Rule of 72 Calculator
Estimate how long it takes to double your money.
How to use this tool
- Enter annual rate in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your years to double and the full breakdown beneath it.
Estimate how long it takes to double your money.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Years to double = 72 ÷ Annual Rate (%)
t = 72 ÷ r where r is the annual interest rate in percent
How it works
The Rule of 72 is a quick mental-math shortcut that estimates the number of years required for an investment to double in value at a fixed annual compound interest rate. Dividing 72 by the annual percentage rate gives a close approximation to the exact doubling time derived from the natural logarithm formula (ln(2) ÷ ln(1 + r)). The approximation is most accurate for rates between roughly 6% and 10% per year.
Because it is an approximation, results will differ slightly from exact compounding calculations, especially at very high or very low interest rates.
Worked example
- Annual rate = 8%
- Years to double = 72 ÷ 8 = 9
Years to double = 9
Common mistakes to avoid
- Entering the rate as a decimal (e.g., 0.07) instead of a percentage (7) — the rule of 72 divides 72 by the percentage rate, so 0.07 gives a nonsensical 1,028-year result.
- Applying the rule to accounts that charge fees or taxes, which reduce the effective rate and make the doubling time longer than the formula suggests.
- Using the rule for very high rates (above 25%) without knowing accuracy degrades — the exact doubling time at 30% is 2.64 years, but the rule of 72 gives 2.4 years (8% error).
Key terms
- Rule of 72
- A heuristic stating that dividing 72 by an annual compound interest rate approximates the number of years needed to double the investment.
- Doubling time
- The period required for a quantity growing at a constant rate to become twice its current size.
- Compound interest
- Interest calculated on both the initial principal and the accumulated interest from previous periods.
- Approximation error
- The small difference between the Rule of 72 estimate and the exact doubling time; it widens for rates far from 8%.
Frequently asked questions
- How accurate is the rule of 72?
- Most accurate for rates between 6% and 10%, where the error is under 1%. At 1% it gives 72 years (exact: 69.7); at 25% it gives 2.88 years (exact: 3.11).
- Can I use the rule of 72 for debt?
- Yes — the same formula tells you how quickly a debt doubles if you make no payments. Credit card debt at 24% APR doubles in about 3 years (72 / 24).
- Why 72 and not 70 or 69?
- 72 is divisible by 1, 2, 3, 4, 6, 8, 9, and 12, making mental arithmetic easier. 69.3 (ln2 x 100) is the mathematically exact base, and 70 is a common rough alternative.