Estimate how long it takes to double your money — or what return you need to double it by a deadline — and compare the rule of 72 with the exact answer.
How to use the rule of 72 calculator
Choose what you want to find. Enter an annual rate of return to see roughly how many years it takes your money to double, or enter a number of years to find the return you would need to double your money in that time. The calculator gives the rule-of-72 estimate, the exact answer, how far apart they are, and — for a given rate — the time to triple and quadruple your money as well.
The rule of 72 formula
The exact doubling time with annual compounding comes from solving (1 + r)t = 2:
That is why the "true" constant for continuous compounding is 69.3. The number 72 is used because it gives better results at typical rates of 6–10% and is divisible by 2, 3, 4, 6, 8, 9 and 12, making mental arithmetic easy.
Worked example
At an 8% annual return, the rule of 72 says your money doubles in 72 ÷ 8 = 9 years. The exact answer is ln 2 ÷ ln 1.08 = 9.01 years — almost identical. So 10,000 invested at 8% becomes about 20,000 after nine years, 40,000 after eighteen and 80,000 after twenty-seven. Going the other way, if you want to double your money in 6 years you need about 72 ÷ 6 = 12% a year; the exact figure is 12.25%.
How accurate is the rule of 72?
| Rate | Rule of 72 | Exact |
|---|---|---|
| 2% | 36.0 years | 35.0 years |
| 5% | 14.4 years | 14.2 years |
| 8% | 9.0 years | 9.0 years |
| 12% | 6.0 years | 6.1 years |
| 20% | 3.6 years | 3.8 years |
The rule is most accurate around 8%. At low rates it slightly overstates the doubling time and at high rates it understates it. Variations include the rule of 70 (often used for low rates and economic growth), the rule of 69.3 (for continuous compounding) and the rule of 114 and rule of 144 for tripling and quadrupling.
Everyday uses
- Investments: at 6% a year, money doubles about every 12 years, so a 30-year-old's savings could double three times before 66. See the full picture with the compound interest calculator.
- Inflation: at 4% inflation, prices double in about 18 years — your money's buying power halves in the same time. Explore it with the inflation calculator.
- Debt: a credit card balance at 24% doubles in about three years if left unpaid.
- Fees: a 2% annual fee consumes half of a portfolio's value over about 35 years (the rule says 36; the exact figure is 34.3) — a striking way to see why low costs matter.
- Economic growth: a country growing 3.5% a year doubles its economy in about 20 years.
Checking claims
The rule is a quick sanity check. If someone promises to double your money in two years, the rule of 72 implies a 36% annual return (exactly 41.4%) — far above long-run market returns and a classic warning sign. To find the actual annual growth of an investment you already hold, use the CAGR calculator.
This is an estimation tool for education; actual returns vary.
Cite or link to this page
Using this rule of 72 calculator in an article, report or homework? Copy a citation or a ready-made link:
Calcivia. (2026). Rule of 72 Calculator. Retrieved October 31, 2026, from https://www.calcivia.com/rule-of-72-calculator/"Rule of 72 Calculator." Calcivia, 2026, https://www.calcivia.com/rule-of-72-calculator/. Accessed 31 Oct. 2026.<a href="https://www.calcivia.com/rule-of-72-calculator/">Rule of 72 Calculator</a> by Calcivia