How does the Rule of 72 work?

Divide 72 by your annual growth rate to estimate the years it takes for money to double. For example, at 8% a year, 72 ÷ 8 ≈ 9 years.

How accurate is the Rule of 72?

It is a close approximation for rates roughly between 6% and 10%. For very high or very low rates, exact compounding (using logarithms) gives a slightly different answer.

Can I use the Rule of 72 in reverse?

Yes. Divide 72 by the number of years you want to double in to find the rate you would need. To double in 6 years, you need about 72 ÷ 6 = 12% a year. This calculator does both directions.

Does it work for inflation as well as investments?

Yes. The same shortcut estimates how long it takes prices to double at a given inflation rate, or how long until savings lose half their buying power. At 3% inflation, prices roughly double in 24 years.

Why 72 and not another number?

72 is chosen because it divides cleanly by many common rates (2, 3, 4, 6, 8, 9, 12) and is close to the exact value (about 69.3) for the rates people use most. Some people use 70 or 69 for continuous compounding.

How do I get the exact doubling time?

The exact figure is ln(2) ÷ ln(1 + rate). At 8% that is 0.693 ÷ 0.0770 ≈ 9.0 years, almost identical to the Rule of 72 estimate of 9 years.

Is anything I enter uploaded?

No — the calculation runs entirely in your browser and nothing you type leaves your device.

Rule of 72 Calculator

Rule of 72 calculator

The Rule of 72 is a quick mental shortcut for compounding: divide 72 by an annual growth rate to estimate how many years money takes to double. This calculator runs it both ways — enter a rate to get the doubling time, or enter a target number of years to find the rate you would need. It is handy for sizing up investment growth, inflation or any figure that compounds, without reaching for a spreadsheet.

How it works

The tool applies two arrangements of the same rule:

Years to double = 72 ÷ rate
Rate needed = 72 ÷ years

It is an approximation of the exact formula ln(2) ÷ ln(1 + rate), and is most accurate for rates roughly between 6% and 10%. For very high or very low rates the exact logarithmic answer drifts a little from the estimate.

Example

At an 8% annual return, money doubles in about 72 ÷ 8 = 9 years — almost exactly the precise answer of 9.01 years. Working backwards, to double in 6 years you would need about 72 ÷ 6 = 12% per year.

Annual rate	Rule of 72	Exact doubling time
3%	24.0 yr	23.4 yr
6%	12.0 yr	11.9 yr
8%	9.0 yr	9.0 yr
10%	7.2 yr	7.3 yr
12%	6.0 yr	6.1 yr

Everything runs in your browser, so nothing you enter is uploaded.