How accurate is the Rule of 72?

The Rule of 72 is most accurate for return rates between 2-15%. For very high or low rates, the Rule of 69 (more precise) or Rule of 70 (for low rates) may be better. The exact formula is ln(2)/ln(1+r).

Rule of 72 Calculator - Investment Doubling Time

Free online Rule of 72 calculator. Quickly estimate how many years it takes for an investment to double. Pure frontend local computation. | No registration · 100% private

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Rule of 72 Estimate

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Rate	Rule of 72 (yrs)	Exact (yrs)	Error (mos)

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What is the Rule of 72?

A simple formula: Years to Double ≈ 72 ÷ Annual Return Rate

Example: 8% return → 72÷8=9 years; 12% return → 72÷12=6 years.

Most accurate for rates between 2%-15%.

Three Rules Compared

Rule of 72: Best for moderate rates (4%-12%)

Rule of 69: More precise for continuous compounding

Rule of 70: Better for low rates (2%-4%)

FAQ

Why does the Rule of 72 work?

It comes from the compound interest formula ln(2)/ln(1+r). When r is small, ln(1+r)≈r, and ln(2)≈0.693. So 0.693/r × 100 = 69.3/r. 72 is used instead of 69.3 because it has more divisors, making mental math easier.

Does this work for all investments?

It works for any investment growing at a fixed annual compound rate. Not suitable for volatile assets or simple interest products.

📈 Rule of 72 Calculator