Rule of 72 Calculator
Estimate how long it takes to double your investment
About the Rule of 72
The Rule of 72 is a simple way to estimate how long an investment will take to double given a fixed annual interest rate.
Formula: Time to Double = 72 ÷ Interest Rate
Example: At 8% interest, your money will double in approximately 72 ÷ 8 = 9 years.
Calculator Inputs
Interest Rate
The annual interest rate your investment earns, expressed as a percentage. This is the rate of return you expect to receive each year on your investment.
Time Period
The length of time your money will be invested. While typically measured in years, you can use any time unit (months, quarters, etc.) as long as it matches the compounding frequency of your interest rate.
Calculation Mode
Choose whether to calculate the time needed to double your investment at a given interest rate, or the interest rate required to double your money within a specific time frame.
Compounding Frequency
This calculator assumes interest compounds once per period. For annual calculations, interest compounds yearly; for monthly calculations, interest compounds monthly. All accrued interest is reinvested and earns additional interest over time.
Rule of 72 Formula
A simplified method to estimate investment doubling time: Divide 72 by your interest rate to find years needed to double, or divide 72 by your time period to find the required interest rate. The formula works as: Interest Rate × Time Period = 72.
Example Calculations
- At 6% annual interest: 72 ÷ 6 = 12 years to double your investment
- To double money in 10 years: 72 ÷ 10 = 7.2% annual interest rate required
- At 0.5% monthly interest: 72 ÷ 0.5 = 144 months (12 years) to double
References
Vaaler, Leslie Jane Federer; Daniel, James W. Mathematical Interest Theory (Second Edition), Washington DC: The Mathematical Association of America, 2009, page 75.
Weisstein, Eric W. "Rule of 72." From MathWorld--A Wolfram Web Resource, Rule of 72.