How to estimate how long money takes to double

1. Enter the annual rate

   "Annual rate %" is the per-year return as a percentage (e.g. 6 for 6% per year). Zero and negative rates are rejected — the Rule of 72 only makes sense for positive growth.

2. Press Run

   Result is yearsToDouble = 72 / annualRatePercent, rounded to 2 decimals. At 6%/year, expect ~12 years; at 9%/year, ~8 years; at 12%/year, ~6 years.

3. Why 72?

   It's a mental shortcut: for compounding rates in the 6–10% range, 72/rate is within ~2% of the exact answer ln(2)/ln(1+r). Other shortcuts use 70 or 69.3 — pick one and stay consistent.

4. Use for sanity checks, not contracts

   Don't quote Rule-of-72 numbers to clients — at very low (<3%) or very high (>20%) rates the error grows. Run compound-interest-calculator for the precise answer.