How this Rule of 72 calculator works
The Rule of 72 is a simple mental shortcut for estimating how long it takes an investment to double at a fixed annual rate. You take the number 72 and divide it by the annual percentage rate of return. The answer is the approximate number of years needed to double your money. Because 72 divides cleanly by many common rates — 2, 3, 4, 6, 8, 9 and 12 — it is easy to do in your head, which is why it is the most popular version of the doubling rule.
You can also run the rule in reverse. If you know how many years you want your money to take to double, divide 72 by that number of years to find the annual rate you would need: rate ≈ 72 ÷ years. This calculator does both — type a rate to get the years to double, or type a target doubling time to get the required rate.
For comparison the tool also shows the exact doubling time, calculated as t = ln(2) ÷ ln(1 + r), where r is the rate written as a decimal. The Rule of 72 is only an approximation; it is most accurate for rates in the 6% to 10% range and drifts further from the exact figure at very low or very high rates. Two related variants exist: the Rule of 70 and the Rule of 69.3, the latter being the mathematically exact constant for continuous compounding.
Reference note: these figures assume a single fixed annual rate with annual compounding and do not account for taxes, fees, dividends, additional contributions, inflation, or risk. Results are an estimate, not financial advice.
Years to double at common rates
| Rate | Rule of 72 (years) | Exact (years) |
|---|---|---|
| 2% | 36.0 | 35.00 |
| 4% | 18.0 | 17.67 |
| 6% | 12.0 | 11.90 |
| 8% | 9.0 | 9.01 |
| 10% | 7.2 | 7.27 |
| 12% | 6.0 | 6.12 |
Frequently asked questions
- What is the Rule of 72?
- The Rule of 72 is a quick mental shortcut for estimating how long it takes an investment to double at a fixed annual rate of return. You divide 72 by the annual percentage rate, and the result is the approximate number of years to double. For example, at 8% per year it takes roughly 72 ÷ 8, or about 9 years.
- How accurate is the Rule of 72?
- It is an approximation that is most accurate for rates in the 6% to 10% range, where it usually lands within a fraction of a year of the exact figure. At very low or very high rates the error grows, so the calculator also shows the exact doubling time, t = ln(2) ÷ ln(1 + r), to compare against.
- How do I use the Rule of 72?
- Divide 72 by your annual interest or return rate as a whole number; the answer is the approximate years to double. You can also work backwards: divide 72 by a target number of years to find the annual rate you would need to double in that time.
- Rule of 72 vs 70 vs 69.3?
- All three estimate doubling time by dividing a constant by the rate. 69.3 is the mathematically exact constant for continuous compounding (100 × the natural log of 2). The Rule of 70 is a close, easy alternative, while 72 is the most popular because it divides cleanly by many common rates such as 2, 3, 4, 6, 8, 9 and 12.
- What rate doubles money in 10 years?
- Using the Rule of 72, divide 72 by 10 to get about 7.2% per year. The exact rate that doubles money in exactly 10 years with annual compounding is about 7.18%, so the rule's estimate is very close in this range.
- Is this financial advice?
- No. This calculator is an informational tool that performs basic Rule of 72 math. It assumes a single fixed rate and ignores taxes, fees, inflation, contributions, and risk, and it is an estimate, not financial advice. Consult a qualified professional before making investment decisions.