Rule of 72 Calculator – Estimate Doubling Time
The Rule of 72 Calculator helps you estimate how long it takes for an investment to double based on its annual rate of return. Instead of calculating exact compound interest formulas, the Rule of 72 divides 72 by the interest rate to give a fast and fairly accurate estimate.
This tool goes further by comparing the Rule of 72 estimate with the exact mathematical doubling time. It also calculates the required rate of return to double your money within a specific time period.
How the Rule of 72 Works
The Rule of 72 states:
If your investment earns 8% per year, doubling time is roughly 72 ÷ 8 = 9 years.
The rule works best for interest rates between 4% and 15%. Outside that range, error increases slightly.
Exact Doubling Formula
Exact doubling time uses natural logarithms:
This tool uses both formulas and shows the error margin.
Mode 1: Years to Double (Given Rate)
- Enter an interest rate
- Calculator shows Rule of 72 estimate
- Exact doubling time
- Error percentage
Example
At a 9% interest rate:
- Rule of 72: 8 years
- Exact: 8.04 years
- Error: less than 1%
Mode 2: Required Rate (Given Years)
- Enter desired doubling time
- Calculator shows required interest rate using Rule of 72
- Exact required rate based on compound formulas
- Error margin shown
Mode 3: Compare Rule of 72 vs Exact
This mode allows you to:
- Enter principal, rate and years
- See exact future value
- See Rule of 72 predicted doubling time
- Compare exact vs estimated results
Why Use This Calculator?
- Quick doubling time estimates
- Check accuracy of Rule of 72
- Compare estimated vs real compound interest
- Ideal for investing, retirement planning and financial education
Related Tools
Rule of 72 Calculator FAQs
Frequently Asked Questions
Find quick answers about doubling time and the Rule of 72.
The Rule of 72 is most accurate between 4% and 15% interest. For very high or low rates, its accuracy decreases slightly. That’s why this calculator includes the exact doubling formula for comparison.
Because 72 has many divisors (2, 3, 4, 6, 8, 9), it provides good approximations for common interest rates without complex math. It also closely fits the logarithmic formula for doubling time.
Yes. Knowing how quickly your money doubles helps you estimate long-term investment growth, especially when paired with a compound interest calculator.