What is the Rule of 72?

The Rule of 72 is a shortcut for estimating how long an investment takes to double: divide 72 by the annual rate of return, and the result is the approximate number of years. At an 8% return, 72 / 8 gives about 9 years to double your money.

The appeal is that you can do it in your head. There’s no need for a compound interest formula or a spreadsheet to get a usable answer, which is why the rule shows up so often in personal finance conversations, investing books, and quick back-of-the-envelope planning.

The Rule of 72 Formula

The formula is a single division:

Years to double = 72 / annual rate of return

Use the whole number for the rate, not the decimal. For an 8% return you divide by 8, not 0.08. You can also run it in reverse: if you want your money to double in a set number of years, divide 72 by that number to find the return you’d need. Doubling in 6 years, for example, requires 72 / 6 = 12% per year.

Rule of 72 Examples

The math is easiest to see across a few common return rates:

• At 6%, your money doubles in 72 / 6 = 12 years.
• At 8%, it doubles in 72 / 8 = 9 years.
• At 10%, it doubles in 72 / 10 = about 7.2 years.

So $10,000 growing at 8% becomes roughly $20,000 after 9 years, then about $40,000 after 18 years, and $80,000 after 27 years. Each doubling stacks on the last, which is what makes compound growth feel slow at first and dramatic later.

Rate of Return and Years to Double

This table shows the estimate across a range of returns:

The pattern is worth internalizing: small differences in return compound into large differences in doubling time. A portfolio earning 10% doubles nearly twice as fast as one earning 6%, which over a multi-decade horizon can mean an extra doubling or two.

Why 72 Works

The exact time to double comes from logarithms: it’s the natural log of 2 (about 0.693) divided by the return, which works out to roughly 69.3 divided by the rate expressed as a percentage. So the mathematically precise number is closer to 69.3 than 72.

72 gets used instead because it’s a friendlier number to divide in your head. It splits cleanly by 2, 3, 4, 6, 8, 9, and 12, which covers most of the return rates people actually care about. The small overshoot from the true value also happens to make the estimate more accurate in the range where most investors operate, which is the next point.

Limitations to Keep in Mind

The Rule of 72 assumes a single, steady rate of return, which real markets don’t deliver. Actual returns arrive as a jagged sequence of up and down years, so the doubling time you experience can vary even when the long-run average matches your assumption.

A few things the rule quietly ignores:

• Volatility. The estimate uses one smooth rate. A portfolio that averages 8% through a rough sequence of returns may double on a different timeline than the formula implies.
• Taxes and fees. Doubling your pre-tax balance isn’t the same as doubling your spendable money. Investment fees and taxes on gains both drag on the rate that actually compounds for you.
• Inflation. A nominal doubling doesn’t mean your purchasing power doubled. To estimate how long it takes to double in real terms, run the rule on your real rate of return (your nominal return minus inflation) rather than the nominal interest rate. At a 7% nominal return with 3% inflation, use 4%, so real doubling takes about 18 years, not the roughly 10 years the nominal rate suggests.

For anything beyond a mental estimate, it’s worth checking the number against an actual projection. You can model the compound growth of your specific accounts, contributions, and return assumptions in ProjectionLab rather than relying on a single-rate shortcut, which also lets you factor in the volatility, taxes, and inflation the rule leaves out.

Related Rules of Thumb

The Rule of 72 has a family of cousins built on the same doubling math, each tuned for a different situation:

• Rule of 70 and Rule of 69.3. Both estimate doubling time more precisely at lower rates. The Rule of 69.3 is closest to the true logarithmic value and is often used for continuous compounding, where interest is credited constantly rather than in discrete periods.
• Rule of 114. Divide 114 by the rate to estimate how long it takes to triple your money.
• Rule of 144. Divide 144 by the rate to estimate how long it takes to quadruple (two doublings).

These follow the same idea: pick a numerator that matches the growth multiple you’re after, then divide by the return.

Frequently Asked Questions

How does the Rule of 72 work? Divide 72 by your annual rate of return, and the answer is the approximate number of years for your investment to double. At 9% a year, 72 / 9 = 8 years. It works because it approximates the logarithmic math behind compound growth, using a number (72) that’s easy to divide in your head.

What is the Rule of 72 formula? Years to double = 72 / annual rate of return, using the whole number for the rate (divide by 8 for an 8% return, not 0.08). Run it in reverse to find the return you’d need for a target doubling period: divide 72 by the number of years.

How accurate is the Rule of 72? Close enough for mental math. It lands within a fraction of a year of the true doubling time for returns in the 6% to 10% range, and only loses a little precision at very low or very high rates.

What is the Rule of 72 vs the Rule of 70? Both estimate doubling time, but the Rule of 70 is more accurate at lower rates and the Rule of 72 is easier to compute because 72 divides cleanly by more numbers. The mathematically exact value is closest to 69.3, so the Rule of 70 sits nearer the true answer while the Rule of 72 trades a little precision for mental-math convenience.