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The square foot (plural square feet; abbreviated sq ft, sf, ft2) is an imperial unit and U.S. customary unit (non-SI, non-metric) of area, used mainly in the United States and partially in Bangladesh, Canada, Ghana, Hong Kong, India, Malaysia, Nepal, Pakistan, Singapore and the United Kingdom. It is defined as the area of a square with sides of 1 foot.
circular inch is a unit of area. it uses in Imperial/US system. it exactly equal π⁄4 sq in and approximately ≈ 5.067075 × 10−4 m2