Rule of 72 Calculator

Approximate how long it will take for an investment to double at a given annual compound rate using the classic mental-math Rule of 72.

Use the expected nominal annual return (before inflation).

Educational estimate, not financial advice.

Examples

  • 6.0% ⇒ 12.00 years
  • 8.0% ⇒ 9.00 years
  • 12.0% ⇒ 6.00 years

FAQ

Why does the shortcut use 72?

72 has many small factors (2, 3, 4, 6, 8, 9, 12), making mental division easy while staying close to the natural logarithm-based result.

How accurate is the Rule of 72?

At 6% the error is under 0.1 years. At 20% it overestimates by about 0.7 years. Use the exact doubling formula if precision is critical.

Can I adjust the constant for other compounding frequencies?

Yes. Investors sometimes use 69.3 (ln 2 × 100) for continuous compounding or 70 for a quick inflation estimate. Choose the constant that best matches your scenario.

Additional Information

  • The Rule of 72 is a shortcut: doubling time ≈ 72 ÷ annual rate (in percent).
  • For higher precision, compare against the exact formula t = ln(2) ÷ ln(1 + r).
  • The heuristic works best for rates between 4% and 15%; outside this range, the approximation error grows.

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