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Mathematics & Statistics / Financial Mathematics
Rule of 72 Calculator (Investment Doubling Time)
S&P 500 historical average: ~10-11% annually
About the Rule of 72
Rule of 72: Years to double = 72 / Annual return rate (%)
Reverse: Required rate = 72 / Target years
Exact formula: Years = ln(2) / ln(1 + r)
Example: At 8% return, investment doubles in 72/8 = 9 years
Why 72? It has many divisors, making mental math easier, and it\'s accurate for typical investment returns (6-10%).
Estimate how long an investment takes to double using a quick mental-math approximation for return-rate planning.
Formula
Doubling Time (years) ~= 72 / annual return rate (%)
Worked Example
annualReturnPercent8
Result: Approx. 9 years to doubleAt 8% return, the rule estimates doubling near 9 years, useful for fast scenario benchmarking.
Frequently Asked Questions
How accurate is Rule of 72?
It is a close approximation for moderate return rates, but exact compound calculations are better for final planning.
Can it be used for debt growth?
Yes. The same logic can estimate how quickly debt may double at a given interest rate.