How the rule of 72 works
Years to double ≈ 72 ÷ yearly rate (in %)
Rate needed ≈ 72 ÷ years
It's a shortcut for compound growth. The exact doubling time is ln(2) ÷ ln(1 + rate), and 72 happens to give answers within a few months for everyday rates between about 4% and 12%, while being easy to divide in your head.
Worked examples
At 7% a year, money doubles in about 72 ÷ 7 = 10.3 years; the exact figure is 10.2. $10,000 becomes $20,000, then $40,000 about ten years later.
It works for costs too: at 3% inflation, prices double, and your cash loses half its buying power, in about 72 ÷ 3 = 24 years.
And for debt: a credit card at 24% doubles an unpaid balance in about three years.
Questions people ask
Why 72 and not 70 or 69?
At low rates, 69.3 is closer to exact (the "rule of 70" is a rounder version). 72 is slightly more accurate around 8% and divides evenly by 2, 3, 4, 6, 8, 9 and 12, which makes mental math easy.
How do I see the full growth with monthly deposits?
Use the compound interest calculator, which adds contributions and shows the result in today's dollars.
Sources
- U.S. Securities and Exchange Commission: Creating Choices (the rule of 72)
- Investor.gov (SEC): what is compound interest?
Reviewed September 28, 2026 by The Dollars World team. How we build and check our calculators · Our data and formulas. This is general information, not financial advice.