Rule of 72 Calculator

The Rule of 72 is a classic mental math shortcut used by investors, economists, and financial advisors: divide 72 by your annual interest or return rate to estimate the number of years it takes for an investment to double in value. The rule works because 72 is a close approximation of 100 × ln(2) ≈ 69.3, adjusted upward to a number that divides cleanly into many common rates like 1, 2, 3, 4, 6, 8, 9, and 12. It applies to any scenario involving compound growth — savings accounts, stock portfolios, inflation, and even compound debt. Enter any rate to instantly see your approximate doubling time without needing a financial calculator or spreadsheet. The rule is equally useful for understanding the corrosive effect of inflation: at 3% inflation, purchasing power halves in about 24 years. At a 24% credit card APR, an unpaid balance doubles in just three years — a powerful illustration of why high-interest debt is so damaging when left unchecked over time.

The Rule of 72 works by dividing the constant 72 by the annual interest or return rate. The result is the approximate number of years needed for an investment to double in value, assuming compound interest. For example, at 8% per year: 72 ÷ 8 = 9 years. The rule is most accurate for rates between approximately 6% and 10%, where the approximation closely matches the exact logarithmic formula. Outside this range — particularly at very high or very low rates — the estimate becomes slightly less precise but still provides a useful ballpark figure. The exact formula for doubling time is ln(2) ÷ ln(1 + r), where r is the decimal rate. The Rule of 72 approximates this with the simpler 72 ÷ rate, which is accurate enough for almost all practical financial planning and quick mental calculations.