Investment Doubling Time Calculator
See how many years it takes money to double and triple at a given return — the quick Rule of 72 estimate next to the exact figure.
How it works
Rule estimate: doubling years ≈ rule ÷ rate% · Exact: doubling years = ln(2) ÷ ln(1 + rate) · Tripling years = ln(3) ÷ ln(1 + rate)
The Rule of 72 divides 72 by the percentage return to approximate how long money takes to double — a mental-math shortcut that is remarkably close for rates between about 6% and 10%. The exact answer uses logarithms, and 72 is chosen over the theoretically purer 69.3 because it has more whole-number divisors. For tripling, the analogous shortcut is the Rule of 114.
Worked example
At an 8% annual return, the Rule of 72 gives 72 ÷ 8 = 9 years to double. The exact figure, ln(2) ÷ ln(1.08), is about 9.0 years — the shortcut nails it. Tripling takes roughly 114 ÷ 8 ≈ 14.3 years.
Sources: Rule of 72 — Wikipedia
Frequently asked questions
How accurate is the Rule of 72?
Very accurate near 8%, where it is almost exact. It drifts a little at the extremes — for low rates the Rule of 69.3 is closer, and for high rates 72 slightly overstates the time. For everyday planning the error is trivial.
Why 72 and not 69.3?
Continuous compounding gives ln(2) ≈ 0.693, so 69.3 is the pure constant. But 72 divides cleanly by 2, 3, 4, 6, 8, 9 and 12, making it far easier for mental math while staying accurate over common return ranges.
Does inflation change this?
To see how long your purchasing power takes to double, use your real (after-inflation) return instead of the nominal return. The same rule applied to a smaller real rate gives a longer doubling time.
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Educational estimate, not investment advice. Returns are hypothetical and not guaranteed.