How it is worked out
Each tile covers its own size plus one grout joint along and one across. The area divided by that gives the tile count, rounded up, and the extra is added on top.
Tiles are sold by the box and usually priced by the square metre, so the result shows both.
Buy the whole job at once where you can. Batches of the same tile can differ slightly in shade and size.
Worked example
The numbers this page opens with: length of the area 3 m, width or height of the area 2.4 m, tile length 600 mm, tile width 600 mm, grout joint 3 mm, extra for cuts and breakages 10 %, tiles per box 4 tiles.
The answer: area 7.20 m²; tiles, exact 20 tiles; tiles with 10% extra 22 tiles; area of tile to buy 7.92 m²; boxes of 4 6 boxes.
Questions
How much extra should I allow?
It depends on the layout and the room. A plain grid in a square room wastes the least. Diagonal and herringbone layouts, large tiles in small rooms and lots of corners all waste more. Ask your tiler what they want on site.
Does it include adhesive or grout?
No. Coverage for those is printed on the bag and depends on the trowel and the joint.