The rule in one line
The rule of 72 is a mental-math shortcut: divide 72 by the yearly return and you get the approximate number of years it takes money to double. At 6% that is 12 years, at 8% it is 9 years, and at 12% it is 6 years. It works because the true doubling time, ln(2) ÷ ln(1 + r), happens to sit very close to 72 ÷ r for the interest rates people usually deal with.
Years to double ≈ 72 ÷ yearly return (%)
The calculator shows both the shortcut and the exact answer so you can see how close they are. Between about 6% and 10% the rule is off by only a few weeks. At very low or very high rates it drifts, which is why some people use 70 or 69.3 instead for low rates.
Why doubling matters for big goals
Thinking in doublings makes long-term growth easy to picture. At 7% money doubles roughly every ten years, so $125,000 at age 35 becomes about $250,000 at 45, $500,000 at 55 and $1,000,000 at 65 without another deposit. That is also why the last doubling is the biggest one in dollars: it adds as much as all the earlier growth put together.
The same rule works against you with debt and inflation. A credit card charging 24% doubles an unpaid balance in about three years, and 3% inflation halves the buying power of cash in about 24 years.
Common questions
How long does it take to double money at 7%?
About 10.2 years exactly, or 10.3 years by the rule of 72.
Is the rule of 72 accurate?
It is very close for returns between about 5% and 12%. For low rates the rule of 70 is slightly better, and for exact answers use the formula shown in the steps.
Does the rule of 72 work for inflation?
Yes. Divide 72 by the inflation rate to see how long it takes prices to double, or the buying power of cash to halve.
What about the rule of 115?
It is the same idea for tripling: divide 115 by the rate to estimate how long money takes to triple.
This calculator gives estimates for planning. Real returns, rates and prices change, so treat the result as a guide rather than a promise. Read how we build and check our calculators.