10,000 at 6%, that is 10,000 × 0.06 × 181 ÷ 365 = $297.53.

Count the days. Skip the start day and count the end day.
Divide the days by 365. That gives the part of a year.
Multiply by the principal and by the yearly rate.

A leap year adds a day. From January 15 to July 15, 2028 is 182 days, since that year has a February 29. The interest is $299.18.

Six months is not a fixed number of days. It can be anywhere from 181 to 184 days. So the same loan can cost a little more or less for "six months". The next section shows three ways to count the days.

360 or 365 days? Day-count conventions

A day count is the rule for turning days into a part of a year. Exact interest divides the real days by 365. Ordinary interest divides them by 360, so each day costs a little more. A third rule, 30/360, treats every month as 30 days. Your contract says which one applies.

Name	Days counted	Year	Also called	Where it shows up
Actual/365	real days	365	exact interest	Bank deposits
Actual/360	real days	360	ordinary interest	Treasury bill discount rates
30/360	30 a month	360	bond basis	Municipal bonds

Regulation DD, the bank deposit rule, sets a daily rate of at least 1/365 of the yearly rate. In a leap year, 1/366 is allowed.

Ordinary interest is also called the banker’s year, or 365/360. US Treasury bill discount rates use it. So does the federal Prompt Payment formula for late bills. The Federal Reserve lists money market rates on a 360-day year too.

Municipal bonds use 30/360. The formula below is written in MSRB Rule G-33.

d = 360\,(Y_2 - Y_1) + 30\,(M_2 - M_1) + (D_2 - D_1)

In words: Count 360 days for each year and 30 for each month. Then add the days left over.

$d$: the days counted
$Y_1, M_1, D_1$: the year, month and day of the start date
$Y_2, M_2, D_2$: the year, month and day of the end date

A start date on the 31st counts as the 30th. So does an end date on the 31st, when the start is the 30th or 31st.

A lending rule, Regulation Z, lets loan papers treat all months as equal. The papers must still count the effect of a 360-day rate charged on 365 days.

Here is the same $10,000 at 6% on real dates, under each rule:

Dates	Actual/365	Actual/360	30/360
Jan 15 to Jul 15, 2027	181 days, $297.53	181 days, $301.67	180 days, $300.00
Jan 15 to Jul 15, 2028	182 days, $299.18	182 days, $303.33	180 days, $300.00
Feb 1 to Mar 1, 2027	28 days, $46.03	28 days, $46.67	30 days, $50.00
Apr 1 to May 1, 2027	30 days, $49.32	30 days, $50.00	30 days, $50.00
Jul 1 to Aug 1, 2027	31 days, $50.96	31 days, $51.67	30 days, $50.00
Jan 1, 2027 to Jan 1, 2028	365 days, $600.00	365 days, $608.33	360 days, $600.00
Jan 1, 2028 to Jan 1, 2029	366 days, $601.64	366 days, $610.00	360 days, $600.00
Jan 31 to Mar 31, 2027	59 days, $96.99	59 days, $98.33	60 days, $100.00

February costs $46.03 or $50.00 on the same loan. Only the rule changed. Under 30/360, each full month costs the same. 2028 is a leap year, so it adds one day of interest under both actual rules.

Over a full year, a 360-day year charges the daily rate 365 times. That is about 1.39% more interest.

r_{\text{real}} = r \times \dfrac{365}{360}

In words: Multiply the stated rate by 365, then divide by 360.

$r_{\text{real}}$: the rate you really pay over a full year
$r$: the stated yearly rate

Stated rate	Real rate	Extra on $100,000 in a year
4%	4.0556%	$55.56
6%	6.0833%	$83.33
8%	8.1111%	$111.11
9%	9.1250%	$125.00
10%	10.1389%	$138.89
12%	12.1667%	$166.67

At 9%, the gap is one eighth of a point. That is the most a stated APR may miss by on a regular loan. The FDIC, a US bank agency, notes this in its exam manual.

One federal student loan servicer divides by 365.25 days.

How a simple-interest car loan works

On a simple-interest loan, interest is charged only on what you still owe. It is counted by the day or by the month. By the day, it is your balance times the yearly rate, divided by 365. Each payment covers that interest first. The rest pays down the balance, so later payments carry less interest.

Here is a made-up example. A $25,000 loan at 7% a year runs for 60 months. The payment is $495.03. On day one, interest builds at $4.79 a day. The first payment comes 31 days in. Of that payment, $148.63 is interest and $346.40 comes off the balance.

M = \dfrac{L \times i}{1 - (1 + i)^{-n}}

In words: Multiply the loan by the monthly rate. Then divide by 1 minus a small number: 1 plus the monthly rate, raised to minus the number of payments.

$M$: the payment each month
$L$: the amount of the loan
$i$: the monthly rate: the yearly rate divided by 12
$n$: the number of monthly payments

I_k = B \times r \times \dfrac{d}{365}

In words: Multiply the balance by the yearly rate and by the days. Then divide by 365.

