How to calculate how many tiles you need
Measure the area first: multiply length × width in feet, and split L-shaped rooms into rectangles and add them up. Next, work out what one tile covers including its grout joint: (tile width + grout) × (tile length + grout) ÷ 144 gives square feet. Multiply the area by 1.10 for a 10% waste allowance, divide by the coverage of one tile and round up. Divide the tile count by the pieces per box and round up again to get the number of boxes.
Worked example: a 10 × 12 ft bathroom floor
The floor is 120 sq ft. A 12 × 24 in tile with a 1/8 in grout joint covers (12.125 × 24.125) ÷ 144 = 2.03 sq ft. With 10% waste you need 132 sq ft ÷ 2.03 = 64.98, so 65 tiles. At 8 tiles per box that is 8.1 boxes, so you buy 9 boxes.
How much extra tile should I buy?
Add 10% for a straight layout in a simple room. Add 15% for diagonal or herringbone patterns, rooms with many cuts, and large-format tile (24 in and up), which breaks more easily when cut. Natural stone and handmade tile vary more from piece to piece, so many installers add 15–20%. Keep a few spare tiles after the job: finding the same color batch years later is difficult.
Common tile sizes and coverage
Coverage per tile before grout: 12 × 12 in = 1 sq ft; 12 × 24 in = 2 sq ft; 24 × 24 in = 4 sq ft; 6 × 24 in plank = 1 sq ft; 3 × 6 in subway = 0.125 sq ft (8 per sq ft); 4 × 4 in = 0.111 sq ft (9 per sq ft). Grout joints make each tile cover slightly more, so the real count is a little lower — the calculator includes this.