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⏳ Rule of 72 Calculator

Find out how many years it takes to double your money at a given rate of return - or what return you'd need to double it within a target timeframe. Includes the mathematically exact answer alongside the Rule of 72 estimate.

⏳ What Do You Want to Find?

Years to Double
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⚖️ Rule of 72 vs. Exact Math

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Rule of 72 Estimate
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Exact Calculation

Doubling Time by Interest Rate

Compare the Rule of 72 estimate against the exact answer across common rates.

RateRule of 72ExactDifference

Rule of 72 Formulas

Years to Double

Years ≈ 72 ÷ Interest Rate (%)

Example: 72 ÷ 8 = 9 years at 8% return

Rate Needed to Double

Rate (%) ≈ 72 ÷ Target Years

Example: 72 ÷ 10 = 7.2% needed to double in 10 years

Exact Formula (for comparison)

Years = ln(2) ÷ ln(1 + rate/100)
Rate = (2^(1/years) − 1) × 100

Frequently Asked Questions

The Rule of 72 is a mental-math shortcut for estimating how long it takes an investment to double at a fixed annual rate of return. Divide 72 by the annual interest rate to get the approximate number of years. For example, at 8% annual return, money doubles in roughly 72 ÷ 8 = 9 years. It can also be reversed: divide 72 by the number of years to find the rate needed.
It's most accurate for rates between roughly 6% and 10%, where it typically lands within a few months of the exact answer. At very low rates (1-3%) or very high rates (20%+), the approximation drifts further from the true value. This calculator shows both the Rule of 72 estimate and the mathematically exact doubling time so you can see the difference for your specific rate.
The mathematically precise constant for continuous compounding is about 69.3 (from the natural logarithm of 2 × 100). 72 is used instead because it's very close to that value while being much easier to divide mentally - 72 is evenly divisible by 2, 3, 4, 6, 8, 9, and 12, making quick mental math practical. Some variations use 70 or 69 for more precision at low rates.
Yes. Applied to inflation, it estimates how long until prices double - at 3% inflation, purchasing power halves in roughly 24 years. Applied to debt, it estimates how quickly an unpaid balance doubles - credit card debt at 24% APR doubles in about 3 years if nothing is paid. It's a useful lens for understanding compounding in either direction.

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Rule of 72 Calculator - Estimate How Long to Double Your Money

The Rule of 72 is one of the most useful mental-math shortcuts in personal finance. Divide 72 by an annual rate of return and you get a close approximation of how many years it takes for money to double. It's simple enough to do in your head during a conversation, yet accurate enough for most practical planning - which is why it's widely taught in finance courses and used by investors as a quick sanity check.

The rule: Years to Double ≈ 72 ÷ Annual Rate (%)
Quick example: At 8% annual return, money doubles in roughly 9 years

When the Rule of 72 Is Most Accurate

The approximation works best for rates between about 6% and 10%, where it's typically accurate within a couple of months. At lower rates (1-3%), the rule slightly underestimates the time needed - using 69 or 70 instead of 72 gives a closer answer there. At very high rates (20%+), the rule starts to overestimate. This calculator always shows the exact mathematical answer alongside the estimate, so you can see the size of the gap for your specific rate.

Beyond Investments: Other Uses

  • Inflation: At 3% inflation, prices double (and purchasing power halves) in about 24 years - a useful reality check for long-term retirement planning.
  • Debt: Credit card debt at 24% APR doubles in about 3 years if nothing is paid down - a stark illustration of why high-interest debt compounds dangerously.
  • Comparing investments: A quick way to translate abstract percentage returns into a tangible timeframe when weighing options.
  • Population or business growth: Any quantity growing at a steady percentage rate follows the same doubling math.

How this calculator works, and where the numbers come from

The Rule of 72 Calculator applies the standard formula for this calculation to the values you enter and updates the result as you type. The calculation itself happens in your browser, and the page explains the method so you can check any result by hand.

Please note: Projections assume the inputs you enter stay constant; real returns vary. Results are estimates, not financial advice.

Sources and further reading

Learn more

Read our guide: Compound Interest and the Rule of 72, Checked Against the Math

Last reviewed: by the CalcQube Editorial Team. See our editorial policy for how we build and check calculators, or report an error.