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Browse all calculatorsFind the interest, principal, rate or time. Enter the term in years, months and days, see the working and a year-by-year table, and compare it with compound interest.Find the interest, principal, rate or time, with the working shown.
Interest is the same every full year because it is charged on the original principal only.
| Year | Interest | Total interest | Balance |
|---|
Private by design. Everything runs in your browser and is never uploaded. Results are estimates, not financial advice.
Simple interest is interest charged only on the original amount, never on interest already added. To find it, multiply the principal by the annual rate as a decimal and by the time in years: I = P × r × t. For 20,000 at 3% for 10 years, the interest is 6,000 and the end amount is 26,000.
This simple interest calculator solves for interest, principal, rate or time. It takes the term as any mix of years, months and days, and accepts a rate per year or per month. Results change as you type, the working puts your numbers into each step of the formula, and a table splits the interest by year or by month. It treats the principal as fixed with no payments, so for savings that earn interest on interest, see the comparison with compound interest below. For other everyday sums, browse the free online calculators.
| Case and working | Interest | End amount |
|---|---|---|
| 20,000 at 3% per year for 10 years20,000 × 0.03 × 10 | 6,000.00 | 26,000.00 |
| 10,000 at 5% per year for 5 years10,000 × 0.05 × 5 | 2,500.00 | 12,500.00 |
| 5,000 at 4% per year for 18 months5,000 × 0.04 × 18 ÷ 12 | 300.00 | 5,300.00 |
| 1,000 at 6% per year for 90 days (365-day year)1,000 × 0.06 × 90 ÷ 365 | 14.79 | 1,014.79 |
| 1,000 at 6% per year for 90 days (360-day year)1,000 × 0.06 × 90 ÷ 360 | 15.00 | 1,015.00 |
| 2,000 at 1.5% per month for 8 months2,000 × 0.18 × 8 ÷ 12 | 240.00 | 2,240.00 |
The first answer is already on screen. Change any number and it updates at once, with no Calculate button.
Interest is the default. Choose Principal, Rate or Time to solve for one of those instead, then say whether you know the interest or the end amount.
Type the starting amount and the rate. Commas and a percent sign are fine. Use the menu beside the rate to set it per year or per month.
Fill in any mix of years, months and days, such as 1 year and 6 months. Months count as one twelfth of a year, and days use a 365-day or 360-day year, as you choose.
See the result, the formula with your numbers and a table by year or month. Copy the result, share a link that opens with the same numbers, or download the table as a CSV file.
Simple interest grows in a straight line, because the base never changes and the same amount is added in every period. With the rate written as a decimal and the time in years, the interest is I = P × r × t and the end amount is A = P + I.
To write a percentage as a decimal, divide by 100, so 4.5% is 0.045 (the percentage calculator handles other percentage sums). Months become years by dividing by 12, so 18 months is 1.5 years, or 3/2 if you prefer fractions (the fraction calculator shows the steps). Days are divided by 365 in the textbook method, and some lenders use 360, which gives slightly more interest for the same number of days. Money is rounded to the nearest cent, with half a cent rounded up, as covered in the rounding calculator.
| To find | Formula |
|---|---|
| Interest | I = P × r × t |
| End amount | A = P + I, or A = P × (1 + r × t) |
| Principal from interest | P = I ÷ (r × t) |
| Principal from end amount | P = A ÷ (1 + r × t) |
| Rate | r = I ÷ (P × t) |
| Time in years | t = I ÷ (P × r) |
| Term as years | years + months ÷ 12 + days ÷ 365 (or 360) |
| Monthly rate to annual | annual rate = monthly rate × 12 |
Interest for one year, rounded to the cent. For a longer term, multiply by the number of years, because the same amount is added every year.
