Conscious Finance
Rule of 72 Calculator
The simplest and most powerful mental model in personal finance. Enter an interest rate to see how long your money takes to double — or enter a timeframe to find the return you need.
The Math Behind the Rule of 72
The Rule of 72 is derived from the compound interest formula. The exact calculation uses the natural logarithm: Years = ln(2) ÷ ln(1 + r). Since ln(2) ≈ 0.693, and for rates between 6–10% the denominator approximates well, dividing 72 by the percentage rate gives a result accurate to within 1–2% of the precise answer.
72 was chosen over 69.3 (the mathematically precise numerator) because it has more integer divisors — making mental math easier. 72 divides cleanly by 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, and 36, covering almost every realistic investment return scenario.