Compound Interest Calculator
Project how your savings grow: enter a starting amount, interest rate, time horizon, and monthly contributions.
Your investment plan
How compound interest is calculated
FV = P(1 + r/n)nt + PMT · (((1 + r/n)nt − 1) / (r/n)), where P is the initial principal, r the annual rate, n compounding periods per year, t years, and PMT the contribution each period. Monthly contributions are converted to the equivalent per-period amount, so any compounding frequency works.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both the original principal and previously earned interest. Over time it makes money grow exponentially rather than linearly.
How does compounding frequency affect growth?
More frequent compounding (monthly vs annually) grows money slightly faster because interest starts earning interest sooner. The difference is small but compounds over decades.
What is the difference between APY and APR?
APR is the annual rate without compounding; APY includes the effect of compounding, so APY is slightly higher than APR for the same nominal rate.
What is the rule of 72?
Divide 72 by the annual interest rate to estimate how many years it takes money to double. At 8%, money doubles in about 9 years.
Is it better to invest a lump sum or contribute monthly?
Both benefit from compounding. A lump sum starts growing immediately, while monthly contributions add discipline and dollar-cost averaging. Doing both is best — this calculator models the combination.
How much will $10,000 grow in 10 years at 7%?
About $19,672 with annual compounding (10,000 × 1.07^10). Add $200/month and it grows to roughly $54,000. Try the numbers above.