Rule of 72 Calculator
This is the handiest mental math in all of finance: 72 ÷ your return rate ≈ the years it takes your money to double. It works both directions — and yeah, it works against you too.
Money doubles every
9.0 years
Return needed
8.0%
Doubling times at common rates
🐻 The rule cuts both ways: at 24% APR, credit-card debt doubles against you every 3 years. The same math that builds wealth on one side of the ledger destroys it on the other — which side of the compounding are you on?
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How this is calculated
Years to double ≈ 72 ÷ annual return. At 8%, money doubles every 9 years; at 4%, every 18. The rule is a close approximation of the exact logarithmic answer (ln 2 ÷ ln(1+r)) for typical rates — accurate within months for rates between 4% and 15%.
Flip it to solve for rate: 72 ÷ years-to-double = required return. Want to double in 6 years? You need about 12% — which tells you instantly whether a goal needs markets, time, or bigger contributions.
The dark side: debt compounds by the same rule. A 24% credit card doubles what you owe roughly every 3 years — the same exponential force, pointed at you.
Common questions
Why 72 and not some other number?
72 is close to the mathematically exact value (≈69.3 for continuous compounding) and divides cleanly by 2, 3, 4, 6, 8, 9, and 12 — making the mental math instant. That convenience is why it's survived centuries.
How many doublings do I have left?
Years until retirement ÷ years-per-double. A 30-year-old at 8% has about four doublings by 66 — and because each doubling equals all previous growth combined, the last one is the big one. That's the real cost of delay.
Does the rule work for inflation too?
Yes — 72 ÷ inflation rate = years for prices to double (or your cash's buying power to halve). At 3% inflation, money under the mattress halves in ~24 years.
Go deeper
Investing fundamentals — count your doublings
This math is taught step-by-step in Module 5 of the Bear Bankroll Wealth System — with interactive worksheets that run the numbers on YOUR situation.
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