Rule of 72 calculator

How it works

Divide 72 by the yearly rate in percent to estimate the years needed to double, or divide 72 by the years to estimate the rate. It works because ln 2 ≈ 0.693 and 72 has many divisors, making mental maths easy.

The estimate is best for rates of about 6% to 10%. The exact answer, assuming yearly compounding, is shown alongside.

Formula

Years ≈ 72 ÷ rate
Exact years = ln 2 ÷ ln(1 + rate ÷ 100)
Exact rate = 2^(1 ÷ years) − 1

Worked example

At 6% a year money doubles in about 72 ÷ 6 = 12 years; the exact figure is ln 2 ÷ ln 1.06 ≈ 11.90 years. To double in 10 years you need about 7.2% a year; exactly 7.18%.