| Principal | 2% | 3% | 4% | 5% | 6% | 8% | 10% |
|---|---|---|---|---|---|---|---|
| 500 | 10.00 | 15.00 | 20.00 | 25.00 | 30.00 | 40.00 | 50.00 |
| 1,000 | 20.00 | 30.00 | 40.00 | 50.00 | 60.00 | 80.00 | 100.00 |
| 2,500 | 50.00 | 75.00 | 100.00 | 125.00 | 150.00 | 200.00 | 250.00 |
| 5,000 | 100.00 | 150.00 | 200.00 | 250.00 | 300.00 | 400.00 | 500.00 |
| 10,000 | 200.00 | 300.00 | 400.00 | 500.00 | 600.00 | 800.00 | 1,000.00 |
| 20,000 | 400.00 | 600.00 | 800.00 | 1,000.00 | 1,200.00 | 1,600.00 | 2,000.00 |
| 50,000 | 1,000.00 | 1,500.00 | 2,000.00 | 2,500.00 | 3,000.00 | 4,000.00 | 5,000.00 |
| Term | Time in years (t) | Interest |
|---|---|---|
| 1 day (365-day year) | 0.0027 | 1.64 |
| 30 days (365-day year) | 0.0822 | 49.32 |
| 30 days (360-day year) | 0.0833 | 50.00 |
| 90 days (365-day year) | 0.2466 | 147.95 |
| 90 days (360-day year) | 0.2500 | 150.00 |
| 6 months | 0.5000 | 300.00 |
| 9 months | 0.7500 | 450.00 |
| 18 months | 1.5000 | 900.00 |
| 2 years and 6 months | 2.5000 | 1,500.00 |
Simple interest is earned on the original principal only. Compound interest is also earned on the interest already added, so the two drift apart the longer the money stays in.
| Years | Simple interest | Compounded yearly | Compounded monthly | Extra from yearly compounding |
|---|---|---|---|---|
| 1 | 10,500.00 | 10,500.00 | 10,511.62 | 0.00 |
| 2 | 11,000.00 | 11,025.00 | 11,049.41 | 25.00 |
| 5 | 12,500.00 | 12,762.82 | 12,833.59 | 262.82 |
| 10 | 15,000.00 | 16,288.95 | 16,470.09 | 1,288.95 |
| 20 | 20,000.00 | 26,532.98 | 27,126.40 | 6,532.98 |
| 30 | 25,000.00 | 43,219.42 | 44,677.44 | 18,219.42 |
After one year the simple and yearly figures are the same, 10,500.00, because no interest has yet been added to earn more interest. By year 5 simple interest gives 12,500.00, yearly compounding 12,762.82 and monthly compounding 12,833.59. The compound formula is A = P × (1 + r ÷ n)n × t, where n is the number of compounding periods in a year.
Which one is better depends on whether you are earning or paying, on the real rate and on the terms of the account or loan. The table shows how big the gap can get, and the choice stays with you. Under every result, this calculator also shows the end amount if the same rate were compounded monthly.
Many simple interest calculators take a principal, a rate and a number of years and stop there. These are the details that change the answer or save you a step.
| Feature | Basic simple interest calculator | This calculator |
|---|---|---|
| What it solves | Often only interest and the end amount | Interest, principal, rate or time, from the interest or the end amount |
| Term | Often years, or years and months | Any mix of years, months and days |
| Rate period | Usually per year | Per year or per month, with both shown |
| Days in a year | Rarely stated | 365-day or 360-day year, your choice |
| Working | The answer, or a general formula | Your numbers put into each step |
| Breakdown | None, or one row per year | One row per year or per month, part periods marked, CSV download |
| Compound comparison | A separate page, or none | A monthly-compounding line under every result |
| Results | Often a Calculate button | Update as you type, with an answer on first load |
| Sharing | Rare | Copy the result or share a link with the same numbers |
Estimate the interest on a loan or note that charges a flat rate on the amount borrowed, and see the end amount owed.
On a fixed balance, one month of interest is the principal times the annual rate divided by 12. For 30,000 at 6% that is 150 a month.
Know the principal, the term and the interest charged? Choose Find the rate to see the rate per year and per month behind those numbers.
Set up I = Prt, then compare each line of your own working with the steps shown. Switch between years, months and days to practise the conversions.
If an agreement states a flat monthly rate on an overdue amount, choose per month and enter the months or days it ran.
