Rule of 72 Calculator
Use this free Rule of 72 Calculator to quickly get accurate results in seconds, free and instantly offline with no signup required ever, trusted by professionals in Austin, Chicago, Miami, Denver, Seattle, and New York for reliable daily use without accounts, subscriptions, limits, or delays on any device for everyday personal and business workflows daily.
Estimate how long it takes money to double at any annual return using the Rule of 72. Instant, accurate results with the exact doubling formula.
Calculator Inputs
About Rule of 72
Use this free Rule of 72 Calculator to quickly get accurate results in seconds, free and instantly offline with no signup required ever, trusted by professionals in Austin, Chicago, Miami, Denver, Seattle, and New York for reliable daily use without accounts, subscriptions, limits, or delays on any device for everyday personal and business workflows daily.
The Rule of 72 is a mental math shortcut that estimates how many years it takes an investment to double when compounding at a fixed annual rate. Divide 72 by the annual rate in percent: an investment earning 8% per year doubles in roughly 72 ÷ 8 = 9 years, and one earning 6% doubles in about 12 years. For a few seconds of arithmetic it gives you a surprisingly good sense of how compounding flows through time.
The number 72 is chosen because it is easy to divide and produces exact estimates in the 6% to 10% range where most long-term investments sit. The precise doubling time is ln(2) ÷ ln(1 + r/100), where r is the annual rate as a percent. At 8%, the exact answer is 9.01 years versus the 9.00 years the rule predicts, which is why the estimate is so reliable in that range. The same logic extends to other multiples: dividing by 115 estimates years to triple, and dividing by 144 estimates years to quadruple.
The Rule of 72 assumes a steady compound growth rate and ignores taxes, fees, and withdrawals, so treat it as a quick planning estimate rather than a precise projection. It works best for positive rates between about 1% and 30%, and it remains one of the most useful mental tools in personal finance for turning a single percentage into a time horizon.
Where it is used
Investors planning retirement and savings goals, financial advisors explaining expected returns, and anyone comparing savings accounts, bonds, or index funds use the Rule of 72 Calculator. It is also a fast way to understand how inflation erodes purchasing power by estimating how many years it takes prices to double at a given inflation rate.
How to use the Rule of 72
- Annual Return Rate — enter the value in % (e.g. 8).
- Calculate — press the Calculate button to see the result instantly above.
Formula
Rule of 72: Years to double ≈ 72 / annual rate. Rule of 115: Years to triple ≈ 115 / annual rate. Exact: ln(2) / ln(1 + r/100)
Examples
Double your money at 8% per year
Growth fund at 12% per year
Frequently Asked Questions
- What does the Rule of 72 tell you?
- It estimates how many years an investment takes to double at a fixed annual compounding rate. Divide 72 by the rate as a percent: at 8% a portfolio doubles in about 9 years, at 6% in about 12 years. It converts a return percentage into a time horizon in seconds.
- Is the Rule of 72 exact?
- It is an approximation. The exact doubling time is ln(2) divided by ln(1 + r/100). At 8% the rule gives 9.00 years while the exact answer is 9.01 years, which is why it is so useful near typical investment returns. For continuous compounding, dividing 69.3 by the rate is more accurate.
- Can the Rule of 72 be applied to inflation?
- Yes. If inflation runs at 3% per year, purchasing power halves in about 72 ÷ 3 = 24 years. It is a handy way to put long-term inflation into perspective when planning retirement spending.
- Are there similar rules for tripling or quadrupling money?
- The Rule of 115 estimates years to triple the investment, and the Rule of 144 estimates years to quadruple it. Both follow the same logic as the Rule of 72 and have the same approximate accuracy at typical return rates.
- Does the Rule of 72 work for very high or low rates?
- It is most accurate for positive rates between about 1% and 30%. At very low rates the error grows, and at very high rates the estimate undershoots slightly, but it remains a reasonable planning guide. For any rate it is safer to rely on the exact formula this calculator reports alongside the estimate.