What Is the Rule of 72? The Fastest Way to Calculate Investment Growth

The Rule of 72 is a mental math shortcut: divide 72 by your annual return rate to find how many years it takes to double your money. Earning 8% per year? 72 ÷ 8 = 9 years to double. Earning 6%? 12 years. It works for any compounding scenario — investments, debt, inflation, or economic growth. It's one of the most useful numbers in personal finance.

The Math Behind the Rule

The Rule of 72 is an approximation of the exact doubling formula: Years = ln(2) ÷ ln(1 + rate) = 0.693 ÷ rate. For rates between 6% and 10%, 72 is a better approximation than 69 or 70 because it's divisible by more numbers (2, 3, 4, 6, 8, 9, 12), making mental math easier. For rates below 3% or above 15%, the rule becomes less accurate — but for the ranges most people actually deal with, it's remarkably precise.

Rule of 72 in Action: Investment Examples

• 4% return (bonds, HYSA): 72 ÷ 4 = 18 years to double
• 6% return (conservative portfolio): 72 ÷ 6 = 12 years to double
• 8% return (balanced portfolio): 72 ÷ 8 = 9 years to double
• 10% return (S&P 500 historical average): 72 ÷ 10 = 7.2 years to double
• 12% return (aggressive growth): 72 ÷ 12 = 6 years to double

Using the Rule of 72 for Debt

The Rule of 72 applies to debt too — and it's sobering. Credit card debt at 24% APR doubles in 72 ÷ 24 = 3 years if you're not paying it off. A $5,000 balance at 24% becomes $10,000 in just 3 years if you only make minimum payments. This is why high-interest debt is financially destructive — the same compounding that makes investments powerful works against you when you're the borrower.

The Rule of 72 and Inflation

Inflation uses the same math. At 3% inflation (the Federal Reserve's rough target), prices double in 72 ÷ 3 = 24 years. That means $100 of groceries today will cost $200 in 2050. At 6% inflation (what the US experienced in 2021-2022), prices double in just 12 years. This is why money in a savings account earning 0.5% — far less than inflation — loses purchasing power over time.

The Power of Small Rate Differences

The Rule of 72 makes the impact of fees and rate differences visceral. Consider two investments: one returning 7% and one returning 5% (2% lower due to fees or worse returns).

• 7% return: doubles in 72 ÷ 7 ≈ 10.3 years
• 5% return: doubles in 72 ÷ 5 = 14.4 years
• A $100,000 portfolio at 7% doubles to $200,000 in ~10 years
• The same portfolio at 5% takes 4 more years to reach $200,000 — and falls further behind with each doubling
• Over 40 years, the 7% portfolio doubles ~4 times (to $1.6M); the 5% portfolio doubles ~2.8 times (to $700K)

Related Rules

• Rule of 114 — divide 114 by the rate to find how long to triple your money (at 8%: 114 ÷ 8 = 14.25 years)
• Rule of 144 — divide 144 by the rate to find how long to quadruple your money (at 8%: 144 ÷ 8 = 18 years)
• Rule of 70 — used by economists for GDP doubling time (slightly more accurate at very low rates like 1-2%)