⏳ Rule of 72 Calculator
Enter an annual rate and the Rule of 72 estimates the doubling time — shown alongside the Rule of 70 and the exact figure.
Rule of 72
9 yrs
to double
Rule of 70
8.75 yrs
Exact
9.01 yrs
ln2 ÷ ln(1+r)
The Rule of 72 estimates the doubling time: years ≈ 72 ÷ rate. It’s most accurate near 8%; the exact figure is ln2 ÷ ln(1+rate). Educational, not advice. 🔒 In your browser.
How the rule of 72 calculator works
The Rule of 72 is a mental-math shortcut: divide 72 by the annual percentage rate to estimate the years to double your money. The tool also shows the Rule of 70 (often used for continuous growth) and the exact doubling time, ln(2) ÷ ln(1 + rate).
The Rule of 72 is most accurate around 6–10%; far from that range the exact figure diverges. Educational — not financial advice.
Frequently asked questions
What is the Rule of 72?
A shortcut for estimating how long an investment takes to double: years ≈ 72 ÷ annual percentage rate. At 8%, money doubles in about 72 ÷ 8 = 9 years.
How accurate is the Rule of 72?
Very close around 6–10%. Outside that range, use the Rule of 70 or 69.3, or the exact formula ln(2) ÷ ln(1 + rate), which the tool also shows.
Why 72 and not 70?
72 has many small divisors (2, 3, 4, 6, 8, 9, 12), making the mental division easy, and it’s a good approximation for annual compounding. 70 is closer for continuous compounding.
Can I use it for inflation?
Yes — dividing 72 by an inflation rate estimates how long until prices double (or your money’s purchasing power halves).
What is the exact doubling time?
ln(2) ÷ ln(1 + rate). At 8% that’s about 9.01 years, very close to the Rule of 72’s estimate of 9.
Educational information, not financial advice. These calculators use standard formulas with the figures you enter; results are illustrations, not guarantees. For decisions about your money, consult a qualified, regulated financial professional.