How accurate is the Rule of 72?

It's quite accurate for interest rates in the roughly 6-10% range, with small deviations outside that range — good enough for quick mental estimates, not for precise financial planning.

Can I use this to figure out what rate I need?

Yes — rearranging the rule, divide 72 by your target number of years to estimate the annual rate needed to double your money in that time.

Rule of 72 Calculator

Use the Rule of 72 — a quick mental-math shortcut — to estimate how many years it takes an investment to double at a given rate, or what rate you'd need to double in a target number of years.

Find

Annual interest rate

Years to Double

12.0

How It Works

The rule

Divide 72 by the annual interest rate (as a whole number, not a decimal) to estimate the number of years to double your money — for example, 72 ÷ 8 = 9 years at 8%.

Why it works

It's a mathematical approximation of the actual compound growth formula, accurate to within a fraction of a year for typical interest rates (roughly 6-10%), and reasonably close outside that range.

Example Calculation

At 8% annual growth, money doubles in about 9 years (72 ÷ 8). At 6%, it takes about 12 years (72 ÷ 6).

Common Mistakes

Expecting exact precision — the Rule of 72 is a quick estimate, not a substitute for the exact compound growth formula, especially at very high or very low rates.
Using a percentage in decimal form (0.08) instead of whole-number form (8) in the calculation.

Frequently Asked Questions

How accurate is the Rule of 72?: It's quite accurate for interest rates in the roughly 6-10% range, with small deviations outside that range — good enough for quick mental estimates, not for precise financial planning.
Can I use this to figure out what rate I need?: Yes — rearranging the rule, divide 72 by your target number of years to estimate the annual rate needed to double your money in that time.