How to use the compound interest calculator
- Initial deposit — the amount you are starting with (can be zero).
- Monthly contribution — what you plan to add each month.
- Years to grow — how long the money stays invested.
- Annual interest rate — the expected yearly rate of return.
- Compounding frequency — how often interest is added to the balance.
The chart compares your growing balance with the money you actually put in. The gap between the two lines is compound interest at work.
Compound interest formula explained
For a single deposit, the classic formula is:
A = P × (1 + r/n)n×t
- A = final amount, P = initial deposit
- r = annual rate (as a decimal), n = compounding periods per year
- t = number of years
Monthly contributions add the future value of a series of payments: PMT × [(1 + i)m − 1] ÷ i, where i is the monthly rate equivalent to your compounding frequency and m is the number of months. The calculator simulates every month so both parts are combined precisely.
Worked example
Investing $10,000 and adding $500 a month for 20 years at 7%, compounded monthly, grows to roughly $300,000. You contribute $130,000; the other ~$170,000 is interest — more than half of the final balance.
How to make compounding work harder for you
- Start early. Time is the most powerful input — ten extra years can double the result.
- Contribute consistently. Automate monthly deposits so you never skip.
- Keep fees low. A 1% annual fee can consume a quarter of long-term growth.
- Reinvest returns instead of withdrawing dividends or interest.
- Planning for later life? Try the retirement calculator.