Rule of 72

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

Watch the 75-second explainer

Questions this answers

  • How long will it take my money to double?
  • What return do I need to double my money in 10 years?
  • How fast does inflation cut my purchasing power in half?
  • How quickly does credit card debt double?
  • How accurate is the Rule of 72?
Calculator Learn FAQs
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
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Rule of 72 vs. exact compound growth
Exact compound growth
Rule of 72 estimate
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.

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Quizzes, guides and answers built to go with this calculator.

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Eight questions on doubling time, debt and inflation.

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A 75 second explainer: divide 72 by your interest rate to see roughly how many years it takes money to double, why the same math makes card debt grow fast, and why starting early beats a higher rate. Try it on the Compound Interest and Rule of 72 calculators.

Music: "Wholesome" by Kevin MacLeod (incompetech.com), licensed under CC BY 4.0.

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Frequently asked questions

What is the Rule of 72?

Divide 72 by an annual growth rate in percent to estimate how many years it takes for something to double. At 8% a year, money doubles in about 9 years. It works for anything that compounds, including investments, debt and prices.

How accurate is it?

It is very close for rates between about 6% and 10%. At low rates the Rule of 70 is slightly more accurate, and at high rates the estimate drifts. The calculator shows the exact doubling time next to the shortcut so you can see the difference.

How does inflation fit in?

Divide 72 by the inflation rate to see how long it takes prices to double, which is the same as your money losing half its purchasing power. At 3% inflation, that is about 24 years.

What is a real return?

A real return is your growth after inflation. The calculator uses the exact formula, (1 + return) / (1 + inflation) minus 1, rather than simply subtracting, then applies the Rule of 72 to show how long it takes your purchasing power to double.

Can I use it for debt?

Yes. A credit card balance at 24% interest doubles in about 3 years if you make no payments, which is why paying down high-rate debt first is so powerful.

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