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The Rule of 72 and How Fast Money Doubles

By the Money Years editors · October 28, 2026

The rule of 72 is the quickest way to understand compound growth without a calculator. Divide 72 by an annual interest rate and the answer is roughly the number of years it takes money to double at that rate. At 8% a year, money doubles in about 72 ÷ 8 = 9 years. At 6%, it takes about 12. It is simple enough to do in your head, and surprisingly accurate for everyday rates.

How accurate it is

The exact doubling time comes from logarithms, which is not something most people want to work out at the kitchen table. The rule of 72 is a shortcut that stays very close across the range of rates that matter for savings and investing.

The rule is most accurate around 8%, and only drifts by about a year at very low rates. For rates far above 20% it becomes less reliable, but those rarely apply to savings. The rule of 72 calculator shows both the estimate and the exact answer for any rate.

Why 72?

The exact formula for doubling time involves the natural logarithm of 2, which is about 0.693. That suggests a rule of 69 or 70, and some people do use those. The number 72 won out because it divides neatly by 2, 3, 4, 6, 8, 9 and 12, which makes mental math easy, and because it happens to give better answers for typical rates between 6% and 10% when interest is added once a year.

Using it for investing

The rule makes the power of long-term investing easy to see. At 7% a year, which is close to the long-run return of a stock-heavy portfolio before inflation, money doubles about every 10 years. Over 40 years that is four doublings: $10,000 becomes about $20,000, $40,000, $80,000 and then about $160,000.

It also shows why small differences in return add up. At 5%, money doubles about every 14 years, so over the same 40 years $10,000 grows to only around $70,000. The difference between 5% and 7% is more than double the final amount. This is why fees matter: a fund charging 1% a year turns a 7% return into 6%, stretching the doubling time from about 10 years to 12.

Using it for debt

The rule works just as well against you. Credit card debt at 24% doubles in about three years if you pay nothing, and at 18% in about four. That is the clearest way to see why high-interest debt should be paid off before investing, since few investments reliably double that fast.

Using it for inflation

Inflation halves the value of money in the same way. At 3% inflation, prices double, and the buying power of a dollar halves, in about 24 years. At 2% it takes about 36 years, and at 6% only 12. Someone retiring at 65 with a fixed pension could see its buying power halve by their late eighties at 3% inflation, which is why inflation-adjusted income matters so much in retirement. The million in future dollars calculator shows what a target will be worth after inflation.

Related rules

The same idea extends to other multiples. Dividing 114 by the rate gives the approximate time to triple your money, and 144 gives the time to quadruple it. At 8%, money roughly triples in 14 years and quadruples in 18.

The bottom line

Divide 72 by the rate to see how fast money doubles, whether it is savings, debt or inflation. It is accurate within a year for the rates most people deal with. For exact numbers including monthly contributions, use the compound interest calculator. All of these are illustrations that assume a steady rate; real investment returns vary from year to year.

This guide is general information for planning, not personal financial advice. Figures were checked on the date shown and can change.