Square footage is the area of a space — length times width for a rectangle. It's what you need for flooring, paint, rugs, and rent comparisons.
Measuring area
Multiply length by width in the same units to get the area. For an L-shaped or irregular room, split it into rectangles, measure each, and add them up. The perimeter (twice length plus width) is handy for baseboards and trim, and the metric conversion helps when materials are sold in square meters.