The Rule of 72: A Quick Way to Estimate Doubling Time
The rule of 72 turns one division into a doubling-time estimate. Here is why it works and how to use it.
What is the Rule of 72 Calculator?
This calculator uses the rule of 72, a handy shortcut for doubling time: years to double = 72 divided by the annual interest rate in percent. It is an approximation that works well for typical rates, and it also shows the same answer in months. A higher rate means a shorter doubling time.
How to use it
- Enter the annual interest rate as a percentage.
- Press Calculate to see the estimated years to double.
- Try different rates to see how doubling time changes.
Worked example
At an interest rate of 8% per year, the tool gives 9 years to double, which is 108 months.
How long does it take money to double at 8%?
Divide 72 by the rate: 72 divided by 8 equals 9 years. That is the rule of 72 estimate, and it is close to the exact answer for annual compounding. At higher rates the doubling time shrinks, and at lower rates it stretches out, which is why starting early matters so much.
Frequently asked questions
Why 72?
Because 72 is easy to divide by many common rates, and it gives a close approximation to the exact doubling time for rates in the usual range.
Is the answer exact?
No, it is an estimate. The exact doubling time depends on compounding, but the rule of 72 is close enough for quick planning at ordinary rates.