Rule of 72 Calculator

The Rule of 72 is a quick mental math approximation for compound growth. Divide 72 by the annual interest or growth rate (as a percentage) to get the approximate number of years for an investment to double. For example, at 8% per year: 72 / 8 = 9 years to double. The Rule of 72 is most accurate for interest rates between 6% and 10%.

The Rule of 72 is an approximation. The exact formula for doubling time is: years = ln(2) / ln(1 + r), where r is the decimal interest rate. At 6%, the exact answer is 11.90 years; the Rule of 72 gives 12 years (error of 0.8%). At 10%, exact is 7.27 years; rule gives 7.2 years (error of 1%). The approximation is sufficiently accurate for financial planning purposes.

The Rule of 114 estimates how long it takes for money to triple: divide 114 by the annual rate. For example, at 6%: 114 / 6 = 19 years to triple. This is a natural extension of the Rule of 72 (doubling) to tripling.

The Rule of 144 estimates how long it takes for money to quadruple (grow to 4x): divide 144 by the annual rate. For example, at 6%: 144 / 6 = 24 years to quadruple. Note that quadrupling = two doublings, so the Rule of 144 is approximately 2 x Rule of 72 at the same rate, which checks out: 2 x 12 = 24.

Yes. The Rule of 72 also shows how fast debt grows. At 18% APR (typical credit card): 72 / 18 = 4 years for a debt to double if unpaid and compounding. This is a powerful illustration of why high-interest debt is so damaging: a $5,000 credit card balance at 18% grows to $10,000 in just 4 years if you make no payments.