Question: My friend in India told me the "Rule of 72" says my money will double in 8 years at 9% annual interest. Is that exactly right, or just an approximation — and if so, how far off is it?
Answer: Rule of 72 gives 8.0 years; the exact doubling time is approximately 8.04 years— a difference of roughly 2 weeks, so the rule is a slight underestimate here.

Supporting evidence:
title: Inline formula / method note
quote: Compound interest formula (exact solution) vs. Rule of 72 approximation — standard financial mathematics
context: Compound interest formula (exact solution) vs. Rule of 72 approximation — standard financial mathematics.