Compound Interest Calculator
See how a lump sum or regular contributions grow over time with compound interest, using your chosen interest rate, compounding frequency, and time horizon.
How it works
- Enter the principal amount, annual interest rate, and compounding frequency.
- Set the investment duration in years.
- Optionally add regular top-up contributions.
- See the final balance broken down into principal, contributions, and earned interest.
What compounding actually does
Compound interest means each period's interest is added to the balance, so the next period earns interest on the interest as well. The calculator applies this period by period: balance = balance x (1 + r / n) + contribution, where r is the annual rate as a decimal and n is the number of compounding periods per year (12 for monthly, 4 for quarterly, 1 for annual). Your monthly top-up is converted to the compounding period (monthly top-up x 12 / n) and added at the end of each period, so a deposit earns interest starting from the following period.
For a single lump sum with no top-ups, this matches the textbook formula A = P x (1 + r / n)^(n x t). With regular top-ups it matches the future value of an ordinary annuity added to the grown lump sum. As a sanity check, the rule of 72 says money doubles in roughly 72 divided by the annual rate in percent years, which is about 10.3 years at 7%.
Two worked examples
Frequency matters, but less than rate or time. Put 10,000 at 7% for 10 years with no top-ups: compounding annually gives 19,671.51, while compounding monthly gives 20,096.61, a difference of 425.10. Annual compounding falls just short of doubling and monthly compounding just passes it, in line with the rule of 72.
Contributions matter more. Start with 10,000 at 7% compounded monthly for 10 years and add 200 a month. The final balance is 54,713.58. You paid in 34,000 (the 10,000 start plus 200 x 120 months), so 20,713.58 of the result is interest earned.
Reading the results
The summary separates what you put in from what the money earned, and the yearly table shows the balance at the end of each year. The earlier a contribution is made and the longer it stays invested, the larger its share of the final balance. Try the same total spread over a longer time with smaller top-ups to see this for yourself.
Assumptions and limits
The rate is a nominal annual rate held constant for the whole period. Real savings and investments rarely work that way: bank rates change, and investment returns vary from year to year and are never guaranteed. The calculator does not include tax, account or fund fees, or inflation, and fees in particular can take a large bite out of long-term growth, so use a net rate if you want to account for them. The number of years is rounded up to a whole year for the yearly table and the final balance, and the maximum is 100 years. Use the result as an illustration of how growth works, not as a forecast.
Frequently asked questions
What is the compound interest formula?
A = P(1 + r/n)^(nt), where P is the starting principal, r is the annual interest rate, n is how many times per year it compounds, and t is the number of years.
Does compounding frequency actually make much difference?
It matters more the longer the time horizon and the higher the rate — daily compounding versus annual compounding on the same rate typically differs by a fraction of a percent per year, but it adds up over decades.
Sources and further reading
Formulas checked against the sources below. Last reviewed . General information only, not professional advice.