Rule of 72
How long until your money doubles, triples, or grows ten-fold? The famous 72-shortcut, checked against the exact answer.
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Enter a return rate above 0% — money can't double at zero growth.
Rule of 72 estimate: — — —
Milestones
Time to reach each multiple, the Rule of 72 estimate, and what your starting amount becomes.
| Growth | Exact | Rule of 72 | Becomes |
|---|
Want to double faster?
You'd need to earn — a year.
Project forward
Grows to — (— your money).
Why 72?
Doubling means solving 2 = (1 + r)t, which gives t = ln 2 / ln(1 + r). For the small rates real investments earn, that curve is very close to 72 / rate% — and 72 divides cleanly by 2, 3, 4, 6, 8, 9 and 12, so the mental math is easy. The shortcut is most accurate around 8%; far from there, trust the exact figure.
The trick generalises. The magic number for any target multiple m is 72 · log₂(m): doubling uses 72, tripling ≈ 114 ("Rule of 114"), quadrupling = 144 ("Rule of 144"). Some people prefer 70 (cleaner division) or 69.3 (exactly right for continuous compounding, since 100 · ln 2 ≈ 69.3).