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About Rule Of 72 Calculator Online
This tool applies the Rule of 72 — a simple mental math shortcut that estimates how long it takes for an investment to double at a given annual rate of return. Divide 72 by the rate, and the result is the approximate number of years until your money doubles.
At 6% annual return, money doubles in about 12 years. At 9%, about 8 years. At 12%, about 6 years. The rule is remarkably accurate for typical investment returns between 4% and 12%, and it works because of a useful approximation in compound-interest math.
Use it to quickly judge whether an investment's rate of return is worth pursuing, to estimate how inflation will erode purchasing power, or to set realistic doubling-time expectations for retirement savings.
How to use this tool
How to estimate how long money takes to double
Enter the annual rate
"Annual rate %" is the per-year return as a percentage (e.g. 6 for 6% per year). Zero and negative rates are rejected — the Rule of 72 only makes sense for positive growth.
Press Run
Result is yearsToDouble = 72 / annualRatePercent, rounded to 2 decimals. At 6%/year, expect ~12 years; at 9%/year, ~8 years; at 12%/year, ~6 years.
Why 72?
It's a mental shortcut: for compounding rates in the 6–10% range, 72/rate is within ~2% of the exact answer ln(2)/ln(1+r). Other shortcuts use 70 or 69.3 — pick one and stay consistent.
Use for sanity checks, not contracts
Don't quote Rule-of-72 numbers to clients — at very low (<3%) or very high (>20%) rates the error grows. Run compound-interest-calculator for the precise answer.