Money Doubling Time Calculator

Finds how many years an investment takes to double at a given annual return, both exactly and with the quick rule of 72.

Enter your numbers

Annual return rate

Example: 12

Result

Currency

Figures are computed on your device; nothing is sent anywhere.

exactYears = ln(2) / ln(1 + ratePercent/100); ruleOf72Years = 72 / ratePercent

The exact method divides the natural logarithm of two by the natural logarithm of one plus the annual rate as a decimal.

That compound formula reflects interest being earned on interest, so it is accurate even at high rates.

The rule of 72 simply divides 72 by the percentage rate, giving a fast mental estimate that is close for moderate rates.

Using simple interest instead of compound interest, which makes the doubling time look longer than it really is.
Trusting the rule of 72 at very high rates, where its shortcut answer drifts away from the exact compound result.

Use it to compare savings accounts or funds by how quickly each one doubles your money.
Use it to set a realistic time horizon for a long-term goal instead of relying on a rough guess.

At 12% a year money doubles in about 6.12 years exactly, and the rule of 72 gives 6.00 years.

Exact years 6.1

Rule of 72 (years) 6.0

Why does the rule of 72 work?

Dividing 72 by the annual percentage rate gives a close approximation of compound doubling time for ordinary rates between about 6% and 12%.

Last updated: 2026-10-03