Divide the room area by the area of one tile (including the grout gap on each edge). For example, a 4 m × 3 m room is 12 m². A 60 cm × 30 cm tile with a 3 mm grout gap covers (0.603 × 0.303) = 0.1827 m² per tile. You need ceil(12 / 0.1827) = 66 tiles without waste. Always add a waste factor (typically 10%) to cover cuts and breakage: 66 × 1.10 = 73 tiles. Round up to whole boxes.

Use 10% for a straight lay (rows parallel to walls). Use 15% for a diagonal lay (tiles at 45°) because every edge tile must be cut at an angle. Use 20% for complex patterns like herringbone, pinwheel, or mosaic.

Add an extra 5% for rooms with many internal corners, doorways, or columns. A small extra batch of the same tile (especially a custom colour) is almost impossible to match exactly later — always buy slightly more than you think you need.

The grout gap increases the effective area each tile occupies on the floor. With a 3 mm gap, a 600 mm × 300 mm tile takes up 603 mm × 303 mm = 0.1827 m², rather than 0.18 m² without a gap. For a 12 m² room that is about 65.7 vs. 66.7 tiles — a small difference for large floor tiles.

For small mosaic tiles (e.g. 50 mm × 50 mm with a 2 mm gap), the grout gap adds 4% to each tile dimension and significantly changes the count. Always enter the actual grout gap for accuracy.

Split the room into two rectangles and calculate each section separately, then add the totals. For example, an L-shaped room can be split into a 5 m × 3 m section and a 2 m × 2 m section: 15 m² + 4 m² = 19 m² total. Plan the tile layout from the centre of the main area outward so that cut tiles at the edges are equal in size. Avoid starting from a wall — you may end up with a narrow sliver (less than half a tile width) at the opposite edge.