The rule of 72: a mental shortcut for doubling your money

Divide 72 by an annual growth rate and you get, roughly, the number of years it takes money to double. At 6% a year, 72 ÷ 6 = 12 years. That’s the whole rule. The rest is knowing when to trust it.

The rule of 72 works for anything that grows at a steady percentage each year: savings earning compound interest, an investment portfolio, even prices rising with inflation. It turns a question that normally needs a calculator or spreadsheet into something you can work out while someone is still talking to you.

How to use it

There are two directions to the rule, and both use the same division.

From rate to years. Divide 72 by the annual growth rate to estimate how long doubling takes. A sum growing at 4% a year doubles in about 72 ÷ 4 = 18 years. At 9%, it’s 72 ÷ 9 = 8 years. The higher the rate, the faster the doubling, and the rule captures that relationship without any real arithmetic.

From years to required rate. Flip the division around. If you want your money to double in 8 years, divide 72 by 8 to get the rate you’d need: 9%. If you’re happy to wait 12 years, you only need 6%. This direction is useful for sense-checking a target: someone claiming they’ll double your money in 3 years is implicitly claiming a 24% annual return, which is worth pausing on.

Both directions rely on the same number, 72, sitting in the middle. You’re never calculating anything new, just rearranging the same relationship depending on which side of it you know.

How accurate is it really

The rule of 72 is an approximation, not an identity, so it’s worth knowing where it holds up and where it strains.

Annual rate	Exact years to double	Rule of 72 estimate
1%	69.7	72.0
2%	35.0	36.0
3%	23.4	24.0
4%	17.7	18.0
6%	11.9	12.0
8%	9.0	9.0
9%	8.0	8.0
12%	6.1	6.0

The estimate is tightest in the 6–10% band, where it’s accurate to a few weeks either way. That’s not a coincidence: it’s the range most savings accounts, bonds and diversified portfolios actually sit in, which is presumably why the rule caught on. Move to the extremes and it drifts. At 1%, the rule overstates the true doubling time by more than two years. At 12%, it understates it slightly. Neither error is large enough to mislead you about the shape of the answer, but neither is free either. Treat the rule of 72 as a fast first estimate, not a substitute for an exact calculation when precision actually matters, such as comparing two similarly priced financial products.

Why 72

Seventy-two isn’t an arbitrary round number picked for memorability. It approximates the underlying logarithmic relationship that governs compound growth, and it happens to divide cleanly by 2, 3, 4, 6, 8, 9 and 12 — the growth rates people actually encounter most often. That combination of a reasonably accurate approximation and a highly divisible number is what makes it useful as mental arithmetic rather than merely as a formula. A number like 70 would be marginally more accurate at very low rates, and some textbooks do use it for that reason, but it divides far less cleanly, which defeats the purpose of a shortcut meant to be done without a calculator.

What it’s actually for

The rule of 72 is not a forecasting tool. It’s an instant sense-check, useful precisely because it fits in your head at the moment you need it.

Someone shows you a pension projection claiming your fund will double in ten years. The rule of 72 tells you that implies roughly a 7.2% annual return, above what a cautious, diversified portfolio typically delivers. Worth a question, not necessarily worth alarm.

A friend mentions inflation is eating into their savings and wonders how long until prices double. At an inflation rate hovering around 6%, the answer is about 12 years. Suddenly the vague worry has a rough timescale attached to it.

A growth claim from an investment pitch sounds impressive until you divide 72 by the stated return and realise the doubling time implied is faster than anything sustained markets have historically managed. That’s not proof the claim is false, but it’s a reason to ask harder questions before relying on it.

None of these are precise answers, and none should be treated as financial advice or a substitute for a proper calculation from a professional adviser when real money and real decisions are on the line. But doubling time worked out in your head, in the moment, beats an exact figure from a spreadsheet you never open. That’s the whole value of the rule of 72: it turns compound interest, inflation and doubling time from abstract concepts into a number you can hold onto during a conversation.