Area items are measured in square feet. Flooring, ceilings, roofing, drywall, paint and insulation are common examples.
A room 20 ft by 30 ft has 600 SF of floor. Wall area is the wall length times the wall height, less large openings.
Know whether you need net area or gross area. A paint estimator may not deduct small openings. A tile estimator adds waste for cuts.
Some materials sell in other units. Carpet often sells by the square yard. Divide square feet by 9 to get square yards.