How Many Tiles Do I Need? Measuring, Waste and Grout

Measure the area

Measure the floor or wall in its simplest shapes. An L-shaped room is two rectangles; a bay can often be approximated as a rectangle plus a triangle. Add the areas together. For walls, subtract windows and doors only if they are large — small openings generate almost as much waste in cuts as they save.

Include the grout joint

Every tile effectively occupies its own size plus one joint width in each direction. For a 12 × 12 in tile with a 1/8 in joint, that is 12.125 × 12.125 = 147 in² rather than 144 in². The difference is small for large tiles but becomes significant for small tiles and wide joints.

Choose a waste allowance

These are common rules of thumb rather than fixed rules. Large-format tile, tiles with a pattern that must be matched, and rooms with many corners all push waste up.

Round up — then buy from one batch

Round tiles up to whole tiles and then to whole boxes. Tiles are produced in batches that can differ slightly in shade and size, so buy everything at once and keep a few spares for repairs.

Don't forget grout and adhesive

Grout quantity depends on the joint volume — tile size, joint width and tile thickness. Adhesive coverage depends on the trowel notch and is given by the adhesive manufacturer. Check both before you start so you do not run out halfway through a wall.

Worked example: a bathroom floor

In metric: a 2.4 × 3.0 m floor is 7.2 m². A 600 × 300 mm tile with a 3 mm joint occupies 0.603 × 0.303 = 0.1827 m², so you need 7.2 ÷ 0.1827 = 39.4 tiles, or 44 with 10 % waste.

• Floor 7 × 9 ft = 63 ft².
• Tile 12 × 24 in with a 1/8 in joint: (12.125 × 24.125) ÷ 144 = 2.031 ft² per tile.
• Tiles needed = 63 ÷ 2.031 = 31.0; with 10 % waste = 34.1, so 35 tiles.
• If a box holds 8 tiles: 35 ÷ 8 = 4.4, so buy 5 boxes (40 tiles) — the 5 spares are useful for future repairs.

Plan the layout before you buy

Dry-lay a row of tiles or draw the layout to scale before ordering. Where you start the first tile decides how big the cut pieces at the walls will be. Starting from a centre line usually gives equal cuts on opposite walls; shifting the layout by half a tile can avoid thin slivers at a doorway or along the most visible wall.

Thin slivers are not only unattractive, they also break easily during cutting, which adds waste. If your plan shows many cuts narrower than about a third of a tile, allow extra or adjust the layout.

Large-format and patterned tiles

Large tiles cover more area per piece but leave larger offcuts that often cannot be reused elsewhere. They also need a flatter substrate: unevenness shows up as lippage, where neighbouring tile edges sit at different heights. Tile manufacturers and industry bodies publish flatness requirements and recommended maximum offsets for running-bond layouts, which can affect both the layout and how much you buy.

Tiles with a printed pattern, such as wood-look planks, look best when similar pieces are not placed side by side. Mixing tiles from several boxes as you lay them hides repeats but means you are less able to choose cuts, so allow a little more waste.