Enter a growth rate to see how long money takes to double — the mental-math rule, the exact answer beside it, and the inflation flip side.
The most useful shortcut in finance
Divide 72 by an annual growth rate and you get, almost exactly, the years for money to double at that rate — no compound-interest formula required. This calculator runs the rule, prints the mathematically exact answer beside it so you can see how good the approximation is, and shows the flip side people forget: the same rule tells you how fast inflation halves your money’s value.
How to use it
- Enter the annual rate — an investment return, or an inflation rate.
- Read the doubling time.
- Compare the exact figure to see the rule’s accuracy.
The formula
A worked example
At 8%, the rule says 9 years; the exact answer is 9.01 — the approximation is that good in the everyday range. The compounding chain is where it gets powerful: at 8%, money doubles in 9, quadruples in 18, is 8× in 27 — and the same arithmetic run on 3% inflation says a currency’s purchasing power halves every 24 years, which reframes “safe” cash held for decades.