Rule of 72 Calculator
The Rule of 72 is one of the most useful mental shortcuts in finance. Divide 72 by the annual return rate (as a percentage) to estimate how many years it takes to double your money. At 8% annual return, 72/8 = 9 years. The rule works for investments, debt, inflation, and any compounding growth scenario. This calculator shows both the Rule of 72 approximation and the mathematically exact doubling time using the natural logarithm formula, so you can see how close the approximation is at different rates. It also shows how much a $1,000 investment becomes after one, two, and three doubling periods.
Rule of 72 formula
Approximate years to double = 72 / rate(%)
Exact years to double = ln(2) / ln(1 + rate) = 0.6931 / ln(1 + rate)
The exact formula uses the natural logarithm and is accurate for any rate. The Rule of 72 approximation works best for rates between 1% and 20% per year. For higher rates, the Rule of 72 modestly underestimates the doubling time.
Quick reference: doubling times by rate
- 2% return: 36 years to double (Rule of 72: 36).
- 4% return: 18 years to double (Rule of 72: 18).
- 6% return: 11.9 years to double (Rule of 72: 12).
- 8% return: 9.0 years to double (Rule of 72: 9).
- 12% return: 6.1 years to double (Rule of 72: 6).
- 18% credit card: 4.2 years for debt to double (Rule of 72: 4).
Frequently asked questions
What is the Rule of 72?
The Rule of 72 is a mental math shortcut: divide 72 by the annual interest or return rate (as a percentage) to estimate how many years it takes to double an investment. At 6% per year, 72/6 = 12 years to double. It works for any compounding growth, not just investments.
How accurate is the Rule of 72?
The Rule of 72 is an approximation. The exact formula is years = ln(2) / ln(1 + r) = 0.693 / r (for small r). At rates between 1% and 20%, the Rule of 72 is accurate to within one or two percent of the exact answer. At higher rates it slightly underestimates the doubling time.
Can I use the Rule of 72 for debt?
Yes. If a credit card charges 18% APR, 72 / 18 = 4 years for unpaid debt to double. This makes the rule a powerful way to understand the compounding cost of high-interest debt as well as the compounding benefit of positive returns.
Does the Rule of 72 work for inflation?
Yes. Divide 72 by the inflation rate to find how many years it takes for prices to double (and for cash to lose half its value). At 4% inflation, 72/4 = 18 years to halve purchasing power.
Why 72 and not 70 or 69.3?
The Rule of 69.3 (using ln(2) = 0.693) is more mathematically precise for continuous compounding. The Rule of 72 is used because 72 has many factors (1, 2, 3, 4, 6, 8, 9, 12) making mental division easy for many common interest rates. The Rule of 70 is also used, particularly for economic growth and inflation calculations.
Official sources
- Federal Reserve FRED: Historical Interest Rate Data.
- U.S. Treasury: Current Treasury Rates.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.