Use the Rule of 72 to estimate how long it takes to double your money. Enter any interest rate and instantly see years to double plus the exact result.
The Rule of 72 is one of the most useful mental math shortcuts in finance. Divide 72 by the annual interest rate, and you get the approximate number of years it takes for your money to double. At 8% return, money doubles in about 9 years. At 12%, it doubles in about 6 years.
Our Rule of 72 Calculator goes further than the simple approximation. It shows both the Rule of 72 estimate and the exact doubling time using the precise compound interest formula, so you can see how accurate the shortcut is at different rates. It also works in reverse — enter your target doubling time and find the required rate.
This calculator is essential for quick investment planning, comparing opportunities, and understanding the power of compound growth without reaching for a spreadsheet. The approximation works best at moderate growth rates between 4% and 15%, which conveniently covers most savings account, index fund, and reasonable business growth scenarios.
The Rule of 72 lets you estimate doubling time in your head, but it becomes less accurate at very high or very low rates. This calculator shows both the approximation and exact result side by side, so you know when the shortcut works and when to use the precise formula.
Rule of 72: Years to Double ≈ 72 / Rate. Exact formula: Years = ln(2) / ln(1 + r) where r is the annual rate as a decimal. Required rate for given years: r = 2^(1/years) - 1.
Result: Rule of 72: 9.0 years | Exact: 9.01 years
At 8% annual return, the Rule of 72 estimates 72/8 = 9.0 years to double. The exact formula gives ln(2)/ln(1.08) = 9.01 years. At this rate, the approximation is remarkably accurate — within 0.01 years of the true answer.
The Rule of 72 reveals the exponential nature of compound growth. At 10% annual return, your money doubles every 7.2 years. That means $100,000 becomes $200,000 in 7 years, $400,000 in 14 years, and $800,000 in 21 years. Each doubling builds on the previous one, creating accelerating wealth over time.
The S&P 500 has historically returned about 10% per year (before inflation), meaning stocks have doubled roughly every 7 years. Treasury bonds have returned about 5%, doubling every 14 years. Savings accounts at 2% take 36 years to double. These comparisons instantly illustrate why long-term investors favor equities.
The mathematical constant behind doubling is ln(2) = 0.693, which gives the Rule of 69.3 for continuous compounding. For discrete annual compounding, 72 is more accurate at typical rates. The Rule of 70 is a compromise used in economics textbooks. For tripling, use 114.9 (approximated as 115).
The exact number for continuous compounding is 69.3 (ln(2) x 100), but 72 is used because it is more divisible and gives better approximations for typical interest rates (6-10%). It divides evenly by 2, 3, 4, 6, 8, 9, and 12, making mental math easier.
Very accurate for rates between 4% and 20%. At 8%, the error is less than 0.1 years. At 2% or 30%, the error grows to about 0.5-1 year. For extreme rates, use the exact logarithmic formula.
The Rule of 72 assumes annual compounding. For monthly or daily compounding, the doubling time is slightly shorter. The exact formula handles any compounding frequency correctly.
No — use the after-tax, after-fee return rate for realistic results. If your return is 8% but you pay 2% in taxes and fees, use 6% in the calculator.
The Rule of 115 estimates the time to triple your money. Divide 115 by the annual return to get the approximate tripling time. At 8%, money triples in about 14.4 years.
To find how long it takes for your money to double in real (inflation-adjusted) terms, subtract the inflation rate from your nominal return. At 8% nominal return and 3% inflation, your real return is about 5%, so real doubling takes 72/5 = 14.4 years.