The rule of 72: how many years it takes your money to double

The rule of 72 is the quickest way to work out in your head how long it takes money to double: divide 72 by the yearly return in percent. At 6% a year your money doubles in about 12 years, at 9% in about 8 years.

Take an example. $10,000 growing at 7% a year should double, by the rule, in 72 ÷ 7 = 10.3 years. The compound interest calculator shows $19,672 after 10 years and $21,049 after 11. The money passes $20,000 during year 11, after exactly 10.2 years.

How many times your money doubles in a lifetime

At 7% a year, money doubles roughly every ten years. After 10, 20, 30 and 40 years you have 2×, 4×, 8× and 15× what you started with: $10,000 becomes $149,745 in 40 years. The higher the return, the more doublings fit into the same time, which is why the gap between a low and a high return keeps widening.

• 4% a year
• 7% a year
• 10% a year

40

• 4% a year$48,010
• 7% a year$149,745
• 10% a year$452,593

Horizontal axis: years since the money was invested. Yearly return, no fees or taxes.

How accurate the rule is

The rule of 72 is a shortcut. The exact doubling time comes from logarithms (ln 2 ÷ ln (1 + return)), and the table shows how far the rule lands from it.

Yearly return	Rule of 72	Exact	Difference (months)
1%	72.0	69.7	+28
2%	36.0	35.0	+12
3%	24.0	23.4	+7
4%	18.0	17.7	+4
5%	14.4	14.2	+2
6%	12.0	11.9	+1
7%	10.3	10.2	0
8%	9.0	9.0	0
10%	7.2	7.3	−1
12%	6.0	6.1	−1
15%	4.8	5.0	−2
20%	3.6	3.8	−2

The rule is most accurate around 8% a year. At low returns it overstates the time: at 1% it says 72.0 years, while 69.7 years is enough, 28 months less. At high returns it understates it: at 20% it gives 3.6 years instead of 3.8 years, 2 months short.

A small tweak fixes most of that: add 1 to 72 for every three percentage points above 8%, and take 1 away for every three points below. At 20% you divide 76 instead and get 3.8 years, the same as the exact figure. For small numbers such as inflation, people often use the rule of 70: at 2% it gives 35 years, as does the exact calculation (35 years).

Why 72

If returns were added continuously, the exact rule would be 69.3 (100 × ln 2). With returns added once a year and rates around 8%, 72 fits better. It also divides evenly by 2, 3, 4, 6, 8, 9 and 12, which makes it easy to use in your head.

How fast stocks, bonds and cash have doubled

US stocks (the S&P 500 with dividends) returned 10.0% a year on average from 1928 to 2025. By the rule of 72 that is a doubling every 7.2 years; the exact figure is 7.3 years. $100 invested at the start of 1928 would have grown to about $1,160,000 by the end of 2025: the money doubled more than 13 times.

Prices rose 18.7 times over the same years, 3.0% a year on average. After inflation, stocks earned 6.8% a year and the buying power of the money doubled every 10.6 years. Over the whole period it grew about 618 times.

Investment	Yearly return	Doubling (years)	Return after inflation	Doubling after inflation (years)
Stocks (S&P 500)	10.0%	7.3	6.8%	10.6
10-year Treasury bonds	4.5%	15.6	1.5%	48
3-month Treasury bills	3.4%	21	0.3%	212

Treasury bonds doubled about every 15.6 years. Three-month Treasury bills, the series closest to a savings account or a money market fund, took 21 years. After inflation, though, bills returned only 0.3% a year, and doubling their buying power would take 212 years. Cash at best holds its value; it does not build wealth.

The rule works for inflation too

The same trick tells you how fast prices double, and so how fast money loses half its buying power. If prices rose exactly at the Federal Reserve's 2% target, they would double in 72 ÷ 2 = 36 years (exactly 35 years).

In practice they rose faster. US consumer prices last doubled in 28 years, from 1997 to 2025. Inflation averaged 2.5% a year over that time, and the rule of 72 gives 72 ÷ 2.5 = 28.6 years. $10,000 kept in a drawer since 1997 buys about what $4,990 bought then.

To keep its buying power, money has to earn more than inflation. The inflation calculator shows how the value of the dollar changed year by year.

Fees stretch the doubling time

A fund's fee comes off its return. A fund that earns 7% and charges 1.5% a year leaves you about 5.4%. By the rule of 72 your money then doubles every 13.3 years instead of every 10.3 years, and after 40 years you end up with 8.2 times what you put in rather than 15 times. You can compare two real funds in the fund fee calculator.

What it means for you

• Divide 72 by the return. You get the years to double. Between 4% and 12% you will be off by a few months at most.
• The rule works against you too. Prices rising 2.5% a year halve the value of cash in 28.6 years. An unpaid debt at 20% doubles in 3.8 years.
• Every percentage point counts. A point less in return, or a point more in fees, stretches every doubling, and over decades you get fewer of them.
• Returns are not guaranteed. The rule assumes a steady return; real markets swing.

See also: A 0.2% or a 1.5% fund fee · Start investing at 25 or 35

Try your own amount, return and time in the compound interest calculator. It shows how the money grows year by year, with regular contributions too.