Rule of 72 Calculator – Estimate Years to Double Your Money
Use this free Rule of 72 calculator to estimate how many years it takes for an investment to double at a given annual compound return rate. Enter the annual rate and get an instant approximation. No sign-up required.
Rule of 72 Calculator
Estimate how long it takes for an investment to double at a given annual return rate. Rule of 72: years to double ≈ 72 ÷ rate.
How the Rule of 72 works
Years to double ≈ 72 ÷ annual rate (%)
The Rule of 72 is derived from the compound interest formula. The exact time to double is ln(2) / ln(1 + r), where r is the decimal rate. For small rates, 72 / rate gives a close approximation. The calculator uses this formula for a quick estimate.
Understanding the Rule of 72 and compound growth
The Rule of 72 is one of the most useful shortcuts in personal finance. It answers a simple question: at a given annual return, how long until my money doubles? Instead of using logarithms or a compound interest formula, you divide 72 by the annual rate. The result is the approximate number of years to double. For example, at 9% annual return, 72 ÷ 9 = 8 years. So an investment of $10,000 at 9% would grow to about $20,000 in roughly 8 years.
The rule assumes compound growth—interest or returns are reinvested and earn more returns over time. It works for investments, savings accounts, and even inflation. When inflation runs at 4% a year, the rule says prices double in about 18 years (72 ÷ 4). That helps you understand why long-term inflation matters for purchasing power.
The Rule of 72 is an approximation. It is most accurate for rates between 6% and 10%. At very high rates (e.g. 20%), the rule overstates the time to double; at very low rates (e.g. 2%), it understates slightly. For exact numbers, use a compound interest or lumpsum calculator. But for quick mental math or back-of-the-envelope planning, the Rule of 72 is hard to beat.
Investors often use the rule to compare different return assumptions. If one investment promises 6% and another 12%, the rule tells you the 12% option doubles in 6 years while the 6% option takes 12 years. That makes the impact of a higher return rate very clear. Remember that past performance does not guarantee future results, and the rule does not account for taxes, fees, or volatility.