The Rule of 72: the mental shortcut for doubling money

The Rule of 72 is the most useful mental math shortcut in personal finance. Divide 72 by your annual return rate, and you get the number of years it takes your money to double. At 8% return, money doubles every 9 years. At 12%, every 6 years. It works for any compounding investment.

How it works

Formula: Years to double = 72 ÷ annual rate. So 72 ÷ 6 = 12 years. 72 ÷ 9 = 8 years. 72 ÷ 4 = 18 years. The approximation is accurate enough for real-world planning — within 1-2% of the true compound doubling time for rates between 4% and 15%.

Why 72?

The exact doubling time at rate r is ln(2) / ln(1+r), but that requires a calculator. 72 is close to 100 × ln(2) ≈ 69.3, and 72 has nicer divisors (2, 3, 4, 6, 8, 9, 12). It's chosen for mental math convenience, not mathematical purity.

Real-world examples

• S&P 500 historical 10% → doubles every 7.2 years. A $10K investment becomes $40K in ~14 years, $160K in ~29 years.
• High-yield savings at 5% → doubles every 14.4 years. Slow but beats 0% checking.
• Credit card debt at 24% → doubles every 3 years. This is why debt compounds into disaster.
• Inflation at 3% → halves purchasing power every 24 years. Same math, opposite direction.

Reversed: the Rule of 72 for inflation

The same formula tells you how fast money loses value. At 3% inflation, purchasing power halves every 24 years. At 7% inflation (like 2022), every 10 years. This is why holding cash over decades is mathematically dangerous.

The Rule of 114 (tripling) and 144 (quadrupling)

Extensions: divide 114 by rate for tripling time, 144 for quadrupling. At 8%, money triples in 14.25 years and quadruples in 18 years. Handy for quick wealth-trajectory estimates.

Plug any scenario into our compound interest calculator at snowballr.io to verify — the Rule of 72 will be within 1-2% of the true answer for almost any rate you care about.