Rule of 72 Calculator
Divide 72 by your annual return. The answer is about how many years it takes to double your money. At 8%: 72 ÷ 8 = 9 years. The exact answer is 9.01 years.
Why 72 works
Money doubles when (1 + r)t = 2. So the exact time is t = ln 2 ÷ ln(1 + r). ln 2 is about 0.693. For small r, ln(1 + r) is close to r. That gives t ≈ 69.3 ÷ rate%. So why 72? The reason is simple. 72 divides cleanly by 2, 3, 4, 6, 8, 9 and 12. It is also more accurate around 6%–10%, where most people use it.
- Rate: 6% → 72 ÷ 6 = 12 years.
- Exact: ln 2 ÷ ln 1.06 = 0.6931 ÷ 0.0583 = 11.9 years.
- Error: 0.1 years, about 0.9%.
Doubling time by rate
| Annual rate | Rule of 72 | Exact | Doublings in 36 years |
|---|---|---|---|
| 1% | 72 yrs | 69.66 yrs | 0.5 |
| 2% | 36 yrs | 35 yrs | 1 |
| 3% | 24 yrs | 23.45 yrs | 1.5 |
| 4% | 18 yrs | 17.67 yrs | 2 |
| 5% | 14.4 yrs | 14.21 yrs | 2.5 |
| 6% | 12 yrs | 11.9 yrs | 3 |
| 7% | 10.3 yrs | 10.24 yrs | 3.5 |
| 8% | 9 yrs | 9.01 yrs | 4 |
| 9% | 8 yrs | 8.04 yrs | 4.5 |
| 10% | 7.2 yrs | 7.27 yrs | 5 |
| 12% | 6 yrs | 6.12 yrs | 5.9 |
The rule works for anything that grows at a steady rate. Prices too. At 3% inflation, prices double in about 24 years. Debt too. On a credit card at 24% APR, an unpaid balance doubles in roughly 3 years. Stop on that number for a second. Three years.
Frequently asked questions
How accurate is the rule of 72?
Very close for typical rates. At 8% it says 9 years. The exact answer is 9.01. It gets less accurate at very high rates. At 20% it says 3.6 years, and the exact answer is 3.8.
What is the rule of 69 or 70?
Same idea, different number on top. 69.3 is the precise constant for continuous compounding. 70 is common for low rates like inflation or GDP growth.
How long does it take to double money in a savings account?
At a 4% APY, about 17.7 years. At 0.5%, about 139 years. That is not a typo.
Can I use it to find the rate I need?
Yes. Divide 72 by the number of years. To double in 6 years, you need about 12% a year.
Not investment advice. These results are estimates. They use the numbers you enter and a constant rate of return. Real returns change every year. They can be negative. They are not guaranteed. Taxes, fees and inflation will change your real results. Talk with a licensed financial professional before you decide.
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Updated: October 2, 2026 · Sources and methodology