Rule of 72 and doubling time calculator
How long money takes to double at a steady annual return, compared with the Rule of 72 shortcut.
Real returns are not steady from year to year. Not financial advice.
How this calculator works
Exact doubling time = ln(2) ÷ ln(1 + r). The Rule of 72 approximates this as 72 ÷ r, and it's most accurate for rates between about 6% and 10%.
Worked example: at 7%, money doubles in 10.24 years exactly, and the Rule of 72 says 10.29. The same maths shows that 3.5% inflation halves the buying power of cash in about 20 years.
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