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See the real, exact compound-interest answer for how long a return rate takes to double your money — right next to the quick Rule of 72 estimate, entirely in your browser.
Enter a positive interest rate to see how long it takes to double.
Whatever annual return you want to check — a savings rate, an expected investment return, anything.
The exact answer depends on how often that rate actually compounds.
See the real answer next to the classic mental-math shortcut, and exactly how far apart they are.
A mental-math shortcut: divide 72 by an interest rate to get a rough estimate of how many years it takes to double, without doing any actual exponent math. It's a genuinely useful approximation, not an exact formula — it happens to line up closely with reality for rates roughly in the 6–10% range, and drifts further off the further a rate sits outside that range.
By solving the actual compound interest equation for time: how many compounding periods it takes for (1 + rate/frequency) raised to that many periods to reach 2, then converting periods back into years. No approximation or rounding shortcut involved.
More frequent compounding means interest starts earning its own interest sooner, so the same nominal annual rate reaches double slightly faster monthly than it does annually. The Rule of 72 doesn't account for this at all — the exact calculation does.
Yes. Every calculation happens entirely inside your browser tab.
No, this is a mathematical projection based on the rate you enter. Actual investment returns vary and are never guaranteed.