Measure each room, leave out islands and hearths, and choose a waste allowance. Get boxes from the coverage on the label, whole tiles from the tile size, or roll length from the roll width.
By the box (laminate, vinyl plank, engineered wood, many tiles): area × (1 + waste) ÷ coverage per box.
By the tile: area × (1 + waste) ÷ one tile’s face area.
By the roll (sheet vinyl, carpet): strips side by side across the room, each the room length plus trim.
About waste
Every room loses material to cuts along walls and around obstacles. Straight layouts in rectangular rooms lose the least; diagonal, herringbone and patterned layouts, and rooms with many corners, lose more. Flooring makers sometimes state a figure for their product. We never add an allowance you didn’t choose.
Worked examples
A 12 × 15 ft room is 180 sq ft. With 10% waste, buy 198 sq ft. At 20 sq ft per box: 9.9 → 10 boxes.
A 10 × 10 ft bathroom (100 sq ft) in 12 × 24 in tiles (2 sq ft each), 10% waste: 110 ÷ 2 = exactly 55 tiles.
The same 12 × 15 ft room in 12 ft wide sheet vinyl, strips running the 15 ft way with 3 in trim: one strip of 15 ft 3 in. Running the 12 ft way instead needs two strips: 24 ft 6 in.