Rule of 72 Calculator
Estimate doubling time from a rate and compare it with exact compounding
Rule of 72 estimate
How we calculated it
About the Rule of 72 Calculator
The Rule of 72 is a quick mental estimate of how long a value may take to double under a constant compound growth rate. It is commonly used for savings, investment returns and inflation because it gives an intuitive approximation without a full logarithmic calculation.
The rule is most accurate across a moderate range of annual rates. At very low or very high rates, the exact doubling-time formula can differ more noticeably.
How to Use the Rule of 72 Calculator
- Enter the annual percentage growth or interest rate.
- Calculate approximate doubling time as 72 divided by the rate.
- If precision matters, compare the estimate with the exact logarithmic doubling time.
- Keep the growth rate on a consistent annual compounding basis.
How the Calculation Works
Rule-of-72 doubling time ≈ 72 ÷ annual percentage rate.
The exact compound doubling time is ln(2) ÷ ln(1+r), where r is the annual rate as a decimal. The number 72 is convenient because it has many divisors and approximates the logarithmic relationship well at common rates.
Worked example
Rule of 72 Calculator Worked Example
8% annual growth
Rule of 72 estimate = 72÷8 = 9 years. The exact annual-compounding doubling time is ln2÷ln1.08 ≈ 9.01 years.
Result guide
Understanding Your Rule of 72 Calculator Result
The rule gives a fast planning estimate, not a guaranteed investment outcome. Real returns vary and may not compound at one stable rate every year.
Before you rely on the answer
Assumptions and Limitations
- Rate is positive and expressed as an annual percentage.
- Growth compounds in a way close enough to the rule’s assumption.
- Fees, taxes and changing returns are excluded.
Practical tip
Using This Result in Practice
Use the rule for rough comparisons, then switch to a compound-interest calculation when actual dollars, irregular contributions or precise timing matter.
Related calculations
Useful Calculations to Continue With
If you want an exact projected balance rather than a mental doubling estimate, use the Compound Interest Calculator.
If you have beginning and ending investment values and need the implied yearly rate, use the Annualized Return Calculator.
Common questions
Rule of 72 Calculator FAQ
Why is it called the Rule of 72?
Dividing 72 by a common annual percentage rate gives a convenient approximation of doubling time.
How long to double at 6%?
About 12 years by the Rule of 72.
Is the Rule of 72 exact?
No. It is an approximation, though it is quite close at many common rates.