Rule of 72 Calculator

What Is the Rule of 72 Calculator?

The Rule of 72 is a widely used mental-math shortcut for estimating how long it takes an investment to double at a given compound annual rate, without needing a full compound-interest calculation. Simply divide 72 by the rate.

How to Use This Calculator

Enter your expected annual rate of return.

The Formula

Worked Example

Important Notes

How Accurate Is the Shortcut?

The exact doubling time comes from dividing the natural log of 2 by the natural log of (1 + rate). Comparing that with 72 divided by the rate shows where the shortcut is strong and where it drifts. At 8% the rule gives 9 years and the exact answer is about 9.01 years, which is nearly identical. At 2% the rule gives 36 years versus about 35.0 years exact. At 20% the rule gives 3.6 years, while the exact figure is about 3.8. At very high rates such as 40%, the rule says 1.8 years but the exact value is about 2.06, so the gap widens. For rates roughly between 6% and 10%, it is a dependable estimate.

Handy Mental Math

• Doubling twice quadruples your money. At 6%, that is about 24 years for a fourfold increase.
• Work backwards: if you want money to double in 8 years, you need about 9% per year (72 divided by 8).
• Estimate tripling with a similar rule of thumb around 115 divided by the rate, though this is less widely used and even rougher.

Using It for Debt and Inflation

Compounding works against you on debt. A credit card balance at 24% with no payments roughly doubles in 3 years (72 divided by 24). This is a strong reminder of why high-rate balances deserve priority. For inflation, at an assumed 3% per year, prices double in about 24 years, so what costs $100 now could cost about $200 then, and money sitting idle loses about half its purchasing power over that time. These figures are only illustrations, and actual inflation varies by year.

Limits and Common Mistakes

• Enter the rate as a whole percentage, for example 8, not 0.08.
• The rule assumes a steady rate compounded once per year with no deposits, withdrawals, fees or taxes, which real accounts rarely match.
• It shows the time to double, not a guaranteed result, since market returns are uncertain and vary from year to year.
• It should not replace an exact calculation when you are making a decision involving large sums.

The rule is best used for quick comparisons, such as judging whether a 4% account and a 7% option make a meaningful difference over a working lifetime.

A Quick Sanity Check

Multiply the result by the rate to confirm you get roughly 72. Then compare with the Compound Interest Calculator: enter a principal of 1,000, the same rate, and the estimated years, and the final value should land near 2,000. If it is far off, check whether you entered the rate as a whole percentage.

Frequently Asked Questions