Calculate how many years it takes to double an investment using the Rule of 72, compared with the exact formula.
What is the Rule of 72 and how do you use it?
The Rule of 72 is the famous compound-interest shortcut: years to double ≈ 72 ÷ annual rate. At 8% a year, money doubles in about 9 years (72÷8).
How accurate is it?
The exact formula is ln(2) ÷ ln(1+r). Between 4% and 12% the error is under 2% — which is why the rule is so useful for mental math. At very high rates (20%+) it drifts, so the calculator shows both values.
Practical uses
Investing: a 7% return doubles a portfolio every ~10.3 years.
Inflation: 3% inflation doubles prices (halving purchasing power) every ~24 years.
Debt: 18% credit-card interest doubles the debt in ~4 years.
💡 Useful Tips
For tripling use the Rule of 114; for quadrupling — 144 (double doubling).
It works in reverse: to double in N years you need roughly 72÷N percent.
Real (after-inflation) returns are what matter for purchasing power.
Frequently Asked Questions
Why 72?
Because 72 ≈ 100·ln(2)≈69.3 and divides evenly by 2, 3, 4, 6, 8, 9 and 12 — perfect for mental math. At low rates the "Rule of 69.3" is more precise.
Does it work with monthly rates?
Yes, but the answer comes out in months: 72 ÷ monthly rate = months to double.
Does it account for tax and inflation?
No. Use your net real return for a purchasing-power view.