What is the Rule of 72?
The Rule of 72 is a mental shortcut: divide 72 by the annual return to estimate how many years money takes to double. At 8%, that is 9 years. At 12%, 6 years.
It is remarkably accurate between about 6% and 10%, drifting slightly at the extremes. The exact answer uses logarithms, and this calculator shows both so you can see how close the shortcut is.
Formula & worked example
Exact: Years = ln(2) / ln(1 + rate)
Worked example: at 8%, the Rule of 72 estimates 9.0 years to double; the exact answer is 9.01 years — near-perfect. Over 30 years that means about 3.3 doublings, turning 100,000 into roughly 1,006,000 — a tenfold increase.
How to use this rule of 72 calculator
- Enter a starting amount and expected annual return.
- Compare the Rule of 72 estimate against the exact figure.
- Set a projection period to see the total growth multiple.
- The table shows each successive doubling — the pattern that makes compounding feel exponential.
Smart tips
- The rule works both ways: at 6% inflation, prices double in 12 years and your money halves in value.
- Use 69.3 instead of 72 for continuous compounding, or 70 for a slightly better fit at low rates.
- The rule reveals why fees matter: earning 7% instead of 8% adds more than a year to every doubling.
- Applied to debt, it shows how quickly balances grow — credit card debt at 24% doubles in just three years.
- Each doubling adds as much as every previous doubling combined, which is why the final years dominate.
Frequently asked questions
What is the Rule of 72?
A shortcut to estimate doubling time: divide 72 by the annual percentage return. At 9% return, money doubles in about 8 years.
How accurate is the Rule of 72?
Very accurate between 6% and 10%. It drifts slightly at very low or very high rates, where the exact logarithmic formula is better.
Can I use it for inflation?
Yes. At 6% inflation, prices double and purchasing power halves in about 12 years.
Why 72 specifically?
Because 72 is divisible by many numbers, making mental arithmetic easy, and it approximates ln(2) × 100 ≈ 69.3 with a correction for discrete compounding.
Want the theory behind the numbers? Read our compounding guides on the Money Blog.
Written by Gurjeet Singh, Sharpen Solutions. Last reviewed 8 Sep 2026. How we check these numbers.