BCBetter Calculators

Rule of 72 Calculator

Estimate how long it takes for an investment to double.

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How It Works

The Rule of 72 works by dividing the constant 72 by the annual interest or return rate. The result is the approximate number of years needed for an investment to double in value, assuming compound interest. For example, at 8% per year: 72 ÷ 8 = 9 years. The rule is most accurate for rates between approximately 6% and 10%, where the approximation closely matches the exact logarithmic formula. Outside this range — particularly at very high or very low rates — the estimate becomes slightly less precise but still provides a useful ballpark figure. The exact formula for doubling time is ln(2) ÷ ln(1 + r), where r is the decimal rate. The Rule of 72 approximates this with the simpler 72 ÷ rate, which is accurate enough for almost all practical financial planning and quick mental calculations.

Examples

Stock Market Average
Estimating doubling time at 8% annual return.
Result: Approximately 9 years to double.
High-Yield Savings
Estimating doubling time at 4.5% APY.
Result: Approximately 16 years to double.
Aggressive growth portfolio
Estimating doubling time at 12% annual return.
Result: Approximately 6 years to double.

Frequently Asked Questions

How accurate is the Rule of 72?
The Rule of 72 is most accurate for annual rates between 6% and 10%, where it differs from the exact formula by less than 1%. At higher rates (say 20–30%), the constant 69.3 is more accurate, but 72 is preferred because it divides evenly into many common rates like 1, 2, 3, 4, 6, 8, 9, and 12.
Can I use this for debt?
Yes — the Rule of 72 applies to any compound growth scenario, including debt. Enter your credit card APR to see how quickly your balance would double if you made no payments. At a typical 24% APR, your balance doubles in just 3 years.
What is the exact formula for doubling time?
The mathematically exact formula is Years = ln(2) ÷ ln(1 + r), where r is the decimal interest rate. The Rule of 72 approximates this by replacing ln(2) ÷ ln(1 + r) with the simpler 72 ÷ rate, which is close enough for most practical financial planning purposes.

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Investment Doubling Time Calculator

Calculate how long it will take for an investment to double using the Rule of 72 and exact compound interest.

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