Rule of 72 Calculator
The Rule of 72 is a simple mental math shortcut used in finance to estimate how many years it takes for an investment to double at a fixed annual rate of return. By dividing 72 by the annual interest rate percentage, you get a close approximation of the doubling time without needing a calculator or logarithm tables.
The reverse calculation is equally useful: if you know how many years you have to reach a financial goal, dividing 72 by that number tells you the annual return rate you need to achieve it. This makes the Rule of 72 a powerful tool for quickly evaluating investment options, comparing interest rates, or understanding the long-term impact of inflation on purchasing power.
How it works
Years to double ≈ 72 ÷ Annual Rate (%). Reverse: Required Rate (%) ≈ 72 ÷ Years. The exact formula uses the natural logarithm: t = ln(2) / ln(1 + r), but 72/r is accurate within 1% for rates between 6% and 10%.
Use cases
- Estimating how long a retirement account takes to double at a given return rate
- Comparing two investment options to see which doubles money faster
- Understanding how quickly inflation can halve your purchasing power
- Setting realistic savings goals based on available investment returns
- Teaching compound interest concepts in personal finance education
Frequently asked questions
How does the Rule of 72 work?
Divide 72 by your annual interest rate percentage to estimate how many years it takes for an investment to double. For example, at an 8% annual return, 72 ÷ 8 = 9 years to double your money. It works because 72 closely approximates the compound growth math and is easy to divide mentally by common rates.
How accurate is the Rule of 72?
The Rule of 72 is accurate to within about 1% for annual rates between 6% and 10%, which covers most realistic investment returns. The exact doubling time comes from the logarithm formula t = ln(2) / ln(1 + r). Outside that range the approximation drifts slightly, but it still gives a useful ballpark estimate.
What return rate do I need to double my money in a set number of years?
Use the reverse calculation: divide 72 by the number of years available. To double your money in 6 years, you would need roughly 72 ÷ 6 = 12% per year, while doubling in 12 years requires only about 6% annually. This helps you judge whether a financial goal is realistic given available investment options.
Can the Rule of 72 be used for inflation?
Yes. Dividing 72 by the annual inflation rate estimates how many years it takes for prices to double, which is the same as your purchasing power being cut in half. For instance, at 6% inflation, money loses half its purchasing power in about 72 ÷ 6 = 12 years.
Does the Rule of 72 apply to simple interest?
No, the Rule of 72 assumes compound interest, where earnings are reinvested and generate further earnings. With simple interest, doubling takes 100 ÷ rate years instead, since interest accrues only on the original principal. Always confirm the product compounds before applying the rule.