Finance Calculator

Compound Interest

See how your money grows over time with compound interest.

Starting amount Yearly interest rate (%) Years of compounding

Compound Interest with Yearly Contributions

Same as above, but add a fixed amount to the investment every year. The invested amount grows over time.

Starting amount Yearly interest rate (%) Years of compounding Amount to add every year

FIRE Calculator

Estimate how much you need to invest per year so your portfolio reaches the required amount by retirement, then see it draw down as you spend in retirement.

Years to retirement Years of retirement Monthly spending in retirement Investment growth rate (%)

Left: years to retirement (portfolio grows as you invest). Right: years in retirement (portfolio decreases as you withdraw).

Rule of 72 — Doubling time

Quick estimate: divide 72 by your interest rate (%) to get years to double. See how accurate it is and where it goes wrong.

How to use

Enter an interest rate (e.g. 7 for 7%). This is the annual growth rate you assume (savings, investment, or inflation).
Read the result: Rule of 72 gives the quick estimate (72 ÷ rate). Actual is the exact doubling time from the compound-interest formula.
Use the chart below to see how the rule’s error changes between 1% and 15% — it’s best around 7–8%.

Interest rate (%)

Error (in years) when using Rule of 72 instead of the exact doubling time. Positive = rule overestimates years to double.