The rule of 72 works best for rates of about 6–10%. It also works for anything growing exponentially – like inflation: at 3% inflation prices double in about 24 years.
How to use the Rule of 72 Calculator
- Annual rate of return
- Or: years in which to double
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the annual rate of return (or growth) and, optionally, a target number of years to double.
Tips
- Start early – time does more of the work than the amount you save.
- Take the full employer match if your plan offers one.
- Plan with a cautious return (4–6% after inflation) and check again every year.
- Know the contribution limits and required minimum distribution rules for your accounts.
Common mistakes to avoid
- Ignoring inflation when setting a retirement target.
- Underestimating how long retirement can last.
- Withdrawing early and paying penalties and taxes.
Inputs explained
| Field | What to enter |
|---|---|
| Annual rate of return | In % |
| Or: years in which to double | A number |
What each result means
- Years to double (rule of 72) = 72÷[Annual rate of return]
- Exact years to double = log(2)÷log(1 + [Annual rate of return]÷100)
- Rule of 70 = 70÷[Annual rate of return]
- Rule of 69.3 (continuous) = 69.3÷[Annual rate of return]
- Rate needed to double in your years (rule of 72) = 72÷[Or: years in which to double]
- Exact rate to double = (2^(1÷[Or: years in which to double]) – 1)×100
Example values
How the main result changes when Annual rate of return changes and everything else stays at the example values above:
| Annual rate of return (%) | Years to double (rule of 72) |
|---|---|
| 4 | 18 |
| 6 | 12 |
| 8 | 9 |
| 10 | 7.2 |
| 12 | 6 |
| 16 | 4.5 |
Formula
Years to double ≈ 72 ÷ rate (%)Exact: years = ln 2 ÷ ln(1 + r)Rule of 69.3 is exact for continuous compoundingRate to double in t years ≈ 72 ÷ t; exact: 2^(1/t) − 1Worked example
Inputs — Annual rate of return: 8 %; Or: years in which to double: 10.
Results — Years to double (rule of 72) = 9; Exact years to double = 9.01; Rule of 70 = 8.75; Rule of 69.3 (continuous) = 8.66; Rate needed to double in your years (rule of 72) = 7.2 %; Exact rate to double = 7.177 %.
Frequently asked questions
What is the rule of 72?
A shortcut: 72 ÷ the annual rate ≈ years for money to double.
How accurate is it?
Within a few months for rates of 4–12%.
Can I use it for inflation?
Yes – 72 ÷ inflation rate is how long until prices double.
What formula does the Rule of 72 Calculator use?
Years to double ≈ 72 ÷ rate (%).