Simple Interest Calculator
Work out the interest and total on a principal from an annual rate and a term, or solve for the principal, rate or time, with the working and a yearly schedule.
Interest: 150.00 Total amount: 1,150.00
| Year | Interest | Total interest | Balance |
|---|---|---|---|
| 1 | 50.00 | 50.00 | 1,050.00 |
| 2 | 50.00 | 100.00 | 1,100.00 |
| 3 | 50.00 | 150.00 | 1,150.00 |
Days count on a 365-day year unless the 360-day one is chosen. This is a mathematical estimate, not financial advice.
How the estimate works
Enter a principal, an annual interest rate as a percent, and a term in years, quarters, months, weeks or days (anything other than years is converted to a fraction of a year, and days count on a 365-day year unless you pick the 360-day one). The Unit select also offers "Start and end dates", which swaps the Time box for two dates and counts the calendar days between them, a leap day included; and with "Rate quoted" set to per month, the rate box becomes "Monthly rate %" and the working first multiplies it up to a yearly rate. The tool applies the simple interest formula I = P × r × t, where P is the principal, r is the annual rate as a decimal (rate% ÷ 100) and t is the number of years. It instantly shows the estimated interest and the total amount, which is simply the principal plus the interest — or, with "Solve for" set to Principal, Annual rate % or Time and the interest earned typed in, the value that was missing. Unlike compound interest, simple interest is calculated only on the original principal and never on interest already earned, so it grows in a straight line over time. Money figures — the interest, the total, a solved principal and every cell of the schedule — are written grouped and to the cent (1,150.00), and a solved rate or term is rounded to at most two decimals, while the calculation underneath keeps full precision. The Currency select starts on "plain numbers", so you read the result in whatever units you entered; pick a currency and the interest, the total and the yearly schedule are written as money in it. While the page is solving for the interest, "Regular contributions" (None by default) adds Additions or Deductions of a fixed amount — monthly, quarterly, half-yearly or yearly, made at the end or the start of each period, as with a regular savings plan — and each one earns simple interest on its own amount for the time left after it is made. The answer gains an "Added" (or "Deducted") line, and the working and the yearly schedule carry the contributions too; deductions that would take out more than the principal are refused.
Estimate only, not financial advice
This is a mathematical estimate that assumes a single fixed rate applied over the whole term with no compounding, fees or taxes, and no payments beyond any regular contributions you enter. Real loans, savings and investments often use compounding or variable rates, so actual figures will differ. Use it to compare scenarios and build intuition, not as a guarantee or a substitute for advice from a qualified financial professional.
Frequently asked questions
What formula does this calculator use?
The simple interest formula I = P × r × t, where P is the principal, r is the annual rate as a decimal (your rate% divided by 100), and t is the time in years. The total amount is P + I.
How is simple interest different from compound interest?
Simple interest is charged only on the original principal, so it grows by the same amount every year. Compound interest also earns interest on previously accumulated interest, so it grows faster over time. This tool calculates the simple version.
Can it work out the rate, the principal or the time instead, and show the working?
Yes. Set "Solve for" to Principal, Annual rate % or Time and type the "Interest earned", and it turns I = P × r × t round — 45 earned on 2,000 over six months is a 4.5% rate. A "Working" block prints the formula with your numbers in it, and a year-by-year "Schedule" shows the interest, the total interest and the balance at the end of each year of the term, for terms up to the length the page states.
Can it take a monthly rate, a start and end date, or a currency?
Yes. Set "Rate quoted" to per month and the rate box becomes "Monthly rate %"; the working first multiplies it up to a yearly rate. Choose "Start and end dates" in the Unit select to give the term as two dates, counted day by day, so a 29 February in between earns its interest. The Currency select, "plain numbers" by default, writes the interest, the total and the schedule as money.
Can it add regular deposits, like a savings plan?
Yes. While the page solves for the interest, set “Regular contributions” to Additions or Deductions, type “Each contribution”, and choose “How often” and whether each is made at the end or the start of each period. Every contribution earns simple interest on its own amount for the time left after it is made, and the answer, the working and the yearly schedule all include it.