Rule of 72 Calculator
Rule of 72 and exact compound growth calculator
Use this tool to estimate how long an investment may take to double or what annual return is needed, then compare with the exact formula.Section 1: Years Required for Principal to Double
Rule of 72 Estimation
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Exact Answer
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Section 2: Required Interest Rate
Rule of 72 Estimation
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Exact Answer
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Rule of 72 Formula and Instructions
Method 1: Years to Double (Rule of 72)
- Years (estimated) = 72 / Interest Rate (%)
- Quick mental estimate. Most accurate around typical interest rates (roughly 6% to 10%).
Method 2: Years to Double (Exact)
- Years (exact) = ln(2) / ln(1 + r), where r = Interest Rate / 100
- Uses exact compound growth math for precise doubling period.
Method 3: Required Interest Rate (Rule of 72)
- Rate (estimated) = 72 / Years to Double
- Fast estimate of the annual percentage needed to double in given years.
Method 4: Required Interest Rate (Exact)
- Rate (exact) = (2^(1 / Years) - 1) x 100
- Returns precise annual compound interest rate required for doubling target.