Rule of 72 Calculator

How long will it take to double your money? Enter an annual return and see the famous Rule of 72 quick estimate next to the exact compound-growth answer — plus how long it takes to triple, 5× and 10×. Updates as you type.

The Rule of 72, and the exact formula

The Rule of 72 is a piece of mental math: divide 72 by your percentage return to estimate the years to double.

years to double ≈ 72 ÷ rate(%)

The exact figure comes from compound growth:

years to grow ×M = ln(M) ÷ ln(1 + rate)

• The rule is closest for returns between about 4% and 15% — the everyday investing range.
• Higher return, faster doubling — and the effect compounds: doubling twice is 4×, not 2×.
• Use your real return — subtract fees and inflation for a purchasing-power view.

This is an educational estimate using a fixed annual return. Real returns vary year to year. Not financial advice.

FAQ

What is the Rule of 72?

The Rule of 72 is a shortcut for estimating how long an investment takes to double: divide 72 by the annual return rate. At 8% a year, 72 ÷ 8 = 9 years to double. It is a mental-math approximation of the exact compound-growth formula and is surprisingly accurate for rates between roughly 4% and 15%.

How accurate is the Rule of 72?

Very close for typical investment returns. The exact doubling time is ln(2) ÷ ln(1 + rate). At 8% the exact answer is about 9.01 years versus the Rule of 72 estimate of 9.0. The rule drifts a little at very high or very low rates, which is why this calculator shows both the quick estimate and the exact figure side by side.

How long to triple or 10× my money?

Use the same compound formula with a different multiple: years = ln(multiple) ÷ ln(1 + rate). At 8% a year it takes about 14.3 years to triple and about 29.9 years to grow 10×. The calculator shows the milestones for 2×, 3×, 5× and 10× automatically.

Does this account for fees, taxes or inflation?

No — it uses the nominal return you enter. Fees and taxes lower your real return (and so lengthen the doubling time), and inflation erodes purchasing power. For a real-terms view, enter your return after fees and minus the inflation rate.

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