Add the interest to a budget in the expense calculator, or copy the result into the online notepad so the numbers stay with your notes.
These points explain most of the gaps between this page and a figure quoted by a lender. The calculator shows an estimate and does not give advice.
A rate of 1% per month is 12% per year. Set the rate period before you read the answer.
A 360-day year gives slightly more interest than a 365-day year for the same number of days. Check which one the agreement uses.
Six months is 0.5 years and 18 months is 1.5 years. Days are added on top as a fraction of the chosen year.
Some loans called simple interest charge each day on the part of the principal still owed after each payment. That gives less interest than a fixed principal for the whole term.
Amounts are rounded to the nearest cent, with half a cent rounded up. A lender may round another way, so a cent or two can differ.
Nothing you type leaves your browser. After the page loads, the calculator keeps working without a connection.
It assumes the principal stays the same for the whole term. It is not a loan payment schedule, and it does not reduce the balance as payments are made.
Only interest on the principal is counted. Fees, taxes and the annual percentage rate of a real loan or account are outside this calculation.
Results are estimates for learning and planning. Day counts and rounding differ between lenders, so confirm the figures with the lender. See day count conventions for how they vary.
The formulas are standard arithmetic. The calculator was checked against the worked examples on this page and against 400 random cases computed separately with exact fractions. The definitions and conventions come from these references.
Short answers to common questions about simple interest.
Multiply the principal by the annual rate written as a decimal, then by the time in years: I = P × r × t. For 20,000 at 3% for 10 years, I = 20,000 × 0.03 × 10 = 6,000, so the end amount is 26,000.
The interest is I = P × r × t and the end amount is A = P + I, or A = P × (1 + r × t). Rearranged, the principal is P = I ÷ (r × t), the rate is r = I ÷ (P × t) and the time is t = I ÷ (P × r).
Divide the months by 12 to get years, then use the usual formula. For 5,000 at 4% for 18 months, t = 18 ÷ 12 = 1.5 and I = 5,000 × 0.04 × 1.5 = 300. In the calculator, type 18 in the Months box, or 1 in Years and 6 in Months.
Divide the days by 365, or by 360 if the agreement uses a 360-day year. For 1,000 at 6% for 90 days, I = 1,000 × 0.06 × 90 ÷ 365 = 14.79 on a 365-day year and 15.00 on a 360-day year. Choose the year length in the Days in a year option.
Multiply a monthly rate by 12 to get the annual rate, which is what the formula needs. A rate of 1.5% per month is 18% per year, so 2,000 for 8 months earns 2,000 × 0.18 × 8 ÷ 12 = 240. Choose per month in the rate menu and the calculator converts it for you.
Choose Principal, Rate or Time at the top, say whether you know the interest or the end amount, and fill in the other boxes. For example, 600 of interest at 3% over 10 years gives P = 600 ÷ (0.03 × 10) = 2,000. The working shows the rearranged formula with your numbers.
Simple interest is earned only on the original principal. Compound interest is also earned on the interest already added, so the balance grows faster. For 10,000 at 5% for 5 years, simple interest gives 12,500.00 and yearly compounding gives 12,762.82.
Neither is better in every case. The gap depends on the rate, the term and whether you are earning or paying, and the comparison table on this page shows how large it can become. Check the terms of the actual account or loan before deciding.
It is used for some short-term loans and notes, some bonds and fixed-rate deposits that pay on the original amount, and in school maths. Many real accounts compound instead, and some loans called simple interest charge each day on the balance still owed, so read the agreement.
No. It assumes the principal stays the same for the whole term and that no payments are made. If each payment lowers the balance, interest is charged on a smaller amount and the true interest is lower than the figure here.
The arithmetic is exact to the nearest cent, with half a cent rounded up, but the result is an estimate. A lender may count days, round or add fees in a different way, so use the figures as a guide and confirm them with the lender.
No. The calculation runs in your browser and nothing is sent to a server. The numbers appear in the page address only after you change something, so a shared link opens with the same figures, and your currency choice is remembered on your own device.