The Rule of 72: How Fast Will Your Money Double?
By Arshi Khan, Founder of Just4Marketing · October 26, 2026 · 5 min read
Divide 72 by the growth rate to estimate doubling time. How accurate the rule is, how doubling stacks up, and how it applies to debt, inflation and fees.
How long will it take your money to double? You don't need a spreadsheet to get a good answer. Divide 72 by the yearly growth rate, and the result is roughly the number of years it takes. That's the rule of 72 — one of the most useful pieces of mental math in personal finance.
How it works
Years to double ≈ 72 ÷ annual rate (as a whole number, so 8% is 8). At 8% a year, money doubles in about 72 ÷ 8 = 9 years. At 6%, about 12 years. It also works in reverse: to double in 10 years, you need about 72 ÷ 10 = 7.2% a year.
How accurate is it?
| Rate per year | Rule of 72 | Exact doubling time |
|---|---|---|
| 2% | 36.0 years | 35.0 years |
| 3% | 24.0 years | 23.4 years |
| 4% | 18.0 years | 17.7 years |
| 6% | 12.0 years | 11.9 years |
| 8% | 9.0 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6.0 years | 6.1 years |
| 18% | 4.0 years | 4.2 years |
| 24% | 3.0 years | 3.2 years |
It's most accurate around 6%–10%, and within a few months across the range most people care about. The exact answer is ln(2) ÷ ln(1 + rate), which is what the table's last column uses. At low rates, dividing 70 instead of 72 is slightly closer, which is why you'll also hear of the rule of 70.
Doubling stacks up
The rule makes compounding easy to see. $10,000 growing at 8% doubles roughly every 9 years:
- after 9 years: about $20,000,
- after 18 years: about $40,000,
- after 27 years: about $80,000 (the exact figure is $79,881).
Starting nine years earlier means one extra doubling at the end — which is why time matters more than almost anything else in long-term saving.
It works for costs too
- Debt: a credit card balance at 24% that you don't pay down doubles in about 3 years.
- Inflation: at 3% inflation, prices double — and the buying power of cash halves — in about 24 years (23.4 exactly).
- Fees: a 1% yearly fee on an investment halves what that money would otherwise grow to over roughly 70 years — and over shorter periods it still takes a noticeable share.
Where the rule breaks down
- It assumes a steady rate. Real investments go up and down, so use a long-run average and treat the answer as a rough guide.
- At very high rates (above about 20%) it underestimates the time a little.
- It ignores money added or withdrawn along the way — for that, use a full compound interest calculation.