How long does it take to double your money? The rule of 72

6 October 2026 · Calcosmo editorial team

Want to know how long it takes for your savings to double? Divide 72 by the interest rate. At 4% it takes about 72 ÷ 4 = 18 years. For an exact calculation, use the compound interest calculator.

How the rule of 72 works

The rule of 72 is a shortcut for compound interest:

years to double ≈ 72 ÷ interest rate in percent

It works the other way round too: to double your money in 10 years, you need about 72 ÷ 10 = 7.2% return per year.

How accurate is it?

Between about 2% and 10%, you are off by a few months at most. The exact formula is years = ln(2) ÷ ln(1 + r), but the rule of 72 is far easier to do in your head.

Why 72?

72 works best around 8% and is divisible by many numbers: 2, 3, 4, 6, 8, 9 and 12. Mathematically, 69.3 would be more accurate at very low rates, but it is much less convenient.

Inflation works the other way

The same rule shows how fast inflation halves your purchasing power. At 3% inflation, your money is worth only half after about 72 ÷ 3 = 24 years. If your savings earn 1.5% while inflation is 3%, you lose purchasing power every year. That is why your real return (interest minus inflation) matters.

It applies to debt too

The rule works just as well for debt. A credit card balance at 24% APR doubles in about 72 ÷ 24 = 3 years if you pay nothing off. A good reason to pay off expensive debt first.

Calculate it exactly

With the compound interest calculator you can use your own starting balance, monthly deposits and term. Want to learn more about how it works? Read what compound interest is.