$I_k$: the interest in one payment
$B$: the balance you still owe
$r$: the yearly rate, as a decimal (5% is 0.05)
$d$: the days since the last payment

“Amortized” names the plan of equal payments. It is not a different kind of interest. One loan can be both.

A payoff amount is not the same as your balance. The CFPB says it adds the interest due through the day you pay the loan off.

A loan also has an APR. That is the yearly cost with some fees counted in.

Turn a loan rate into APR or APY with the APR calculator →

See how a loan counts against what you own with the net worth calculator →

What paying early or late does

Interest builds each day, so the day you pay changes the cost. Pay early and fewer days pile up. Pay late and more do. Take a made-up $25,000 loan at 7% for five years. Paying 10 days early each month cuts the interest by about $68. Adding $50 a month cuts it by about $523.

How you pay	Payments	Total interest	Against on time	Last payment
On the due date	60	$4,701.51	—	$494.74
10 days early each month	60	$4,633.91	−$67.60	$427.14
10 days late each month	60	$4,769.08	+$67.57	$562.31
On time, plus $50 a month	54	$4,178.57	−$522.94	$291.98
On time, plus $100 a month	49	$3,762.26	−$939.25	$200.82

Extra principal helps far more than early days. Ten days of timing moves about $68. An extra $50 a month moves about $523 and ends the loan 6 payments sooner.

This example leaves out late fees. A lender may also round or count days in its own way. Check your contract before you pay extra, since some loans have rules on early payoff.

Simple vs precomputed interest: what does add-on mean on a loan?

With precomputed interest, all the interest is worked out on day one and spread over the payments. Add-on interest is one kind: the lender uses I = Prt on the full loan for the full term. You pay interest on money you have already paid back, so the real yearly rate is far higher.

M = \dfrac{L\,(1 + r\,t)}{n}

In words: Work out the interest on the full loan for the full term. Add it to the loan. Then divide by the number of payments.

$M$: the payment each month
$L$: the amount of the loan
$r$: the flat add-on rate, as a decimal
$t$: the time, in years
$n$: the number of monthly payments

Here is a made-up example: $20,000 for 48 months at 8%.

Kind of loan	Payment	Total interest	Real yearly rate
Simple interest at 8%	$488.26	$3,436.48	8.00%
Add-on interest at 8%	$550.00	$6,400.00	14.35%

The next table turns a flat add-on rate into the real yearly rate. No fees are counted.

Flat add-on rate	12 months	24 months	36 months	48 months	60 months
5%	9.10%	9.32%	9.31%	9.24%	9.15%
8%	14.45%	14.68%	14.55%	14.35%	14.13%
10%	17.97%	18.16%	17.92%	17.60%	17.27%

An add-on rate costs a bit under twice the same simple rate.

What if you pay off early? A federal law, 15 U.S.C. 1615, says the lender must refund interest it has not yet earned. On precomputed consumer loans longer than 61 months, the refund must be at least as good as the actuarial method. That method counts interest on the balance you still owe. State law may add more. This is not legal advice.

An older method, the Rule of 78, gives a smaller refund. In the example above, paying off after 12 payments would cost $161.80 more under it.

The CFPB says this kind of interest is not common. It helps the lender if you pay more than the minimum or pay the loan off early.

Interest taken off up front: the discount basis

Some loans take the interest off at the start. You get less than the face amount and pay back the full amount later. This is called the discount basis. Because you hold less money than the face amount, the real simple rate is higher than the stated discount rate.

r = \dfrac{d}{1 - d\,t}

In words: Divide the discount rate by 1 minus the discount rate times the years.

$r$: the real simple rate, as a decimal
$d$: the discount rate, as a decimal
$t$: the time, in years

Here is a $1,000 face amount at a few made-up discount rates:

Discount rate and time	You get	You repay	Real simple rate
6% for 6 months	$970.00	$1,000.00	6.19%
8% for 1 year	$920.00	$1,000.00	8.70%
10% for 1 year	$900.00	$1,000.00	11.11%
12% for 1 year	$880.00	$1,000.00	13.64%
10% for 2 years	$800.00	$1,000.00	12.50%

The higher the discount rate and the longer the time, the wider the gap.

A US Treasury bill sells at or below its face value and pays the face value at the end. Its price formula uses a 360-day year.

Price a bill and see its yield with the T-bill calculator →

Compare with a bank CD using the CD calculator →

Simple interest in Excel or Google Sheets

Put the principal in A2, the yearly rate in B2 and the years in C2. Then type =A2*B2*C2 in another cell. For two dates in D2 and E2, use =A2*B2*(E2-D2)/365. The result is the same as the one on this page.

