Tile calculator: tiles, boxes, adhesive and grout per m²

The number of tiles is the area plus a wastage allowance, divided by the area of one tile. For 10 m² of 60 × 60 cm tiles laid straight with 10% extra: 10 × 1.1 ÷ 0.36 = 31 tiles.

Tiles: the area plus a wastage allowance

Tiles cut along walls and around obstacles leave offcuts you cannot reuse, so you order more than the area. UK retailer Tile Mountain advises a 10% extra tile allowance for a straight lay and 15% for diagonal, brick bond and herringbone patterns. The calculator uses those two figures, and lets you enter your own if the room has lots of cuts.

Example: a 20 m² kitchen floor in 60 × 60 cm tiles, 0.36 m² each. Straight lay: 20 × 1.10 = 22 m², and 22 ÷ 0.36 = 61.1, so 62 tiles. Diagonal: 23 m² and 64 tiles. For 10 m² of 30 × 30 cm tiles laid straight, 123 tiles.

Boxes: round up to whole boxes

Tiles are sold by the box, with the coverage printed on the pack. With boxes of 1.44 m² (four 60 × 60 tiles), 22 m² takes 22 ÷ 1.44 = 15.3, so 16 boxes. Always work from the area to order, allowance included, not from the room area.

Tile adhesive: kilos per millimetre of bed

Mapei UK gives the consumption of Keraflex Maxi S1, a deformable cement-based adhesive in 20 kg bags, as 1.2 kg/m² per mm of thickness, for beds up to 15 mm. Adhesive use therefore follows the bed you leave under the tile, which depends on the notched trowel and the tile. For the 20 m² floor with a 3 mm bed: 20 × 3 × 1.2 = 72 kg, so 4 bags; with a 6 mm bed, 144 kg, 8 bags. Enter your own bed thickness in the calculator; it gives no default, because no figure links a trowel size to a bed depth on this sheet.

The same sheet asks for back-buttering, spreading adhesive on the back of the tile as well, for exterior installation, large format tiles, all floor applications and pools or tanks. Tiles can be adjusted within 45 minutes and should be kept out of rain for at least 24 hours. Each bag is mixed with about 5.0 to 5.4 litres of water for the grey version.

Grout per m² by tile size and joint width

Worked out with the formula on the Mapei Keracolor FF data sheet (factor 1.5), unrounded. Example: 60 × 60 cm tiles 10 mm thick with a 3 mm joint need 0.15 kg/m², or 3.0 kg for 20 m².

Grout: the manufacturer's formula

Grout fills the gap between tiles over their full thickness. Mapei UK's data sheet for Keracolor FF, a grout for joints up to 6 mm sold in 25 kg bags, gives the formula: kg/m² = (A + B) ÷ (A × B) × C × D × 1.5, where A and B are the tile length and width, C its thickness and D the joint width, all in millimetres. For 30 × 30 cm tiles 8 mm thick with a 3 mm joint: 600 ÷ 90,000 × 8 × 3 × 1.5 = 0.24 kg/m², or 4.8 kg for 20 m². Large 60 × 60 cm tiles 10 mm thick need only 0.15 kg/m², because there are fewer joints per square metre.

Skirting

Skirting is counted in linear metres: the room perimeter minus door widths. A 5 × 4 m room has 18 m of perimeter; with 2.4 m boards, that is 8 boards. No allowance is added here, as the sources used give none.

Common mistakes

• Ordering the exact area: the last row comes up short.
• Using the straight-lay allowance for herringbone.
• Skipping back-buttering on a floor: the data sheet asks for it on all floor applications.
• Leaving out tile thickness when working out grout.
• Counting boxes from the room area instead of the area to order.

Before tiling, the screed calculator gets the floor flat, the concrete calculator covers an outdoor base, and the block calculator handles a new wall.