TimeValue

Rule of 72 Calculator

The classic mental-math shortcut for estimating doubling time — no calculator needed, but here's the precise version.

Annual growth rate

Years to double

9.0 years

72 ÷ 8%

What you entered

Annual growth rate: 8%

1
Divide 72 by the annual growth rate
72 ÷ 8%= 9.0 years

Result

Years to double: 9.0 years

At a steady 8% annual return, money roughly doubles every 9.0 years. It's a mental-math shortcut, not exact — for a precise figure use a compound interest calculator instead.

Years to double your money, by annual return

The Rule of 72 estimates a doubling time by dividing 72 by the annual growth rate. The second column is that estimate; the third is the mathematically exact answer from ln(2) / ln(1 + r), shown so you can see where the shortcut holds and where it drifts.

Annual return	Rule of 72 estimate	Exact years to double
1%	72.0 years	69.7 years
2%	36.0 years	35.0 years
3%	24.0 years	23.4 years
4%	18.0 years	17.7 years
5%	14.4 years	14.2 years
6%	12.0 years	11.9 years
7%	10.3 years	10.2 years
8%	9.0 years	9.0 years
10%	7.2 years	7.3 years
12%	6.0 years	6.1 years
15%	4.8 years	5.0 years
20%	3.6 years	3.8 years

The rule is at its most accurate around 8%, where it is exact to a tenth of a year, and it drifts either side of that: it is roughly two years pessimistic at 1% and a couple of tenths optimistic above 15%. For the 5% to 12% band that covers most long-term investing questions it is within about a tenth of a year, which is far more precision than the underlying return assumption deserves. The same arithmetic works on inflation - at 3% inflation, prices double in about 24 years.

Where the '72' comes from

It's a close approximation of the exact formula ln(2)/ln(1+r), which isn't easy to do in your head. 72 divides evenly by many common rates (6, 8, 9, 12), which is why it stuck as the mental shortcut.

Accuracy range

The Rule of 72 is most accurate for rates between about 6% and 10%. Outside that range, the estimate drifts further from the true compound-interest answer — use a compound interest calculator for precision.

Quick doubling times to memorize

At 4%: 18 years. At 6%: 12 years. At 8%: 9 years. At 10%: 7.2 years. At 12%: 6 years. These mental benchmarks let you evaluate any investment opportunity instantly — if someone promises 10% returns, you know your money should double in about 7 years, and tripling takes about 11.5 years (Rule of 115).

Frequently asked questions

What is the Rule of 72?

Divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 6% annual return: 72 ÷ 6 = 12 years to double. It works for any compounding growth — investments, inflation, population, even debt.

How accurate is the Rule of 72?

Very accurate between 4-12%. At 6%, it says 12.0 years; the exact answer is 11.9 years. At 2% it's slightly off (says 36, exact is 35), and at 20% it's off more (says 3.6, exact is 3.8). For everyday financial planning, it's close enough for quick decisions.

Can the Rule of 72 be used for inflation?

Yes — it works in reverse too. At 3% inflation, your money's purchasing power halves in 72 ÷ 3 = 24 years. At 7% inflation, it halves in about 10 years. This is why leaving cash idle during high inflation is so damaging.