Very close for rates between roughly 5% and 10%; it drifts at extremes.

Those variants are more precise for continuous compounding; 72 divides cleanly for mental math.

How accurate is the Rule of 72?

Very close for mid-range rates; it drifts at very low or very high rates.

Can it estimate inflation's bite?

Yes — 72 ÷ inflation rate estimates years for prices to double.

Home › Savings & Investing › Rule of 72 Calculator

Free Rule of 72 calculator. Estimate how many years it takes for an investment to double.

The Rule of 72 is a quick mental shortcut: divide 72 by the return rate to estimate doubling time.

The Rule of 72 estimates doubling time by dividing 72 by the annual return percentage — a quick mental shortcut for compounding.

At an 8% return, money doubles in about 9 years (72 ÷ 8).

How accurate is it?: Very close for rates between roughly 5% and 10%; it drifts at extremes.
Rule of 70 or 69?: Those variants are more precise for continuous compounding; 72 divides cleanly for mental math.
How accurate is the Rule of 72?: Very close for mid-range rates; it drifts at very low or very high rates.
Can it estimate inflation's bite?: Yes — 72 ÷ inflation rate estimates years for prices to double.