Rule of 72 Calculator
About Rule of 72 Calculator
What Is the Rule of 72?
The Rule of 72 is the simplest mental math shortcut in all of finance. Divide 72 by your annual rate of return and you get the approximate number of years it takes to double your money. Earning 8%? Your money doubles in about 9 years. Getting 4%? Plan on 18 years. It works remarkably well for rates between 2% and 20%, and it’s a favorite of financial planners, teachers, and investors who need a quick sanity check without pulling up a calculator.
The Formula
The formula is simply Years to Double = 72 / Annual Interest Rate. You can also flip it: Required Rate = 72 / Desired Years to Double. Want to double your money in 6 years? You need roughly 12% annual returns. This calculator also shows the exact doubling time using the precise logarithmic formula for comparison: Exact Years = ln(2) / ln(1 + r).
How to Use This Calculator
Enter an annual interest rate or expected return. The calculator instantly shows the estimated doubling time using the Rule of 72 and the exact doubling time using logarithms. You can also enter a target doubling period to find the required rate of return. It’s a two-way tool that works for both questions.
Frequently Asked Questions
How accurate is the Rule of 72?
Very accurate for rates between 6% and 10%. At 8%, the Rule of 72 gives 9 years while the exact answer is 9.01 years. Accuracy decreases slightly at very low or very high rates.
Does the Rule of 72 work for debt too?
Yes. If your credit card charges 18% interest, your unpaid balance doubles in about 4 years (72 / 18 = 4). It’s a powerful way to visualize how quickly debt grows.
Why 72 and not some other number?
The number 72 is used because it is close to the mathematical constant needed for doubling (ln(2) x 100 = 69.3) and is divisible by many common numbers (2, 3, 4, 6, 8, 9, 12), making mental math easier.
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