Compound Interest Calculator
See how your savings or investment grows over time with compound interest, including regular contributions.
How compound interest works
Compound interest is interest earned on your interest. Instead of only your original deposit earning a return, the return itself starts earning too — so the balance grows faster and faster the longer you leave it. Over long periods this snowball effect does most of the heavy lifting, which is why starting early matters so much.
Adding a regular monthly contribution accelerates it further: each contribution starts compounding the moment it lands, so small, consistent amounts add up to far more than the total you put in.
The formula
Future value = P × (1 + r/n)^(n×t), where P is the starting amount, r is the annual interest rate, n is the number of times interest compounds per year, and t is the number of years. Regular contributions are added and compounded on top.
Worked example
| Item | Value |
|---|---|
| Starting amount | $10,000 |
| Annual rate | 6% |
| Compounding | Monthly |
| Time | 10 years |
| Future value (lump sum only) | $18,194 |
| With $200/month added | Substantially more — enter yours above |
Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both your original amount and the interest already added, so your balance grows at an accelerating rate over time.
How often should interest compound?
More frequent compounding (monthly vs annually) produces a slightly higher return for the same rate. This calculator lets you choose monthly, quarterly or annually.
Does this account for tax or inflation?
No. It shows nominal growth at a constant rate. Real returns will vary and may be reduced by tax and inflation.