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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.

Does the stock market really double that often?

The rule assumes a steady rate, but markets never deliver one, so it is worth testing it against real history. The S&P 500 has returned roughly 10% a year on average since 1928 with dividends reinvested, which the rule of 72 turns into a doubling about every 7 years. Using yearly total returns from 1928 to 2025, we checked every start year from 1928 to 2015 and counted how long a dollar invested at the start of that year took to double.

The median answer was exactly 7 years, so on average the rule held up remarkably well. The spread around it, though, was wide. The fastest doublings took only 2 years, starting just before strong runs. The slowest took 22 years, for money invested at the start of 1929, just before the crash. Other slow starts cluster around 1965 to 1969, when high inflation hurt stocks, and 1998 to 2002, when the dot-com bust and later the 2008 crisis meant 12 to 17 years passed before money doubled.

Chaining the doublings together tells the same story. Starting in 1928, an investment doubled by the end of 1945, again by 1952, again by 1956, and kept going, with eleven full doublings by 2020. The S&P 500 since any year calculator shows the exact path for any start year. The lesson is that the rule of 72 is a good guide to the long-run average, but any single decade can be much better or much worse.

Your age and the last doubling

Because each doubling is bigger than the one before, the final doubling before retirement is the most valuable, and you only get it by starting early. At 7% a year, money doubles about every 10 years. A single $10,000 investment made at 25 grows to about $150,000 by 65. Made at 35, it reaches about $76,000, and at 45 only about $39,000.

Waiting ten years does not cost you a tenth of the result; it costs you a whole doubling, which is half of the final amount. That is the simplest argument for starting to invest as soon as you can, even with small amounts, and it is why the cost of waiting calculator often surprises people.

When the rule does not apply

The rule of 72 describes a single lump sum growing at a fixed rate. It does not work for regular contributions, because each deposit has a different amount of time to grow, and it ignores taxes, fees and withdrawals. For a savings plan with monthly deposits, the investment calculator or the years to a million calculator gives the real timeline.

It also becomes misleading when rates change over time, such as a variable-rate loan or an account whose promotional rate ends. In those cases it is better to work out each period separately than to rely on a single average.

Common questions

How long does it take to double money at 10%? About 7.2 years by the rule of 72, or 7.3 years exactly.

What rate do I need to double my money in 5 years? About 72 ÷ 5 = 14.4% a year, which is well above typical long-run stock returns.

Does the rule of 72 work for inflation? Yes. Divide 72 by the inflation rate to see roughly how long it takes for prices to double and the value of money to halve.

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.