The Rule of 72 is a quick mental math shortcut: divide 72 by the annual interest rate to estimate how many years it takes for an investment to double in value.
How the Rule of 72 Works
The formula is simple: Years to Double ≈ 72 ÷ Interest Rate. At 8% annual return, your money doubles in approximately 72 ÷ 8 = 9 years. At 6%, it takes 72 ÷ 6 = 12 years. The rule also works in reverse: if you want to double your money in 10 years, you need 72 ÷ 10 = 7.2% annual return.
Why It Matters
The Rule of 72 makes the abstract concept of compound interest tangible. It helps investors quickly evaluate opportunities, compare rates, and understand the time value of money — all without a calculator.
Real-World Example
The S&P 500 has historically returned ~10% per year. Rule of 72: 72 ÷ 10 = 7.2 years to double. So $50,000 invested becomes ~$100,000 in 7 years, ~$200,000 in 14 years, and ~$400,000 in 21 years. The exact answer (using compound interest) is 7.27 years — the rule is remarkably accurate.
Frequently Asked Questions
How accurate is the Rule of 72?
Very accurate for rates between 4-12%. At 8%, it predicts 9.0 years vs the exact answer of 9.01 years. Accuracy decreases at very low (<2%) or very high (>20%) rates. The Rule of 69.3 is more precise mathematically but 72 is easier to divide mentally.
Can the Rule of 72 be used for inflation?
Yes. At 3% inflation, prices double every 72 ÷ 3 = 24 years. This means something costing $50 today will cost $100 in 2050. It works for any exponential growth or decay process.
What is the Rule of 72 for tripling?
To estimate tripling time, use the Rule of 115 instead: divide 115 by the interest rate. At 10%, your money triples in approximately 115 ÷ 10 = 11.5 years.