Milo Quell

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Rule of 72 Calculator

See how long it takes to double your money at any return — the fastest mental math in investing.

%

Rule of 72 estimate

9.0 yrs

years to double

Exact answer (for comparison)

9.0 yrs

The Rule of 72 is a mental shortcut: divide 72 by the rate to estimate the doubling time. It's remarkably close to the exact figure for typical returns of 6–10%.

The doubling shortcut worth memorising

The Rule of 72 is the single most useful piece of mental math in personal finance. It turns an abstract return rate into a concrete timeline: an 8% return doubles your money in about nine years, a 6% return in twelve, a 12% return in six. Once you can do this in your head, every investment decision gets a sense of scale.

It also works as a warning. Inflation at 3% halves your buying power in roughly 24 years, and a credit card at 24% doubles your debt in about three. The same rule that shows compounding working for you shows exactly how fast it works against you.

Frequently asked

What is the Rule of 72?

The Rule of 72 is a mental shortcut for estimating how long an investment takes to double. Divide 72 by the annual return rate and you get the approximate number of years. At 8% a year, for example, money doubles in about 72 ÷ 8 = 9 years.

How accurate is the Rule of 72?

Surprisingly accurate for typical returns. Between roughly 6% and 10%, it lands within a few months of the exact figure from the compound interest formula. It becomes less precise at very high or very low rates, which is why this tool also shows the exact answer alongside the estimate.

How do you use the Rule of 72 in reverse?

To find the return needed to double in a set number of years, divide 72 by the years. To double your money in 6 years you would need about 72 ÷ 6 = 12% a year. Switch this calculator to "I know the timeframe" to solve it that way.

Why 72 and not another number?

72 is chosen because it divides cleanly by many common rates (2, 3, 4, 6, 8, 9, 12) and closely matches the true mathematics of compounding for everyday interest rates. It is a convenient approximation, not an exact law.

Keep going

See the full picture of how compounding grows a balance year by year.

Compound Interest Calculator →
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