Compound Interest Calculator
See how your money grows with compound interest — enter a starting amount, rate, years, compounding frequency and monthly contribution to get the future value and interest earned.
How compound interest builds wealth
Compound interest pays you interest on your interest. Each period, the rate is applied to your whole balance — principal plus everything earned so far — so growth speeds up the longer you stay invested. The formula for a lump sum is A = P(1 + r/n)nt, where P is the principal, r the annual rate, n the number of times it compounds per year and t the years. This calculator applies it and also adds your monthly contributions, which compound as they go in.
Time, rate and contributions
Three levers drive the result: how long you invest, the rate, and how much you add each month. Time is the most powerful — starting earlier beats a higher rate later. Compounding frequency (daily vs annually) helps a little too. To project a specific account, try the 401(k) calculator or the dividend calculator.
Frequently asked questions
How does compound interest work?
Compound interest earns interest on both your original money and the interest already added, so growth accelerates over time. The more often it compounds (daily vs annually) and the longer you leave it, the more you earn. Enter your numbers above to see the future value and how much of it is interest.
What is the compound interest formula?
For a lump sum it is A = P(1 + r/n)^(nt), where P is the principal, r the annual rate, n the times it compounds per year, and t the years. This calculator uses that formula and also adds your monthly contributions, compounding them as they are paid in.
How much will $10,000 grow with compound interest?
At 7% compounded monthly, $10,000 grows to about $20,100 in 10 years on its own. Adding $100 a month brings it to roughly $37,500, because the contributions compound too. Try your own figures above.
Does compounding frequency matter?
Yes, but less than time and rate. Daily compounding earns a little more than monthly or annual at the same rate, since interest is added more often. The difference grows with larger balances and higher rates — switch the frequency above to compare.