Rule of 72 Calculator

1. Enter either the annual rate or the years to double.
2. Leave the one you want to find blank.
3. Read the result from the rule of 72.
4. Compare it with the exact doubling time shown alongside.

Here is what each result means:

The rule of 72 is a quick mental shortcut for how long it takes an investment to double at a fixed annual rate. You simply divide 72 by the annual percentage rate, and the answer is the approximate number of years. At 8 percent, money doubles in about 72 divided by 8, which is 9 years.

The rule works in reverse too. Divide 72 by the number of years in which you want your money to double, and you get the annual rate you would need. To double your money in 10 years, you need about 72 divided by 10, which is 7.2 percent a year. This makes the rule handy for comparing savings rates and investments in your head.

It is an approximation of the maths of compound interest, which is why the number 72 is used rather than something exact. The true figure depends on the rate, but 72 is easy to divide and gives a close answer across the range of rates most people meet, so it has become a favourite rule of thumb in personal finance.

It divides 72 by whichever value you know to estimate the other.

1. To find the years to double, divide 72 by the annual rate.
2. To find the rate needed, divide 72 by the years.
3. Compare with the exact time from compound interest, shown alongside.

For the precise figure and full growth, see the compound interest calculator and the simple interest calculator.

You earn 8 percent a year on an investment.

Calculation: years = 72 / 8 = 9 years to double. The exact compound-interest answer is about 9.01 years, so the rule of 72 is very close. To double in 10 years instead, you would need 72 divided by 10, which is 7.2 percent.

One rule, used in either direction.

Both forms come from the same rule, so knowing either the rate or the target time gives the other. Because the numbers are small and 72 divides neatly by many of them, the rule of 72 is easy to use without a calculator, which is its main appeal.

The rule is an estimate, and its accuracy depends on the rate.

The rule of 72 is most accurate for rates between about 6 and 10 percent, where it is within a fraction of a year of the truth. At very low or very high rates it drifts a little, and some people switch to 70 or 69.3 for continuous compounding, but 72 stays the most popular for everyday use.

The Rate

A higher rate doubles money faster. Doubling the rate roughly halves the years needed.

Compounding

The rule assumes compound growth. More frequent compounding shortens the exact time slightly.

Fees and Inflation

The rule uses the nominal rate. Fees and inflation lower the real rate and lengthen the true doubling time.

Quick Estimates

Judge how fast an investment or savings rate doubles your money in your head.

Comparing Options

Turn different interest rates into doubling times to compare them at a glance.

Understanding Inflation

See how many years it takes for prices to double at a given inflation rate.

1. Using a Decimal Rate

Divide 72 by the rate as a whole percent, so use 8, not 0.08.

2. Expecting Exact Answers

The rule is an estimate. For precise figures use a compound interest calculator.

3. Applying It to Simple Interest

The rule assumes compounding. Simple interest doubles at a different, steady pace.

4. Ignoring Inflation

Doubling your money is less impressive if prices also double. Use the real rate for buying power.

5. Using It at Extreme Rates

The rule drifts at very low or very high rates, where the exact formula is better.

The rule is an approximation; this tool also shows the exact compound-interest doubling time for comparison.

The rule of 72 is a shortcut for how long money takes to double at a fixed rate. Divide 72 by the annual percentage rate to get the approximate number of years.

Divide 72 by the annual rate for the years to double, or divide 72 by the years for the rate needed. At 8 percent, money doubles in about 9 years.

It is very accurate for rates between about 6 and 10 percent, usually within a fraction of a year. At very low or high rates it drifts, so the exact formula is better there.

About 7.2 percent, found by dividing 72 by 10. This calculator shows it when you enter 10 years and leave the rate blank.

Because it approximates the compound-interest maths and divides neatly by many common rates. It gives a close answer across the range of rates most people use.

Yes. Divide 72 by the inflation rate to estimate how many years until prices double. At 3 percent inflation, prices double in about 24 years.

Compound interest. It assumes the money grows on itself each year. Simple interest doubles at a steady, different pace.

The exact time is the natural log of 2 divided by the natural log of one plus the rate. This calculator shows it beside the rule-of-72 estimate.

No. The calculation runs in your browser and nothing you enter is stored or sent anywhere, unless you choose Save, which keeps the result only on this device.

• Rule of 72 (Wikipedia).
• Compound interest (Wikipedia).
• Doubling time (Wikipedia).