Estimate how long it takes to double your money with the Rule of 72. Enter an annual interest or return rate to see the doubling time — plus tripling and quadrupling time and the exact figure.
How long to double your money
The Rule of 72 is a quick mental shortcut for estimating how many years it takes an investment to double at a fixed annual rate of return: simply divide 72 by the interest rate. At 8% a year, your money doubles in about 72 ÷ 8 = 9 years. It works because of compound interest, and it's remarkably accurate for rates between roughly 4% and 15%. For tripling money use the Rule of 114, and for quadrupling use the Rule of 144.
The rule is a favorite of investors and financial educators because it turns abstract percentages into a tangible timeline. A 6% return doubles money in 12 years; a 9% return doubles it in 8 years; a 12% return in just 6 years. That single percentage-point difference compounds into enormous gaps over a lifetime, which is exactly why minimizing fees and maximizing returns matters so much. For the precise figure, this calculator also shows the exact doubling time using logarithms.
Divide 72 by your annual return to estimate doubling time. At 8%, money doubles in ~9 years.
It approximates the math of compound growth. Most accurate for rates between about 4% and 15%.
Use 114 ÷ rate to triple your money and 144 ÷ rate to quadruple it — the same logic extended.
The Rule of 72 is a mental shortcut for doubling time: divide 72 by the annual percentage rate and you get the years required, without a calculator or a logarithm. It works because the exact answer, ln(2)/ln(1+r), is very nearly 0.693/r for small rates, and 72 is a convenient numerator that divides cleanly by 2, 3, 4, 6, 8, 9 and 12. Accuracy is excellent between roughly 6% and 10% and degrades at the extremes — at 2% the rule is about 3% optimistic, and at 20% it is noticeably pessimistic. For precision at unusual rates, use the exact logarithmic form.
Years to double ≈ 72 ÷ rate (as a whole-number percent)Exact: years = ln(2) ÷ ln(1 + r)where:
Assumptions: Assumes a constant rate compounded annually with no contributions, withdrawals, fees or tax. Doubling time in real terms requires using a rate net of inflation.
Check the shortcut against the exact calculation to see how close it really gets.
Result9 years to double — exact answer 9.006 years
The rule also runs backwards: to double in 5 years you need 72 ÷ 5 ≈ 14.4% a year. And it applies to erosion as well as growth — at 3% inflation, prices double in 72 ÷ 3 = 24 years.