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Rule of 72 Calculator

Use the Rule of 72 to estimate how long it takes to double your money at a given return rate.

Last Updated: May 5, 2026
2 min read

Rule of 72

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Rule of 72

Years to double = 72 ÷ Annual Return Rate

A quick mental math shortcut for estimating doubling time. Works best for rates between 6%–10%.

Years to Double (Rule of 72)

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Exact Years

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Difference

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The Rule of 72 is a quick mental math shortcut that estimates how many years it takes to double your money at a given compound interest rate. Divide 72 by the annual return rate and you get the approximate doubling time. This calculator extends the concept to show exact values and compare multiple rates.

How to Use This Calculator

  1. Enter the annual interest rate or return rate (e.g., 8 for 8%).
  2. Click Calculate to see the estimated doubling time using Rule of 72, Rule of 69.3 (more precise), and exact compound calculation.
  3. Optionally enter an initial amount to see projected values at each doubling interval.

What This Calculator Measures

  • Doubling time — How many years until your initial amount doubles.
  • Rule of 72 — The fast mental approximation: Years = 72 ÷ Rate.
  • Rule of 69.3 — Slightly more accurate for continuous compounding: Years = 69.3 ÷ Rate.
  • Exact doubling time — Using the precise formula: Years = ln(2) ÷ ln(1 + r).

Formula or Logic

Rule of 72: Years = 72 ÷ Annual Rate (%)

Exact: Years = ln(2) ÷ ln(1 + r) where r is the decimal rate

The Rule of 72 is most accurate for rates between 6% and 10%. At 2% or 30%, use the exact formula for precision.

Example Calculations

Example 1: 6% annual return → 72 ÷ 6 = 12 years to double. Exact = 11.9 years. $10,000 → $20,000.

Example 2: 9% annual return → 72 ÷ 9 = 8 years to double. At 18%, money doubles every 4 years — three doublings in 12 years turns $10,000 into $80,000.

Understanding Your Results

Doubling time is powerful because it reveals how compounding accelerates wealth: at 9%, money doubles every 8 years. Over 40 years, that's 5 doublings — turning $1 into $32. At 6% it's only 3.3 doublings → $10.

Common Mistakes to Avoid

  • Applying the Rule of 72 to simple interest — it only works for compound interest.
  • Forgetting that the "return rate" must be the real, after-fee, after-inflation rate for meaningful projections.
  • Assuming consistent annual returns — actual investments fluctuate; 72 gives an average-case approximation.
  • Not accounting for taxes, which can cut effective investment returns by 20–30% depending on account type.