How to calculate tiles, adhesive and grout

For a floor, the area is simply length × width, minus anything that won't be tiled: a bathtub on feet is tiled under, a built-in bath or a shower tray is not.

For walls, work around the room: the perimeter, 2 × (length + width), multiplied by the tiling height. Then subtract the parts that stay bare — the door, a mirror that will be fixed directly to the wall, a bath panel or a shower enclosure. If you only tile some walls, calculate just those walls or add the untiled ones to the openings.

Waste is not just broken tiles: it's every cut piece at the edge that can't be reused elsewhere. The larger the tile, the bigger each offcut — which is why large formats need a higher allowance even in a simple room.

Keep a few spare tiles after the job. Tiles from a later production run can differ in shade and calibre, and replacing a cracked tile years later is much easier with one from the original batch.

Divide the area to order by the area of one tile and round up: 13.2 m² of 60 × 60 cm tiles (0.36 m² each) is 37 tiles. Tiles are usually sold by the box, so divide the area by the coverage printed on the box and round up again.

Always buy the whole quantity in one order and check that every box shows the same batch (shade) and calibre (size) code.

Adhesive consumption is set by the notched trowel. As a rough guide, small wall tiles are laid with a 6 mm notch, standard floor tiles with 8 mm, and large tiles with 10–12 mm. Consumption rises with the notch — from roughly 2.5–3 kg/m² for small tiles to 5–6 kg/m² or more for large formats laid with back-buttering.

The data sheet on the bag gives the consumption for that product and notch. Use a flexible adhesive outdoors, on underfloor heating and on substrates that move, such as wooden floors.

Grout manufacturers use this formula: consumption (kg/m²) = (A + B) ÷ (A × B) × C × D × 1.6, where A and B are the tile length and width in mm, C the tile thickness in mm and D the joint width in mm. The 1.6 is the density of the hardened grout.

The formula shows why tile size matters so much: 60 × 60 cm tiles with 3 mm joints need only about 0.13 kg/m², while 20 × 20 cm tiles with the same joints need about 0.38 kg/m² — three times as much. Add around 10% for mixing and cleaning losses.

A 3 × 2 m bathroom, walls tiled to 2 m with 30 × 60 cm tiles, floor in 60 × 60 cm tiles.

• Walls: 10 m perimeter × 2 m = 20 m², minus 2 m² of openings = 18 m²; with 10% waste, 19.8 m² — 110 tiles.
• Floor: 6 m² minus a 1.2 m² shower tray = 4.8 m²; with 10% waste, 5.3 m² — 15 tiles.
• Adhesive: about 18 m² of wall at 3–4 kg/m² and 4.8 m² of floor at 4 kg/m² — roughly 75–90 kg, so three or four 25 kg bags (check the data sheet).
• Grout: walls (30 × 60, 8 mm thick, 2 mm joints) ≈ 0.13 kg/m² and floor ≈ 0.13 kg/m² — about 3 kg in total, plus 10%.

• Waterproofing in showers and wet zones before tiling, as specified by the system manufacturer.
• Primer on absorbent or very smooth substrates.
• Edge trims, corner profiles, movement joints and silicone for the corners and the joint between floor and wall.
• A tile levelling system for large formats, and spacers matched to the joint width.