Rule of 72 Calculator
Estimate how many years an investment takes to double at a given rate, and compare the Rule of 72 to the exact compound formula.
Enter the rate as a percent (6 for 6%). Numbers stay in your browser. The default is 6%, where the rule says 12 years and the exact compound figure is just under 12.
Quick answer: if the annual rate is in B2 as 6% or 0.06:
=72/(B2*100)
=LN(2)/LN(1+B2)Write =72/B2 instead when B2 already holds the whole number 6. The first formula is the Rule of 72 estimate; the second is the exact years for annual compounding.
6%, 8%, and 10%
At 6%, 72 ÷ 6 = 12 years. In Google Sheets on 12 November 2026, =72/6 returned 12, and the exact formula =LN(2)/LN(1+0.06) returned 11.895661045941875 years. The rule is about 0.10 years (a little more than a month) on the high side.
At 8%, =72/8 returned 9, and =LN(2)/LN(1.08) returned 9.006468342000588 — as close as the rule usually gets. At 10%, =72/10 returned 7.2, a little below the exact 7.272540897341713.
The shortcut comes from the fact that ln(2) ≈ 0.693, and 72 is a convenient number near 69.3 that also divides evenly by 2, 3, 4, 6, 8, and 9. Some people use 69.3 or 70 for a tighter estimate at low rates; this page sticks to 72, which is the version most spreadsheets and textbooks quote.
When not to use it
The rule assumes a constant annual rate and annual compounding, with no extra deposits. Monthly compounding, or a regular contribution, needs the compound interest calculator instead. Simple interest does not double in 72/r years: at 6% simple, 100 grows by 6 a year and takes 100/6 ≈ 16.7 years to double, not 12.
A rate of 0 cannot be used as a divisor. Very high rates make the rule worse: at 72% it claims 1 year, while money compounding annually at 72% more than doubles in a year. For a growth rate over many years, the CAGR calculator works backwards from the start and end values.
Questions that come up
What is the Rule of 72 formula in Excel or Google Sheets?
If the annual rate is stored as 6% or 0.06 in B2, the estimate is =72/(B2*100) when B2 is a decimal, or =72/B2 when B2 already holds the percent number 6. That is the years to double. The exact compound figure is =LN(2)/LN(1+B2) with B2 as 0.06.
How accurate is the Rule of 72?
It is closest around 8%. At 8%, 72 ÷ 8 = 9 years, and the exact figure in the test workbook was 9.006468342000588 years. At 6% the rule is a little high (12 vs 11.895661045941875). At 10% it is a little low (7.2 vs 7.272540897341713).
Does the Rule of 72 assume compound interest?
Yes. It is a shortcut for annual compounding. It is not a substitute for a full compound-interest schedule with monthly deposits, and it does not apply to simple interest, which grows in a straight line and does not double in 72/r years.