At a 7% annual return, money doubles in about 10 years. At 4% it takes about 18 years, and at 10% just over 7. You can work any of these out in your head with the Rule of 72: divide 72 by the yearly return, and the answer is roughly how many years it takes to double.
Here’s a table for common rates, how accurate the shortcut really is, and the one situation where the same rule works against you.
How the Rule of 72 Works
Years to double ≈ 72 ÷ annual rate of return (as a percentage)
- At 6%: 72 ÷ 6 = 12 years
- At 8%: 72 ÷ 8 = 9 years
- At 9%: 72 ÷ 9 = 8 years
It works because of compound growth: each year you earn returns not just on your original money but on the returns already added. The shortcut is an approximation of the exact compound-interest formula, chosen because 72 divides neatly by so many common rates (2, 3, 4, 6, 8, 9 and 12).
Doubling Time Table: Rule of 72 vs the Exact Answer
| Annual return | Rule of 72 | Exact (annual compounding) |
|---|---|---|
| 1% | 72 years | 69.7 years |
| 2% | 36 years | 35.0 years |
| 3% | 24 years | 23.4 years |
| 4% | 18 years | 17.7 years |
| 5% | 14.4 years | 14.2 years |
| 6% | 12 years | 11.9 years |
| 7% | 10.3 years | 10.2 years |
| 8% | 9 years | 9.0 years |
| 9% | 8 years | 8.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6 years | 6.1 years |
| 15% | 4.8 years | 5.0 years |
The rule is almost exact between about 6% and 10%, which is where most long-term investment return assumptions sit. At very low rates it slightly overstates the time, and at high rates it slightly understates it.
The Exact Formula
If you want the precise figure, compound interest doubles money in:
Years = ln(2) ÷ ln(1 + r)
where r is the annual rate as a decimal (7% = 0.07) and ln is the natural logarithm, the “ln” button on any scientific calculator. For 7%: ln(2) ÷ ln(1.07) = 0.6931 ÷ 0.0677 = 10.24 years. Our free Scientific Calculator has an ln button if you’d like to try it.
Compounding more often speeds things up slightly: at 7% compounded monthly, money doubles in about 9.9 years instead of 10.2.
Rule of 72, 70 or 69?
You’ll sometimes see the Rule of 70 or Rule of 69 instead. They’re the same idea with a different number. 69.3 is mathematically exact for continuous compounding, and 70 works well for low rates, such as when estimating how fast inflation or a population doubles. For everyday investing at 5% to 12%, 72 is the most accurate and the easiest to divide.
When the Rule Works Against You: Inflation and Debt
The same maths applies to anything that compounds, including things you don’t want to grow:
- Inflation halves your money’s buying power. At 3% inflation, prices double in about 24 years (72 ÷ 3), which means cash that earns nothing loses half its purchasing power over that time.
- Debt doubles too. An unpaid balance at 24% interest doubles in about 3 years (72 ÷ 24). That’s why high-interest credit card debt is usually worth paying off before investing.
A Reality Check on Returns
The rule assumes a steady return every year. Real investments rise and fall, so actual doubling times vary, and past returns don’t guarantee future ones. Fees and taxes also reduce your effective rate: a 7% return with 1% in annual fees behaves like 6%, stretching doubling time from about 10 years to 12. Treat the rule as a planning estimate, not a promise.
To see the full year-by-year growth, including regular monthly contributions, use our Compound Interest Calculator. For monthly investing plans, try the SIP Calculator.
Frequently Asked Questions
How long does it take to double your money at 7%?
About 10.2 years with annual compounding. The Rule of 72 gives 10.3 years (72 ÷ 7), and monthly compounding brings it down to about 9.9 years.
How long does it take to double your money at 10%?
About 7.3 years. The Rule of 72 estimate is 7.2 years.
What interest rate doubles money in 10 years?
About 7.2% a year (72 ÷ 10). Exactly, it’s 7.18% with annual compounding.
Does the Rule of 72 work for simple interest?
No. It’s for compound growth. With simple interest, money doubles in 100 ÷ rate years, so 14.3 years at 7% instead of about 10.
See how your own savings grow, with contributions and compounding, in the free Compound Interest Calculator.
This article is for general education and isn’t financial advice.
Sources: Nebraska Department of Banking and Finance; U.S. News.