Rule of 72 Explained: How Long to Double Your Money

The Rule of 72 is a quick mental math shortcut:

Years to double ≈ 72 ÷ annual return (%)

To run exact scenarios and compare contributions, use:

• Rule of 72 Calculator
• Future Value Calculator

Why the Rule of 72 works

Compounding is exponential, and “72” is a convenient constant that approximates doubling time for moderate interest rates (roughly 4%–12%).

Example:

• 8% return → 72 / 8 = 9 years to double
• 6% return → 72 / 6 = 12 years to double

When it’s accurate vs when it’s not

The approximation is most accurate for common investing return rates. It can be less accurate when:

• rates are very low (near 0%)
• rates are very high (20%+)
• contributions occur over time (not a single lump sum)

For contribution-based planning, use the Compound Interest Calculator.

Example: doubling $10,000 at 8%

Rule of 72 estimate: ~9 years.

Exact compounding depends on the return path. Model a single-lump-sum scenario using the Future Value Calculator.

FAQ

Does the Rule of 72 work for debt too?

Yes. It can estimate how quickly a balance grows when interest compounds (like credit cards), but debt terms can be more complex due to payments and APR rules.

What if my return rate changes over time?

The Rule of 72 assumes a steady rate. Real returns vary; treat it as a planning shortcut.

Is 72 the only constant?

You’ll sometimes see 69 or 70 used for mathematical accuracy, but 72 is easy to divide by many numbers (2, 3, 4, 6, 8, 9, 12).

Can I use this for inflation?

Yes. If inflation is 3%, your money’s purchasing power halves roughly in 72/3 ≈ 24 years.

What’s better: Rule of 72 or future value?

Use the Rule of 72 for quick estimation. Use Future Value Calculator for real planning with contributions and time horizon.