Interest Calculator
Calculate simple or compound interest on any principal, rate and time period.
The Interest Calculator answers one straightforward question: how much interest will my money earn? Enter a principal amount, an annual interest rate and a time period, then switch between two modes — simple interest, where interest is calculated only on the original amount, and compound interest, where interest earns interest every month.
The difference between the two modes is the single most important concept in personal finance. Over three years at 5%, $10,000 earns $1,500 in simple interest but about $1,616 when compounded monthly — and the gap widens dramatically over decades. Use the mode switch to see it with your own numbers, whether you are comparing savings accounts, calculating a loan's flat rate, or simply curious what an investment would return.
Results are calculated instantly in your browser and shown before any taxes or fees. Interest is assumed to be credited as described; check your actual account or loan terms for how often interest really accrues.
Simple interest
$1,500.00
Total amount
$11,500.00
Principal $10,000.00 over 3 yrs
How to use the Interest Calculator
- Choose Simple interest or Compound interest from the dropdown.
- Enter the principal amount you are depositing or borrowing.
- Type the annual interest rate as a percentage.
- Enter the time period in years (decimals like 0.5 are fine for 6 months).
- Read the interest earned and the total amount (principal + interest).
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it grows in a straight line: $10,000 at 5% earns $500 every year. Compound interest also earns on previously accumulated interest, so the balance grows faster each year. Over long periods compounding can more than double the total compared with simple interest.
How do I calculate simple interest?
Simple interest = principal × rate × time, with the rate as a decimal and time in years. For example, $5,000 at 4% for 3 years earns 5000 × 0.04 × 3 = $600, giving a total of $5,600.
What rate should I use for a savings account?
Use the APY (annual percentage yield) if your bank quotes it — APY already reflects compounding, so it is the truest comparison figure. If the bank quotes only an APR-style nominal rate, choose compound mode with monthly compounding to approximate what you will actually earn.
Are the results after tax?
No — results are gross, before income tax on interest and before inflation. Depending on where you live, interest above a small allowance is taxable, so your after-tax earnings will be lower than shown.
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