What is the Rule of 72?

A shortcut for estimating how long an investment takes to double: divide 72 by the annual return. At 8%, money doubles in about 9 years (72 ÷ 8). It’s a quick approximation you can do in your head.

How accurate is the Rule of 72?

Very close for typical returns between about 6% and 10%. For lower rates, 69.3 is slightly more precise; for higher rates, the estimate drifts a bit. For everyday planning, 72 is accurate enough.

Can I use it for inflation?

Yes — divide 72 by the inflation rate to see how fast prices double, or how fast cash loses half its value. At 3% inflation, that’s about 24 years.

Investing & Retirement

Rule of 72 Calculator

The Rule of 72 is a mental-math shortcut for how long money takes to double. Divide 72 by your annual return — this tool does it for you and shows the pattern.

Your return

Enter an annual rate to see how long your money takes to double.

Annual return

Starting amount (optional)

How fast your money doubles

Years to double

9.0 years

$10,000 becomes $20,000 in about 9.0 years.

Years to double at various annual returns.
Annual return	Years to double
2%	36.0
4%	18.0
6%	12.0
8%	9.0
10%	7.2
12%	6.0

Divide 72 by the annual return to estimate the doubling time. It's a mental-math shortcut — accurate enough for rates in the normal single-to-low-double-digit range.

Doubling, in your head

The Rule of 72 turns compound growth into simple division. Want to know how long an investment takes to double? Divide 72 by the annual return. It's not exact, but it's remarkably close and instantly useful for comparing options.

Why doubling time matters

Small differences in return have huge long-term effects because of doubling. Money at 8% doubles roughly every 9 years; at 4% it takes about 18. Over a lifetime, that gap is the difference between a few doublings and many.

See it compound

Watch the full curve with the compound interest calculator, project a portfolio with the investment calculator, or flip the idea around to see how inflation erodes value over time.

Related calculators

Compound Interest CalculatorWatch a lump sum and monthly deposits snowball over the years.Investment CalculatorProject the future value of an investment with regular contributions.Inflation CalculatorWhat today’s money will be worth years from now.Retirement CalculatorAre you on track? Project your nest egg to retirement age.

Frequently asked questions

What is the Rule of 72?: A shortcut for estimating how long an investment takes to double: divide 72 by the annual return. At 8%, money doubles in about 9 years (72 ÷ 8). It’s a quick approximation you can do in your head.
How accurate is the Rule of 72?: Very close for typical returns between about 6% and 10%. For lower rates, 69.3 is slightly more precise; for higher rates, the estimate drifts a bit. For everyday planning, 72 is accurate enough.
Can I use it for inflation?: Yes — divide 72 by the inflation rate to see how fast prices double, or how fast cash loses half its value. At 3% inflation, that’s about 24 years.