Years to Double
9 years
Projected Value (at double)
$5,120,000.00
How it works
The Rule of 72 Calculator is a mental math shortcut that estimates how long it takes an investment to double — by dividing 72 by the annual interest rate or return. Enter any rate to see the doubling time, and any doubling time to see the required rate. The tool also shows the more accurate Rule of 69.3 (continuous compounding) for comparison.
The Rule of 72 is one of the most useful tools in financial literacy. At 6% annual return, your money doubles in 72/6 = 12 years. At 12%, it doubles in 6 years. At 3% inflation, prices double in 24 years. Understanding doubling time makes compound interest intuitive and visceral — and helps evaluate any investment or debt scenario quickly.
How to use it: enter the annual rate (%). The calculator returns the doubling time in years. Enter a target doubling time to calculate the required annual rate. The Rule of 72 approximation is shown alongside the exact value (using the log formula).
Why 72?: 72 is the most convenient approximation because it is divisible by 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36 — making mental arithmetic easy for most common interest rates.
Accuracy: the Rule of 72 is most accurate between 6% and 10% annual rates. Below 5%, Rule of 69.3 is more accurate. Above 15%, the error grows — the exact formula should be used. The calculator shows the error percentage alongside the approximation.
Debt application: credit card at 24% APR — debt doubles in 72/24 = 3 years if only minimum payments are made and the balance isn't paid down. This context makes the cost of carrying debt immediately concrete.
Privacy: calculations run in the browser.
Frequently Asked Questions
- The Rule of 72 is most accurate between 6–10% interest rates, with an error of less than 0.1 years. At 2% rate, the Rule of 72 gives 36 years vs. exact 35.0 years (error: 2.9%). At 20%, it gives 3.6 years vs. exact 3.8 years (error: 5%). For precise calculations outside the 4–12% range, use the exact formula: t = ln(2) / ln(1 + r).
- Divide 72 by the inflation rate to find how long it takes for prices to double (or equivalently, for your money's purchasing power to halve). At 3% inflation: 72/3 = 24 years. At 6% inflation: 72/6 = 12 years. This makes the real impact of inflation visceral — at 3%, a retiree's fixed income buys only half as much in 24 years.
- Yes. Credit card at 24% APR: 72/24 = 3 years to double. An unpaid $5,000 credit card balance becomes $10,000 in 3 years if only minimum payments are made and no new charges added. This is one of the most effective ways to communicate the true cost of credit card debt.
- For low interest rates (below 5%), Rule of 70 is slightly more accurate: 70 / r. The mathematically exact constant for continuous compounding is 69.3 (from ln(2) × 100). Rule of 72 uses 72 because it's more divisible by common interest rates (2, 3, 4, 6, 8, 9, 12) than 69.3 or 70. The calculator shows all three alongside the exact value.