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.
A worked example
A room 12 feet by 10 feet is 120 square feet (12 × 10), with a perimeter of 44 feet (2 × [12 + 10]) and a metric area of about 11.15 square meters. When buying flooring, add roughly 5–10% for cuts and waste, so you'd order material for about 130 square feet rather than exactly 120.
Measuring spaces that aren't rectangles
For an L-shaped room, split it into two rectangles, work out each area, and add them. A bay or a nook is just another rectangle to add on. For circular or triangular areas you'd use their own formulas, but most rooms reduce to a few rectangles.
Match the measurement to the material: area covers paint, flooring, and rugs, while perimeter covers baseboards, trim, and crown molding. Wall paint is figured from wall area (height × width), which is separate from the floor's square footage — so measure walls on their own.