Rule of 72 Calculator
Estimate how long it takes to double your money at any interest rate.
Rule of 72 vs Exact Calculation
| Annual Rate | Rule of 72 (years) | Exact (years) | Difference |
|---|---|---|---|
| 2% | 36.0 | 35.00 | 1.00 yrs |
| 4% | 18.0 | 17.67 | 0.33 yrs |
| 6% | 12.0 | 11.90 | 0.10 yrs |
| 8% | 9.0 | 9.01 | -0.01 yrs |
| 10% | 7.2 | 7.27 | -0.07 yrs |
| 12% | 6.0 | 6.12 | -0.12 yrs |
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Request a ToolHow to Use the Rule of 72 Calculator
This calculator has two modes. Choose the one that fits your question:
- Years to Double. Enter the annual interest rate or expected return on your investment. The calculator instantly shows how many years it will take for your money to double. For example, at 7% annual return, the Rule of 72 estimates roughly 10.3 years to double.
- Rate Needed. Enter the number of years in which you want your money to double. The calculator shows what annual return you would need. For example, if you want to double your money in 10 years, you need roughly a 7.2% annual return.
Both modes show the Rule of 72 estimate alongside the exact mathematical result so you can see how close the approximation is. Results update instantly as you type. Use the Share button to send a pre-filled link, or Copy to grab the result.
About the Rule of 72
The Rule of 72 is a simple mental math shortcut for estimating how long it takes an investment to double at a given compound interest rate. Divide 72 by the annual rate, and you get the approximate number of years. The rule works best for rates between 2% and 15%, where it closely matches the exact formula: years = ln(2) / ln(1 + r).
The number 72 was chosen because it has many factors (1, 2, 3, 4, 6, 8, 9, 12), making the mental math easy. At very high or very low rates the estimate drifts from the exact answer, but for typical investment returns it is remarkably accurate. All calculations run entirely in your browser. No data is sent anywhere.
Frequently Asked Questions
What is the Rule of 72?
The Rule of 72 is a quick formula for estimating how many years it takes for an investment to double. You divide 72 by the annual interest rate. For example, at 6% annual return, 72 / 6 = 12 years to double. It is a mental math shortcut that has been used by investors and financial planners for decades.
How accurate is the Rule of 72?
The Rule of 72 is most accurate for interest rates between 2% and 15%. At 8%, the rule estimates 9.0 years while the exact answer is 9.01 years. The accuracy decreases at very high or very low rates. At 1%, the rule says 72 years while the exact answer is 69.66 years. For typical investment scenarios, the rule is accurate enough for quick estimates.
Does the Rule of 72 work for any interest rate?
It works for any positive rate, but accuracy varies. For rates between 2% and 15%, it is very close to the exact calculation. For very low rates (under 2%), the Rule of 69.3 is more accurate. For very high rates (over 20%), the Rule of 72 increasingly overestimates doubling time. This calculator shows both the estimate and exact result so you can see the difference.
Can I use the Rule of 72 for inflation?
Yes. The Rule of 72 works for any constant rate of growth, including inflation. If inflation is 3% per year, prices will roughly double in 72 / 3 = 24 years. This means something that costs $100 today would cost about $200 in 24 years. It is a useful way to understand how inflation erodes purchasing power over time.