Rule of 72

A mental-math shortcut for doubling time — works for investments, debt, inflation, and real returns.

Investment growth

Rule of 72 Divide 72 by an annual rate to estimate doubling time. Divide 72 by years to get the rate needed to double in that time. Accurate within ~1–2% for rates between 2% and 20%. Below or above that range, the exact value is also shown so you can see the gap.

Modes: Investment / Debt / Inflation / Real Return The same math applies to any compounding rate. A 20% credit card doubles your balance in about 3.6 years. Inflation at 3% halves what your money buys in about 24 years. Real return is what is left after inflation, calculated with the proper Fisher formula.

What are you calculating?

Solve for

Annual return rate 7.0%

Example starting amount $10,000

Starting amount is illustrative — doubling time itself does not depend on principal.

Retirement context

Doublings to retirement A rough count of how many times your money can double before you retire. We also show the exact future value — because partial doublings matter. 29 years at 10-year doubling time is closer to 7.5x, not 4x.

Starting amount Affects the dollar values shown but not the doubling time itself. The math is the same regardless of starting balance.

Your current age 35

Retirement age 65

10.3

years to double your money

72 ÷ 7% = 10.3 years

Exact: 10.2 years (ln 2 / ln 1.07)

Starting amount

$10,000

After one doubling

$20,000

Buying power becomes

$5,000

2x in

10.3 yrs

4x in

20.6 yrs

8x in

30.9 yrs

Rule of 72 vs. exact compound growth

Exact compound growth

Rule of 72 estimate

Real (inflation-adjusted)

Doubling milestones

This calculator is for educational and informational purposes only. The Rule of 72 is a mental-math approximation; results are estimates based on the inputs you provide and should not be taken as professional advice. Always consult a qualified professional for decisions involving investments, taxes, or financial planning.

Rule of 72

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