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Rule of 72 Calculator — How Fast Does Your Money Double?

Divide 72 by your interest rate to estimate doubling time

How much is Rule of 72 Calculator — How Fast Does Your Money Double??

The Rule of 72 is a quick formula to estimate how long an investment takes to double. Divide 72 by the annual interest rate to get the approximate number of years. For example, at 8% interest, your money doubles in about 9 years. This shortcut works best for rates between 2% and 15%.

Rule of 72 — Doubling Time at Various Rates

The Rule of 72 is a simple mental math shortcut: divide 72 by the annual interest rate to estimate how many years it takes for your investment to double.

Interest Rate Rule of 72 Estimate Actual Years (Monthly Compounding)
2% 36.0 years ~35 years
3% 24.0 years ~24 years
4% 18.0 years ~18 years
5% 14.4 years ~14 years
6% 12.0 years ~12 years
7% 10.3 years ~10 years
8% 9.0 years ~9 years
9% 8.0 years ~8 years
10% 7.2 years ~7 years
12% 6.0 years ~6 years
15% 4.8 years ~5 years

How to use it: Want to know how long until your money doubles at 8%? Just calculate 72 / 8 = 9 years. At 6%? That's 72 / 6 = 12 years. The formula works for any interest rate between 2-15%.

The Math Behind the Rule of 72

The Rule of 72 comes from the natural logarithm of 2 (ln(2) = 0.693). The exact doubling time is:

t = ln(2) / ln(1 + r) ≈ 72 / (r × 100)

The number 72 is used instead of 69.3 because it has more whole-number divisors (1, 2, 3, 4, 6, 8, 9, 12), making mental math easier. The approximation is most accurate for rates between 6-10%.

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Frequently Asked Questions

The Rule of 72 is a simple formula to estimate how many years it takes for an investment to double. Divide 72 by the annual interest rate to get the approximate doubling time. For example, at 6% interest: 72 / 6 = 12 years to double.

The Rule of 72 is remarkably accurate for interest rates between 2% and 15%. At 8%, it predicts 9 years to double, and the actual time with annual compounding is 9.01 years. Accuracy decreases slightly at very high or very low rates.

Yes. The Rule of 72 works regardless of the initial amount because doubling is about the percentage growth, not the dollar amount. $1,000 and $1,000,000 both double in the same number of years at the same interest rate.

The Rule of 69.3 (sometimes called the Rule of 70) is mathematically more precise for continuous compounding. The number 72 is used in the popular version because it has more divisors (1,2,3,4,6,8,9,12), making mental math easier. For practical purposes, both give very similar results.

Related Calculations

How Long for $10,000 to Double at 7% Interest?

Result: $21,549.40

$10,000 at 7% Interest for 10 Years

Result: $20,096.61

$10,000 at 10% Interest for 10 Years

Result: $27,070.41

$10,000 at 5% Interest for 10 Years

Result: $16,470.09

Related Reading

Compound Interest Calculator: How It Works and Why It Matters → Rule of 72 Explained: How Long to Double Your Money → Compound Interest vs Simple Interest: What's the Difference? →