What is the rule of 72?

The rule of 72 estimates how long it takes money to double by dividing 72 by the annual return rate. At 6%, your money doubles in about 12 years.

How accurate is the rule of 72?

It is a close approximation for moderate rates, most accurate around 6% to 10%. At very low or very high rates it drifts, but it is excellent for quick mental math.

Can I use the rule of 72 for debt?

Yes, in reverse: divide 72 by your debt's interest rate to see how fast the balance doubles if unpaid. A 24% rate doubles debt in about three years, showing the cost of carrying balances.

The Rule of 72 Explained

Key takeaway

The Rule of 72 makes compounding tangible—the real money is made in the later doublings, not the early ones.

The Rule of 72 estimates how long it takes money to double

The Rule of 72 is a quick mental math shortcut to estimate how long it will take for your money to double at a given rate of return. You divide 72 by the annual interest rate or rate of return, and the result is the approximate number of years it will take for your money to double.

It's not perfect, but it's remarkably accurate for most reasonable rates of return, and it works in reverse too—you can use it to estimate what rate of return you need to double your money in a certain number of years.

How the Rule of 72 works

The basic formula is simple:

Years to double = 72 / Annual rate of return

For example:

At 6% return: 72 / 6 = 12 years to double
At 8% return: 72 / 8 = 9 years to double
At 10% return: 72 / 10 = 7.2 years to double
At 12% return: 72 / 12 = 6 years to double

You can also reverse it to find the rate you need:

Required rate of return = 72 / Years to double

If you want your money to double in 8 years: 72 / 8 = 9% annual return needed.

Why it works (and when it's most accurate)

The Rule of 72 is a simplification of the natural logarithm formula for compound interest. It's most accurate for rates between 6% and 10%, which covers the typical range of expected returns for many investments.

For rates outside that range, you can adjust slightly:

For rates below 6%, subtract 1 from 72 for every 3 percentage points below 6%
For rates above 10%, add 1 to 72 for every 3 percentage points above 10%

But for most everyday estimates, the plain Rule of 72 works well enough to be useful.

The Rule of 72 shows the power of compounding

The Rule of 72 makes the power of compounding tangible. It helps you see:

Why time matters more than rate: Money doubling every 9 years at 8% is powerful not because 8% is an amazing return, but because it can double three or four times over a 30-year period.
Why small differences in rate add up: A 2% difference in return (6% vs 8%) means a full extra doubling over 30 years.
Why starting early beats starting late: Starting at 25 vs 35 can mean one or two extra doublings by retirement.

The rule turns abstract percentages into concrete timelines you can actually visualize.

The Rule of 72 applied to investing

For investors, the Rule of 72 helps answer common questions:

"If I invest $10,000 at 7%, when will it become $20,000?" About 10.3 years.
"What return do I need to turn $50,000 into $100,000 in 10 years?" About 7.2%.
"If my portfolio averages 8%, how many times will it double in 30 years?" About 3.3 times—turning $10,000 into roughly $100,000.

Remember that the rule applies to the overall growth rate, not just the price return of an investment. For stocks, that means including dividends; for bonds, including interest.

The Rule of 72 applied to debt

The Rule of 72 works against you when you're in debt, because it shows how quickly debt can double if you're not paying it down:

A credit card at 24% APR: 72 / 24 = 3 years for the balance to double (if you make no payments)
A personal loan at 12% APR: 72 / 12 = 6 years for the balance to double
Even a car loan at 6% APR: 72 / 6 = 12 years for the balance to double

This is why high-interest debt is so dangerous—left unchecked, it can grow faster than your ability to pay it off.

The Rule of 72 applied to inflation

You can also use the Rule of 72 to estimate how long it will take for inflation to cut your purchasing power in half:

At 2% inflation: 72 / 2 = 36 years for prices to double
At 3% inflation: 72 / 3 = 24 years for prices to double
At 4% inflation: 72 / 4 = 18 years for prices to double

This is why investing for growth matters even if you're not trying to get rich—it helps your money keep up with or beat inflation over time.

What the Rule of 72 doesn't tell you

The Rule of 72 is a useful estimate, but it has limits:

It assumes a constant rate of return, which real investments rarely provide. Markets go up and down from year to year.
It doesn't account for taxes, fees, or inflation (unless you use it specifically for inflation as above).
It's an estimate, not a guarantee. Actual results may vary.

The rule is best used for quick mental checks and setting expectations, not for precise financial planning.

The Rule of 72 helps you think in decades, not days

The biggest value of the Rule of 72 is that it helps you think about money in the long term. It shows why consistent, patient investing usually beats trying to chase quick returns—because the real money is made in the later doublings, not the early ones.

When you understand that money can double every 7-10 years at reasonable rates of return, you start to see why starting early and staying invested is more powerful than any hot stock tip.

Use the Rule of 72 Calculator to explore different rates and timelines, then try the Compound Interest Calculator to see the full effect of compounding beyond just doubling. If you're planning for retirement, check the Retirement Calculator to see how multiple doublings fit into your long-term plan.