How Long to Double Money at 8%?
At an 8% annual return compounded monthly, investments double in about 8.69 years.
How to use this tool
- Enter annual return rate and compounding in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your exact doubling time and the full breakdown beneath it.
Calculate the doubling time for a long-term investment earning an 8% average annual return.
Frequently asked questions
- What is the Rule of 72?
- The Rule of 72 is a mental math shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money. For example, at 8% it takes about 9 years (72 ÷ 8 = 9).
- What is continuous compounding?
- Continuous compounding assumes interest compounds infinitely often. The doubling time is ln(2) / r, which is the theoretical minimum. In practice, daily compounding is very close to continuous.
- Which rule is more accurate — 72 or 69.3?
- The Rule of 69.3 is more mathematically precise (ln(2) × 100 ≈ 69.3). The Rule of 72 is more popular because 72 has many factors, making mental math easier for common interest rates.