numvana

Rule of 72 Calculator

Estimate how many years it takes to double an investment using the Rule of 72, compared against the exact compounding formula.

Years to double (Rule of 72)
9
Years to double (exact)
9.01

How it works

The Rule of 72 is a mental-math shortcut: years to double ≈ 72 ÷ annual rate (as a percentage, not a decimal). It's a well-known approximation of the exact answer, which comes from solving the compound interest formula for time: years = ln(2) ÷ ln(1 + r). This calculator shows both, so you can see how close the shortcut gets.

FAQ

Why 72 specifically?

72 divides evenly by many small numbers (1, 2, 3, 4, 6, 8, 9, 12...), which makes it convenient for quick mental math, and it happens to closely approximate ln(2) × 100 across the interest rate ranges most savings and investments actually use.

Related calculators