Rule of 72 Calculator
The fastest mental math in finance: divide 72 by your annual return to estimate how many years money takes to double.
The Rule of 72 approximates the exact doubling time ln(2)/ln(1+r) remarkably well for ordinary return rates: at 8%, the rule says 9.0 years and the exact answer is 9.01. The approximation works because ln(2) ≈ 0.693 and small-rate adjustments nudge the numerator to about 72 for rates between 5% and 12%, the range most investing questions live in.
Frequently asked questions
How accurate is the Rule of 72?
Within about a month of the exact answer for rates between 4% and 12%. At very high rates it drifts, at 24% the rule says 3.0 years but the true answer is 3.22. This page shows both so you can see the gap.
Does it work for debt too?
Yes, uncomfortably: a credit card at 24% APR doubles what you owe in roughly 3 years if nothing is paid. The same math that builds wealth in your favor works against you on high-interest debt.
Why 72 and not 69.3?
ln(2) suggests 69.3, but 72 has more small divisors (2, 3, 4, 6, 8, 9, 12) making mental math easy, and it happens to correct for compounding at typical rates almost perfectly.
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Calculators model hypothetical outcomes from the inputs you provide. They are informational only, not financial, investment, tax, or legal advice.