Rule of 72 Doubling Time

The Rule of 72 is a famous mental math shortcut used by investors to quickly estimate how many years it takes for an investment to double at a given annual rate of return without complex financial equations.

This calculator compares the approximate Rule of 72 and Rule of 70 against the exact logarithmic compound growth formula, showing accuracy differences across low, moderate, and high interest rates.

Rule of 72 Approximation: Years ≈ 72 / Annual Interest Rate (%). Exact Doubling Time: Years = ln(2) / ln(1 + r/100). The Rule of 72 is most accurate for interest rates between 6% and 10%.