Rule of 72
Divide 72 by the rate for a quick doubling time: 12 years at 6%, 8 years at 9%.
Your numbers
Years to double
10.3yr
Result
10.24yr
Formula
Years to double ≈ 72 ÷ Annual rate
The shortcut is most accurate between about 6% and 10%. The exact figure is shown alongside so you can see the gap.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Annual rate (%)Interest quoted per year; the monthly rate is this over 12. | 7 | 0.1 and up |
Step by step
Quick reference table
Results when only Annual rate (%) changes and everything else stays put.
| Annual rate (%) | Years to double (yr) | Result (yr) |
|---|---|---|
| 3.5 | 20.6 | 20.15 |
| 5.25 | 13.7 | 13.55 |
| 7 | 10.3 | 10.24 |
| 10.5 | 6.9 | 6.94 |
| 14 | 5.1 | 5.29 |
What each result means
| Result | At default values |
|---|---|
| Years to double (yr)Years for the principal to double if the rate holds. | 10.3 |
| Result (yr)What the formula gives back — the answer this page is for. | 10.24 |
Common mistakes
The shortcut is most accurate between about 6% and 10%. The exact figure is shown alongside so you can see the gap.
Glossary
- Annual rate
- Interest quoted per year; the monthly rate is this over 12.
- Years to double
- Years for the principal to double if the rate holds.
- Result
- What the formula gives back — the answer this page is for.
Frequently asked questions
Q. How is Rule of 72 calculated?
Years to double ≈ 72 ÷ Annual rate — Divide 72 by the rate for a quick doubling time: 12 years at 6%, 8 years at 9%.
Q. Can you walk through an example?
With Annual rate 7%, the answer is Years to double 10.3yr.
Q. What do I need to enter?
Enter Annual rate. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Annual rate (%) moves the answer to Annual rate (%) 3.5 → Years to double (yr) 20.6 and Annual rate (%) 14 → Years to double (yr) 5.1. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
The shortcut is most accurate between about 6% and 10%. The exact figure is shown alongside so you can see the gap.
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