How to calculate square footage
Multiply the length by the width in feet. A 12 ft × 15 ft room is 180 sq ft. If a measurement has inches, convert them to feet first: 12 ft 6 in = 12.5 ft. For a triangle use base × height ÷ 2, and for a circle π × (diameter ÷ 2)².
L-shaped and irregular rooms
Split the room into rectangles that do not overlap, calculate each one and add them up. For example, an L-shaped room of 12 × 15 ft plus 6 × 8 ft is 180 + 48 = 228 sq ft.
Why add a waste allowance
Flooring, tile and sod are cut at walls and corners. Contractors usually add 5–10% for straight layouts and 15% or more for diagonal patterns or many corners.