Rule of 72 Calculator
Find out how many years it takes to double your money at a given interest rate — exactly and with the rule of 72.
Assumes a fixed rate with all returns reinvested and no tax, fees or withdrawals. Real investment returns vary from year to year. General information, not financial advice.
About this tool
Enter an annual interest rate or return to see how long it takes an amount to double, or enter a number of years to see the rate you need. The calculator gives the exact doubling time for yearly, monthly, daily or continuous compounding, compares it with the rule of 72, 70 and 69.3, and shows when the money has grown four and eight times.
Frequently asked questions
A shortcut for compound growth: divide 72 by the yearly interest rate in percent to get roughly the number of years it takes money to double. At 8% a year, 72 ÷ 8 = 9 years; the exact answer with yearly compounding is ln 2 ÷ ln 1.08 = 9.006 years.
It is most accurate for rates around 6–10% compounded yearly. At 9% it gives 8 years against an exact 8.04. At low rates it overestimates: at 2% the exact doubling time is 35.0 years, so the rule of 70 (70 ÷ 2 = 35) fits better than the rule of 72 (36). For continuous compounding the exact rule is 69.3, because ln 2 ≈ 0.693.
With the rate r as a decimal and interest added k times a year, the doubling time is t = ln 2 ÷ (k × ln(1 + r ÷ k)). With yearly compounding that is ln 2 ÷ ln(1 + r): 6% gives 11.90 years. Adding the interest monthly shortens it to 11.58 years, and continuous compounding gives ln 2 ÷ 0.06 = 11.55 years.
Yes. At 3% inflation a year, prices double in ln 2 ÷ ln 1.03 = 23.4 years (the rule of 72 says 24). That is also how long it takes for the purchasing power of money kept in cash to fall to half.
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