How do convert millimeter to square feet?

Answer 1

You don't.

Millimetres are a measure of distance while square feet are a measure of area. The two measure different things and, according to basic dimensional analysis, conversion from one to the other is not valid.

Answer 2

None, since there can be no conversion. A millimetre is a measure of length in 1-dimensional space while a square foot is a measure of area in 2-dimensional space. The two measure different things and, according to basic principles of dimensional analysis, any attempt at conversion from one to the other is fundamentally flawed.

Answer 3

You cannot do that. Millimeter is measure of LENGTH -or how long something is.
Square feet is a measure of AREA, length times width. You cannot mix these two