Rule of 72 Calculator

Estimate how many years it takes to double money at a given rate with the Rule of 72, and compare it with the exact doubling time.

Estimate years required to double an investment using the Rule of 72.

Interest rate

Years to double (Rule of 72)

9 years

Exact years to double

9.01 years

Results are estimates for informational purposes only.

How to use this calculator

How the Rule of 72 works

The Rule of 72 is a shortcut for estimating how long money takes to double at a fixed annual rate: divide 72 by the rate. At 8% a year, money doubles in about 72 ÷ 8 = 9 years. The calculator also shows the exact doubling time so you can see how close the shortcut is.

The rule works because doubling time depends on the natural logarithm of 2, about 0.693. For typical rates, 72 approximates that math well and is easy to divide, since it divides evenly by 2, 3, 4, 6, 8, 9, and 12.

How accurate is it?

The shortcut is most accurate around 8%, where it is off by only a few days. At lower rates it slightly overestimates the time: at 2% it says 36 years versus an exact 35. At high rates it underestimates: at 20% it says 3.6 years versus an exact 3.8. For quick decisions, it is almost always close enough.

Variations exist. The Rule of 70 is slightly better for low rates, and the Rule of 69.3 is exact for continuous compounding. The Rule of 72 stays the most popular because it is easy to do in your head.

Practical uses

Investments: at a 6% return, money doubles about every 12 years, so $10,000 invested at age 30 could double three times, to about $80,000, by 66.

Inflation: at 3% inflation, prices double in about 24 years, which means a dollar’s purchasing power halves over that time.

Debt: an unpaid credit card balance at 24% APR doubles in about 3 years, which is why high-interest debt is usually the first thing worth paying down.

You can also review Compound Interest Calculator and Investment Growth Calculator to compare scenarios.

Formula

Years to double ≈ 72 ÷ annual rate (in percent)
Exact years to double = ln(2) ÷ ln(1 + rate ÷ 100)
Rate needed to double in a given time ≈ 72 ÷ years

Example: 8% a year

Rule of 72: 72 ÷ 8 = 9 years.
Exact: ln(2) ÷ ln(1.08) ≈ 9.01 years.
The estimate is off by only a few days.
To double in 6 years instead, you would need about 72 ÷ 6 = 12% a year.

Doubling time by rate

Rule of 72 estimate compared with the exact doubling time, compounded once a year.

Interest rate	Years to double (Rule of 72)	Exact years to double
2%	36 years	35 years
3%	24 years	23.45 years
4%	18 years	17.67 years
5%	14.4 years	14.21 years
6%	12 years	11.9 years
7%	10.29 years	10.24 years
8%	9 years	9.01 years
9%	8 years	8.04 years
10%	7.2 years	7.27 years
12%	6 years	6.12 years
15%	4.8 years	4.96 years
20%	3.6 years	3.8 years

Frequently asked questions

How long does it take to double money at 10%?

About 7.2 years by the Rule of 72. The exact figure with annual compounding is 7.27 years.

Does the Rule of 72 work for inflation?

Yes. Divide 72 by the inflation rate to estimate how long prices take to double, which is also how long it takes your money’s purchasing power to fall by half. At 3% inflation, that is about 24 years.

Why 72 and not 70?

Both are approximations. 72 is slightly more accurate for rates in the common 6% to 10% range with annual compounding, and it divides evenly by many numbers. 70 works a little better for low rates.

Can I use it to find the rate I need?

Yes. Divide 72 by the number of years instead. To double your money in 10 years, you need roughly 7.2% a year.