This below is to find the square footage for painting a room.

Take the perimeter (the length in feet around the room) then
multiply it by the height of the ceiling. The ceiling, which is
like a 5th wall that you look up at, is measured by length X width.
Same thing with the floor. Subtract about 28 sq feet per normal
size door. Always get a little more paint to make sure you get the
project done correctly.

If the room is not a complete square, like if it has jogs in it
then you want to find each box seperately by multiplying the length
of the room by the width. Do this for every jog in the room then
subtract each jogs' sq/ft from the sq/ft of the entire room and
that with give you the rooms final sq/ft.

Very simple - measure one side, then measure the side next to it
and multiply one by the other.

measure LENGTH x WIDTH = square footage

It is very simple if the sides envolved are say a square or
rectangle. If the area has for example an angular wall, measure and
calculate the rectangle areas first. To get the square footage of
the floor at the angled wall, measure the length and width as if
the diagonal wall split your rectangle from oposite corners of your
rectangle. In other words: length times width divided by two equals
the square footage of this area. Then add it to all your other
calculated rectangles.