Rule of 72 Calculator
The fastest mental math shortcut in finance. Find out how long it takes to double your money at any interest rate — or what rate you need to double in a specific time.
Rule of 72
Years to double = 72 ÷ Annual interest rate (%)Two calculation modes
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Avg savings: 4.5% | S&P 500: ~10%$
Years to double
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Doubled amount
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Exact calculation
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Doubling comparison by rate
| Interest rate | Rule of 72 estimate | Exact years | $10,000 becomes |
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Frequently asked questions
What is the Rule of 72? ›
The Rule of 72 is a quick mental math shortcut that estimates how long it takes for an investment to double at a given interest rate. Simply divide 72 by the annual interest rate percentage. At 6%, your money doubles in approximately 12 years (72÷6). At 9%, it doubles in 8 years (72÷9). It works because 72 is close to 100×ln(2)≈69.3, but divisible by more numbers.
How accurate is the Rule of 72? ›
It's very accurate for interest rates between 4% and 12%. At 6%, the Rule of 72 gives 12 years versus the exact answer of 11.9 years — less than 1% off. At very low rates (below 2%) or very high rates (above 20%), the rule becomes less precise, but it's always a useful quick estimate.
Can the Rule of 72 apply to inflation? ›
Yes — the Rule of 72 works for any exponential growth or decay, including inflation. At 3.5% inflation, purchasing power halves in about 20.6 years (72÷3.5). This is why modest inflation erodes wealth significantly over retirement spans.
What about the Rule of 69 or Rule of 70? ›
The Rule of 69 (or 69.3) is mathematically more precise for continuous compounding. The Rule of 70 is often used for inflation and GDP growth estimates. The Rule of 72 remains most popular because 72 has more integer factors (1,2,3,4,6,8,9,12), making mental division easier.
About this Rule of 72 calculator
The Rule of 72 is one of the most useful shortcuts in personal finance. This calculator extends it by showing the exact doubling time alongside the approximation, and compares multiple interest rates in a table so you can quickly see how rate differences compound over time.
Use it to evaluate savings account rates, compare investment options, understand the cost of debt, or estimate the erosion of purchasing power from inflation.