How to calculate rule of 72
Estimate = 72 ÷ rate · Exact = ln(2) ÷ ln(1 + rate)
Worked example. Years to double (exact): 9 years.
Example inputs — Annual return (%): 8; Starting amount: $10,000. Starting values are illustrative.
From George Guju, founder of Calculyn · Educational calculator guidance, not professional advice.
How it works
The Rule of 72 is a quick mental shortcut: divide 72 by your yearly return to estimate how long money takes to double. We also show the exact answer.
Assumptions & limitations
- Results are rounded for display.
Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators
Frequently asked questions
How accurate is the Rule of 72?
Very close for returns between about 6% and 10%. It drifts at very low or very high rates.