Time given as	Formula	Result
Years	=60000.054	1200.00
Dates, 365-day year	=100000.06(E2-D2)/365	297.53
Dates, 360-day year	=100000.06YEARFRAC(D2,E2,2)	301.67
Dates, 30/360	=100000.06YEARFRAC(D2,E2,0)	300.00

In the date rows, D2 holds January 15, 2027 and E2 holds July 15, 2027.

YEARFRAC is a built-in sheet function. Its last number picks the day count: 2 means a 360-day year and 0 means 30/360.

Type the rate as 5% or 0.05, never as 5. The tool above writes the formula for your own numbers.

How this calculator works

What it counts. Interest on the amount, the rate and the time you enter. The interest is never added back to earn more.

Days and dates. We leave out the start day and count the end day. Dates are plain calendar days, with no time zones.

The 30/360 rule. We use the formula in MSRB Rule G-33. It has no special rule for the end of February, so a spreadsheet can differ by a day or two.

The loan tab. The payment is rounded to cents. Interest builds each day on a 365-day year and is rounded to cents at each payment. The last payment clears what is left.

What it leaves out. Fees, taxes, late charges and each lender’s own rounding.

Private. All of this runs on your device. What you type is not saved to an account.

Questions people ask

Is simple interest monthly or yearly?

The rate is usually stated per year. You can still count the interest by the month or by the day. Divide the yearly interest by 12 or by 365. On $6,000 at 5%, that is $300 a year, $25 a month, or about 82 cents a day.

Which is better for me, simple interest or compound interest?

It depends on which side you are on. A borrower pays less with simple interest at the same rate, and a saver earns less. On $6,000 at 5% for 4 years, simple interest is $1,200. With interest added back once a year, it is $1,293.04.

Can simple interest be more than the principal?

Yes. Interest passes the principal once the rate times the years is more than 1. At 8% a year, that takes 12.5 years, because 0.08 × 12.5 = 1. After 13 years at 8%, the interest is 104% of the amount you started with.

How long does it take to double your money with simple interest?

Divide 100 by the rate. At 5% a year, simple interest doubles your money in 20 years, because 100 ÷ 5 = 20. With interest compounded once a year, the same rate takes about 14.2 years. The rule of 72 gives a quick guess for that case.

Do car loans use simple interest?

Most do. The CFPB, a US consumer agency, says simple interest is far more common. The charge is based on the balance you still owe. It is counted by the day or by the month. Your loan contract says which way it works.

Does simple interest compound if it is counted each day?

No. It never earns interest on itself. That is true even when it is counted each day. Each day’s interest is worked out on the principal alone. Now say unpaid interest is added to the balance. If that balance is then charged interest, it has become compound interest.

What do the terms ordinary interest and exact interest mean on a loan?

Ordinary interest uses a 360-day year, and exact interest uses a 365-day year. The same days cost a little more with ordinary interest. On $10,000 at 6% for 90 days, ordinary interest is $150.00 and exact interest is $147.95.

Is a simple interest rate the same as APR?

Only when the loan has no fees. APR is the yearly cost of a loan with certain fees counted in, so fees push the APR above the interest rate. With no fees, the two numbers are about the same.

More calculators

Compound interest calculatorSee how interest earns interest at a fixed rate.APR to APY calculatorTurn an APR into APY and a daily rate, or go back.Rule of 72 calculatorCheck the rule of 72 against the exact time to double.Investment calculatorPick an amount, a time and a real past average rate. See how it could add up.Retirement calculatorTest a plan against every stretch of market history since 1928.Budget calculatorAdd up your pay and your spending. See what is left each month.

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Reviewed by Tavish Sharma, CFA · Last reviewed Nov 12, 2026

Published by Amaltash Advisors LLC, an SEC-registered investment adviser

Sources

What’s the difference between a simple interest rate and precomputed interest on an auto loan?, Consumer Financial Protection Bureau
What is a payoff amount and is it the same as my current balance?, Consumer Financial Protection Bureau
What is the difference between a loan interest rate and the APR?, Consumer Financial Protection Bureau
Treasury Bills, TreasuryDirect, US Department of the Treasury
Understanding Pricing and Interest Rates, TreasuryDirect, US Department of the Treasury
Treasury Notes, TreasuryDirect, US Department of the Treasury
Regulation DD § 1030.7, Payment of interest, Consumer Financial Protection Bureau
Regulation Z § 1026.17, General disclosure requirements, Consumer Financial Protection Bureau
Consumer Compliance Examination Manual, V-1: Truth in Lending Act, FDIC
Selected Interest Rates (Daily), H.15, Federal Reserve Board
5 CFR § 1315.17, Formulas (Prompt Payment), US Government Publishing Office
Rule G-33, Calculations, Municipal Securities Rulemaking Board
15 U.S.C. § 1615, Prohibition on use of “Rule of 78’s”, US Government Publishing Office
Student Loan Interest, MOHELA, a servicer for Federal Student Aid
Historical Returns on Stocks, Bonds and Bills, 1928–2025, NYU Stern School of Business (Aswath Damodaran)