Rule of 72 Calculator

Estimate how long it takes an investment to double.

Years to Double (Rule of 72)

10.0 yrs

Exact Doubling Time

10.0 yrs

Implied Rate

7.20%

Doubles To

$20,000

Value

Estimates only, not financial, tax, or legal advice. See our Terms and Privacy Policy.

The Rule of 72 is a mental-math shortcut for compound growth: divide 72 by your annual rate of return and you get a close estimate of how many years it takes money to double. It turns an exponential calculation into simple division you can do in your head, which makes it a favorite for quick sanity checks.

Want the exact growth curve instead of just the doubling estimate? Our compound interest calculator shows the full year-by-year balance, contributions included.

This calculator shows both the Rule-of-72 estimate and the exact doubling time, so you can see just how good the approximation is at your rate.

How does this calculator work?

Enter your expected annual return. Dividing 72 by that rate estimates how many years it takes an investment to double.

The calculator also shows the exact doubling time from compound growth, so you can see how good the approximation is.

Worked example

Compare the Rule-of-72 estimate to the exact doubling time at a few common return rates.

At 6%: rule estimate: 12.0 yrs (exact 11.9)
At 8%: rule estimate: 9.0 yrs (exact 9.0)
At 10%: rule estimate: 7.2 yrs (exact 7.3)
At 2%: rule estimate: 36 yrs (exact 35.0)

How the numbers work

Each estimate is just 72 divided by the rate: 72 ÷ 6 = 12 years, 72 ÷ 8 = 9, 72 ÷ 10 = 7.2, 72 ÷ 2 = 36.

The exact doubling time comes from logarithms: ln(2) ÷ ln(1 + rate), which gives 11.9, 9.0, 7.3, and 35.0 years respectively.

The two line up almost perfectly through the middle of the range and drift apart only at the extremes, which is why the rule is trusted for quick mental estimates between about 6% and 10%.

Between roughly 6% and 10% the Rule of 72 is almost exact. It drifts a little at very low or very high rates; at 2% it slightly overestimates, but for everyday planning it's remarkably accurate.

The same rule works in reverse: divide 72 by a doubling time to back out the implied return, or apply it to inflation to see how fast prices erode your money's value.

Rule of 72 Calculator glossary

Interest Rate: Your expected annual rate of return.
Years to Double: The Rule-of-72 estimate of how long it takes the investment to double.
Implied Rate: Working backward from a number of years, the return that would double your money in that time.
Doubles To: What your entered dollar amount grows to after doubling.
Compound Growth: Growth that earns returns on prior returns, producing the exponential doubling the rule approximates.
Rule of 70 / 69.3: Variants of the same shortcut; 69.3 is the mathematically exact constant, while 72 is used because it divides evenly.
Inflation Doubling: Applying the rule to an inflation rate shows how fast prices double, or how fast money loses half its value.

Rule of 72 Calculator FAQs

How accurate is the Rule of 72?+

Very accurate for rates between about 6% and 10%. At higher or lower rates it drifts slightly from the exact compound-growth figure.

Why 72?+

72 has many small divisors (2, 3, 4, 6, 8, 9, 12), making the mental math easy, and it closely matches the exact doubling formula in the common rate range.