Work out interest in seconds. This free interest calculator handles both simple and compound interest — enter your principal, rate and time to see how much you’ll earn and your final balance.
Results update automatically as you type. Compound interest is compounded annually.
Use this free interest calculator to work out how much interest you’ll earn or pay. Enter a principal, an annual rate and a time period, then switch between simple and compound interest to see the difference instantly.
An interest calculator tells you how much a sum of money grows from interest over time. Enter your principal, annual rate and number of years, and it returns your interest earned and final balance for either simple interest or compound interest.
It works for savings, deposits, bonds or any situation where you need to know the interest on a fixed amount — without regular contributions.
Simple interest is calculated only on your original principal. Compound interest is calculated on your principal plus all the interest already earned, so it grows faster. Over $10,000 at 5% for 10 years, simple interest earns $5,000 while compound interest earns about $6,289.
| Simple | Compound | |
|---|---|---|
| Interest on | Principal only | Principal + past interest |
| Growth | Linear | Accelerating |
| $10k @ 5% / 10yr | $15,000 | $16,289 |
| Common use | Some loans, bonds | Savings, investments |
Simple interest:
Interest = P × r × t
Compound interest (annual):
Final balance = P × (1 + r)t
Where P is the principal, r is the annual rate as a decimal, and t is the number of years.
| Term | What it means |
|---|---|
| Principal | The original amount of money you start with. |
| Simple interest | Interest earned only on the principal. |
| Compound interest | Interest earned on principal plus accumulated interest. |
| Rate | The annual percentage applied to your balance. |
| Term | The length of time the money earns interest. |
Simple interest is calculated only on your original principal. Compound interest is calculated on your principal
plus the interest already earned, so it grows faster over time. On $10,000 at 5% for 10 years, simple interest
earns $5,000 while compound earns about $6,289.
Multiply the principal by the annual rate and the number of years: Interest = P × r × t. For $10,000 at 5% for
10 years, that's $10,000 × 0.05 × 10 = $5,000.
Use Final balance = P × (1 + r)t, where P is the principal, r is the annual rate as a decimal, and t
is the years. The interest earned is the final balance minus the principal.
If you're saving or investing, compound interest works in your favor and grows faster. If you're borrowing,
simple interest costs you less than compound. The toggle lets you compare both for the same numbers.
Yes. More frequent compounding (monthly or daily) earns slightly more than annual compounding. This calculator
compounds annually; for monthly or daily options, use a dedicated compound interest calculator.
Yes, this interest calculator is completely free, needs no sign-up, and gives instant results directly in your browser.