The Rule of 72 is a famous mental-math shortcut: divide 72 by your annual growth rate and you get the rough number of years for money to double. This calculator shows the Rule-of-72 estimate, the mathematically exact doubling time, and the years to triple and quadruple.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: compound-growth doubling math, recomputed in code.
The rule & the exact math
Compound growth doubles a balance once the accumulated factor reaches 2. The exact answer uses logarithms, but 72 is chosen because it's close to 100 × ln 2 (about 69.3) and divides neatly by many rates. The approximation is most accurate near 8%, drifting slightly at very low or very high rates. Sister rules use 114 for tripling and 144 for quadrupling.
Worked examples
6% growth:
Triple & quadruple at 6%:
Reverse — double in 10 years:
The same math works for inflation: at 3% a year, prices double — and purchasing power halves — in about 72 ÷ 3 = 24 years. It's a quick way to feel the power, and the cost, of compounding.
Frequently Asked Questions
Divide 72 by the annual rate for years to double. 72 ÷ 8 = 9 years.
Very close for 6–10%. At 6% it gives 12 vs exact 11.9; it drifts at extreme rates.
About 7.2% — run it in reverse, 72 ÷ 10 = 7.2.
Triple ≈ 114 ÷ rate; quadruple ≈ 144 ÷ rate. At 6%: 19 and 24 years.
Yes. At 3%, prices double in ~24 years and purchasing power halves.