Compound Interest Calculator
Compound interest is interest earning interest: every month your balance grows, and the growth itself starts growing. Enter a starting amount, an optional monthly contribution, a rate and a time horizon — you get the future value plus a year-by-year table that shows exactly when compounding starts doing the heavy lifting.
The detail line also applies the rule of 72 — the classic mental math that divides 72 by your rate to estimate the doubling time. At 7%, money doubles roughly every decade; the yearly table lets you watch that doubling happen. Pair this with the savings goal calculator when you have a target date instead of a horizon.
How to use
- Enter your starting amount, annual rate and time in years — the future value updates as you type.
- Add an optional monthly contribution to model regular investing; the table breaks the growth down year by year.
- Share the projection with one tap — the link carries your inputs so the recipient sees the same numbers.
Frequently asked questions
How is compound interest calculated monthly?
Each month the balance grows by rate/12, then contributions are added — the same order banks use. Over a year that lands within a fraction of a percent of the true annual-compounding figure, and it matches how savings accounts and index plans actually post interest.
What is the rule of 72?
Divide 72 by your annual rate to estimate how many years it takes money to double: 72 / 7 ≈ 10 years at 7%, 72 / 4 = 18 years at 4%. The calculator applies it for you and the yearly table shows the real curve behind the shortcut.
How much is $5,000 at 7% for 10 years?
About $10,040 left alone — the classic doubling. Add $200 monthly and the same decade grows to roughly $44,700, because contributions feed the compounding engine every month. Try it above and check the year-by-year table.
Is this calculator free and private?
Completely free with no sign-up, and the math runs entirely in your browser. Inputs are remembered locally on your device only — nothing is transmitted or stored on a server.