Rule of 72 Calculator
Estimate how long it takes an investment to double using the Rule of 72, or find the rate needed to double your money in a target number of years.
Rate → Years to Double
%
Years to Double
9.0
Years → Required Rate
Yr
Required Annual Rate
8.00%
How the Rule of 72 Works
The Rule of 72 is a quick mental-math shortcut for estimating how long it takes an investment to double at a fixed annual rate of return, or the reverse — what rate you'd need to double your money in a target number of years. Divide 72 by the annual interest rate to get the years to double; divide 72 by the years available to get the required rate.
Required Rate = 72 ÷ Years to Double
Example
At an 8% annual return, an investment doubles in about 72 ÷ 8 = 9 years. Conversely, if you want your money to double in exactly 9 years, you'd need an annual return of about 72 ÷ 9 = 8%.
Common Use Cases
- Quickly estimating how long savings or investments take to double.
- Comparing the growth speed of different investment options without a calculator handy.
- Setting a target return rate needed to hit a doubling goal by a certain age.
- Understanding how compounding interest accelerates wealth growth over time.
FAQs
How accurate is the Rule of 72?
It's a close approximation for annual rates roughly between 6% and 10%, with the exact doubling time given by ln(2) / ln(1 + r). Outside that range, the estimate drifts slightly, but it's accurate enough for quick mental math in almost all practical cases.
Does the Rule of 72 assume compounding?
Yes — it assumes the return compounds annually. It does not apply cleanly to simple (non-compounding) interest, where growth is linear rather than exponential.
Can I use it for inflation too?
Yes — the same shortcut estimates how long it takes prices to double at a given inflation rate. For example, at 3% annual inflation, prices roughly double every 24 years.