Finance Shortcut

Rule of 72

The Rule of 72 is a fast estimate for how many years it takes an investment to double. Enter an annual return and compare the shortcut to the exact compounding math, plus rough tripling and quadrupling timelines.

Doubling timeline

The Rule of 72 is the shortcut. The exact result uses annual compounding math for a cleaner comparison.

Rule of 72 estimate 0 years A quick mental-math estimate for doubling time.

Exact doubling time 0 years Calculated with annual compounding.

Difference 0 years How close the shortcut is at this rate.

Rule-of-thumb check Good fit The Rule of 72 tends to work best in the mid-single-digit to low-teen range.

Growth milestones

These bars show rough timelines for common milestones using exact annual compounding at your selected rate.

Double your money 0 years

Triple your money 0 years

Quadruple your money 0 years

At 8.0%, money roughly doubles every 9 years. That means a long horizon matters. A small rate change can shave years off your doubling time.

Compare the shortcuts

Different rules are used for different back-of-the-envelope estimates. This gives you the quick mental-math reference points.

0 years Rule of 69.3 (continuous compounding style estimate)

0 years Rule of 70 (slightly conservative shortcut)

0 years Rule of 72 (popular balance between speed and accuracy)

Rule of 72

Rule of 72 tells you roughly how many years it takes to double your money at a given rate of return. Type a rate or drag the slider, with presets for typical savings, index and aggressive returns.

How to use it

Type an annual return rate, or drag the slider anywhere from 0.1% to 30%.

Use the presets for common cases: savings at 3%, index at 7%, aggressive at 10%.

Read the years to double.

Why 72, and where it stops working

Divide 72 by the annual percentage return and you get the approximate years to double. At 8% that is 9 years, and the true answer from the compounding formula is 9.01, so the shortcut is remarkably close.

The approximation is most accurate between about 6% and 10%. It drifts outside that band: at 2% the rule says 36 years and the real figure is 35, while at 20% the rule says 3.6 years and the real figure is closer to 3.8. It is a mental-arithmetic tool, not a precise calculation.

Worked example

Three common return rates:

Savings at 3%72 / 3 = 24 years
Index at 7%72 / 7 = about 10.3 years
Aggressive at 10%72 / 10 = 7.2 years

Result: The gap between 3% and 7% is not twice as fast, it is more than twice, which is the intuition the rule is good at delivering.

When it helps

Getting a fast sense of a return rate without a calculator.

Comparing the long-run impact of two rates in your head.

Understanding what inflation does, by applying the same rule to how fast prices double.

Explaining compounding to someone quickly.

Common mistakes

Using it for precise planning. It is an approximation and drifts at the extremes.

Applying it to volatile returns as though they were steady. Markets do not deliver the average every year.

Forgetting it works on inflation too. At 3% inflation, prices double in 24 years.

What this tool handles

A rule of thumb, not financial advice or a guarantee of any return.