The Rule of 72

To find how many years it takes to double an investment at interest rate r%, simply divide 72 by r. At 6% annual interest, money doubles in about 12 years. At 9%, about 8 years. This rule of thumb comes from a logarithmic approximation: the exact answer is ln(2)/ln(1+r) ≈ 0.693/r, and 72 ≈ 100 × 0.693 × 1.04, slightly adjusted to make mental arithmetic easier. The rule is mentioned by Luca Pacioli in 1494 — 530 years ago — and remains one of the most useful mental shortcuts in personal finance.