Understanding Rule of 72
How we calculate. Approximate years to double = 72 ÷ annual percent return. See our methodology and accuracy policy .
Real-world scenario: Expected return 8% per year. Use those inputs in the Rule of 72 form to verify the on-page formula.
What is Rule of 72?
A quick mental-math approximation for compound growth. It is less accurate at very high or very low rates than the exact log formula.
- Assumes steady annual compounding
- Ignores taxes and fees
- Use exact CAGR math for precision
The Formula
Worked Example
Common Use Cases
- Investor education: compounding intuition
- Goal planning: rough timelines
- Compare rates: 6% vs 9% doubling speed
Pro Tips
- Best near mid-single to low-double-digit returns
- Inflation shortens real doubling
- Fees reduce effective return
| Scenario | When to use |
|---|---|
| Percent of a number | Finding a part of a whole (tax, tip, score) |
| Percent change | Comparing old vs new values |
Limitations: Rule of 72 results are educational finance aids—not loan offers, investment advice, or tax counsel. Confirm figures with a qualified professional and your contract.
Common mistakes
- Swapping part and whole: The denominator must be the full total, not a subset.
- Rounding too early: Carry extra decimal places through multi-step work before rounding the final percent.
- Mixing percent and decimal forms: Enter rates in the format the calculator labels expect.
The worked example for rule of 72 was verified with the calculator form on this page—enter the same values to confirm the result.
When to use this calculator
- Use this page for rule of 72 when you are comparing gain or return to money invested.
- Use the margin calculator when profit should be a percent of selling price, not of capital invested.
- Use the discount calculator when you only need a sale price from list × percent off.
- After ROI, compare borrowing cost with the loan interest calculator or multi-year growth with the CAGR calculator.
Still unsure about rule of 72? Start with the quick answer above, then open the linked calculator that matches your wording.
❓ Frequently Asked Questions
How do I use this rule of 72 calculator?
For rule of 72, enter the values labeled on the form, then press Calculate. Compare the result to the worked example on this page.
What formula does this page use?
See the quick answer and formula box above for rule of 72—the same percentage calculator expression is applied to your inputs.
What if my result looks wrong?
When checking rule of 72, re-check part vs whole (or rate vs base), rounding, and whether percents were entered as 25 rather than 0.25 when the form expects percents.
Authoritative References
For lending and investing concepts, consult:
- CFPB — consumer lending and mortgage education
- Investopedia — finance formula explainers
- SEC Investor.gov — investor education