Rule of 72 Calculator

Estimate doubling time or the annual return needed to double your money, and compare the Rule of 72 shortcut with exact annual compound growth.

How long will it take your money to double? Compare the Rule of 72 mental-math shortcut with exact compound-growth mathematics, or find the annual return needed for a doubling time.

What would you like to find?

Enter a constant annual return. Zero or negative returns (down to −100%) do not double a positive investment.

Enter a positive amount, or leave blank. It does not change doubling time or required return.

How the Rule of 72 works

Divide 72 by the annual return expressed as a percentage: at 8%, 72 ÷ 8 = about 9 years to double. Reverse the shortcut to estimate a required return: doubling in 10 years suggests 72 ÷ 10 = about 7.2% per year.

The exact comparison uses growth = (1 + annual return / 100)^years. The shortcut is an approximation; its error varies with the return. It becomes less reliable at very low or high returns.

The Rule of 72 can also estimate how long it takes prices to double with inflation. At 3% annual inflation, the shortcut estimates approximately 24 years.

Assumptions and limitations

Both comparisons assume a constant positive return, all earnings reinvested, and no additional contributions or withdrawals. Your starting dollar amount does not affect doubling time, so no balance is needed. A 0% return never doubles the starting balance; a constant negative return does not double a positive investment.

Exact means exact for the stated mathematical model, not a forecast. We use annual compounding and its fractional-year mathematical extension. If interest is credited only at whole-year anniversaries, the balance first reaches double at the next whole-year anniversary. Actual investment returns vary and may be negative. Taxes, fees and inflation are excluded, so these are nominal, pre-cost results rather than purchasing-power estimates. This is an educational estimate, not a promise of returns or personalized investment advice.

Source: Investor.gov: Rule of 72.

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