Rule of 72 Calculator: how long to double your money
Divide 72 by your interest rate and you get roughly the years it takes money to double. This calculator runs the famous shortcut alongside the exact formula, in both directions.
Maintained by TheFinanceSection · Methods and corrections
Rule of 72 estimate
10.3 yrs
72 ÷ 7 — $10,000 becomes $20,000
Exact answer
10.24 yrs
ln(2) ÷ ln(1 + r), with annual compounding
| Rate | Rule of 72 | Exact doubling time |
|---|---|---|
| 1% | 72.0 yrs | 69.7 yrs |
| 2% | 36.0 yrs | 35.0 yrs |
| 4% | 18.0 yrs | 17.7 yrs |
| 7% | 10.3 yrs | 10.2 yrs |
| 10% | 7.2 yrs | 7.3 yrs |
| 12% | 6.0 yrs | 6.1 yrs |
What this calculator does
Give it a rate and it returns the doubling time — 72 ÷ rate as the mental-math estimate, and ln(2) ÷ ln(1 + r) as the exact compound-interest answer. Flip the mode and it solves the other way: how big a return you need for your money to double within a set number of years.
Why 72 works
Doubling at a compound rate r takes ln(2)/ln(1+r) years, and for realistic rates that is approximately 0.693/r when r is expressed as a decimal. The number 72 stuck because it's nearly as accurate in the 4–12% range and divides cleanly by 2, 3, 4, 6, 8, 9, and 12. At 8%, the rule says 9 years; the exact answer is 9.01. The calculator shows both results.
Use it on inflation too
The rule cuts both ways: at 3% inflation, prices double — meaning your cash's buying power halves — in about 24 years. See that effect on real dollars with the inflation calculator, and see what realistic bank rates do to your balance with the savings interest calculator.
Frequently asked questions
What is the Rule of 72?
A mental-math shortcut: 72 divided by an annual growth rate approximates the years for a sum to double. At 6%, money doubles in about 72 ÷ 6 = 12 years. It works for investment returns, savings interest, inflation, or any compound growth.
How accurate is the Rule of 72?
It is an approximation, most useful around moderate rates. At 4%, it gives 18 years versus about 17.67 exactly; at 8%, it gives 9 versus about 9.01. This calculator shows the exact compound-growth result alongside the shortcut.
How long does money take to double in a savings account?
At an assumed constant 4% APY, the shortcut gives 18 years and the exact calculation gives about 17.67 years. These examples exclude taxes and fees. Savings rates can change, so the result is a scenario, not a prediction.
Does the Rule of 72 work for inflation?
Yes — divide 72 by the inflation rate to see how quickly prices double, which is the same as how quickly cash loses half its buying power. At 3% inflation, that's roughly every 24 years.
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