Tile calculator for walls and floors

How the tile calculator gets its numbers

The calculator adds up every area you enter, adds your wastage, and divides by the space one tile takes up including its joint. It rounds that up to whole tiles, then to whole boxes. Adhesive and grout are worked out on the area you are actually tiling, using the coverage figures BAL publishes on its own data sheets, and rounded up to whole bags.

Each tile is counted with half a joint on every side. A 600 × 600mm tile with a 3mm joint covers 603 × 603mm of wall or floor, which is why the count comes out slightly lower than dividing by the bare tile.

A worked example, using the calculator's starting figures: a kitchen floor 3.6m by 2.8m, in 600 × 600mm porcelain 9mm thick, with 3mm joints, laid straight with 10% wastage. The tiles come in packs of 3 covering 1.08m², which is how Wickes packs its 600 × 600mm porcelain.

• Area: 3.6 × 2.8 = 10.08m². Plus 10% = 11.088m².
• One tile with its joint: 0.603 × 0.603 = 0.3636m².
• Tiles: 11.088 ÷ 0.3636 = 30.5, so 31 tiles.
• Boxes: 1.08m² ÷ 0.36m² = 3 tiles a box. 31 ÷ 3 = 10.3, so 11 boxes, which is 33 tiles.
• Adhesive: a floor laid with a 10mm notched trowel on a solid bed, at 3.85kg/m². 10.08 × 3.85 = 38.8kg, so 2 bags of 20kg.
• Grout: 0.126kg/m² for this tile and joint (the formula is further down). 10.08 × 0.126 = 1.27kg, so one 2.5kg bag.

How many extra tiles to order

Order 10% extra for a straight layout and 15% for diagonal. B&Q's tiling calculator recommends at least 10% for errors, damage and obstacles, and Wickes's tile pages say to add 10% for breakages and repairs. Tiles360's tile calculator suggests raising that to 15% for diagonal or complicated layouts.

Herringbone is where the advice splits. The calculator offers 15% for it, the figure Tiles360 gives for complicated layouts. NC Tiles works on 25% for herringbone and also suggests more than 10% for large format tiles, because every cut on a big tile wastes a lot of tile. The calculator's pattern options stop at 15%, so for a layout with lots of cuts against walls, units or a bath, add the extra yourself. At 25%, the example floor needs 35 tiles, or 12 boxes.

On the example floor, switching from straight to diagonal at 15% moves the count from 31 tiles to 32. The box count stays at 11, because the extra tile was already sitting in the last box. That happens often: rounding up to whole packs quietly gives you some of the wastage you allowed for.

• Keep a few spares back. A cracked tile in two years' time is easy to replace if you kept three from the same batch, and close to impossible if the range has gone.
• Buy it all at once. Wickes warns that ceramics and stone vary between production batches, so a top-up order can look different from the first lot. Check the batch number on every box before you open one.

How much tile adhesive per square metre

Adhesive use depends on the trowel notch and on whether you need a solid bed, meaning full contact with no voids behind the tile. The calculator uses the coverage table on the BAL Rapid-Flex One data sheet, a 20kg flexible tile adhesive for walls and floors. BAL gives its coverage in square metres per kilo, and the calculator turns each figure into kilos per square metre.

The same sheet sets the minimum trowel by job: 4mm notched for mosaics up to 100 × 100mm, 6mm for internal walls with tiles up to 300 × 300mm, a 6mm solid bed in wet areas, and a 10mm notched solid bed for floors and anything outside. BAL also says larger tiles should be back buttered with a thin layer of adhesive, which uses more than its table allows. On the example floor, two bags leave barely a kilo spare, so with 600mm tiles a third bag is sensible.

These are BAL's figures for one product. Other makers publish their own. Weber's weberset SPF lists 3kg/m² with a 6mm notch, and 9kg/m² for a 6mm thick solid bed. The calculator only offers BAL's four rates. For a different adhesive, multiply your area by the kg/m² on its bag or sheet.

How much grout you need

Grout fills the joints, so the amount depends on how many metres of joint there are and how wide and deep they are. Small tiles have far more joint per square metre than big ones. The calculator works it out with this formula, using millimetres throughout:

Grout (kg/m²) = (tile length + tile width) ÷ (tile length × tile width) × tile thickness × joint width × 1.4

The 1.4 is the density of mixed grout. BAL gives the specific gravity of its Grout Flex as 1.40. The formula lands within about 2% of the coverage table on the BAL Micromax3 Eco data sheet: BAL gives 8.01m² per kg for 600 × 600 × 9mm tiles with 3mm joints, and the formula gives 7.94. Micromax3 comes in 2.5kg, 5kg and 10kg bags.

A wall example: a bath surround made of one wall 1.7m by 1.8m and two ends 0.7m by 1.8m, in 600 × 300mm wall tiles 8.5mm thick with 2mm joints, 10% wastage, packs of 6 covering 1.08m².

• Area: 3.06 + 1.26 + 1.26 = 5.58m².
• Tiles: 5.58 × 1.1 ÷ (0.602 × 0.302) = 33.8, so 34 tiles, which is 6 packs of 6.
• Adhesive: a wet wall, so a 6mm solid bed at 2.5kg/m². 5.58 × 2.5 = 13.95kg, one 20kg bag.
• Grout: (600 + 300) ÷ (600 × 300) × 8.5 × 2 × 1.4 = 0.119kg/m². 5.58 × 0.119 = 0.66kg, one 2.5kg bag with plenty to spare.

Which joint width to use

Use at least 2mm on walls and 3mm on floors. BAL's adhesive data sheet gives those minimums, following the British Standard for tiling, BS 5385, and says the joint may need to be wider depending on the tile and where it is going. Joints that are too tight leave the tiles nowhere to move as the building heats and cools.

Wider joints use a lot more grout. Going from 2mm to 4mm doubles the grout, and a thicker tile adds more again, because the joint is deeper. For rustic or handmade tiles that vary in size, check the tile maker's recommended joint before you buy the grout, and make sure the grout you choose is rated for that width. BAL sells a separate wide joint grout for joints of 3mm to 20mm.

Measuring up and what is not included

Measure each wall or floor as its own rectangle and add it as a separate area. Take off windows, doors, a bath panel you are not tiling and fitted units, either by measuring the wall in sections around them or by taking them off before you type in the size. An L-shaped floor is two rectangles.

Check what the wall can carry. BAL limits tiling onto plaster to 20kg per m² and onto plasterboard to 32kg per m², and does not recommend either in commercial wet rooms or for domestic power showers. Large porcelain plus a solid bed of adhesive can get close, so check the tile weight on the box.

Once it is grouted, BAL's sheet, citing BS 5385, says not to use a new shower for at least two weeks. Worth telling the customer before they book the plumber back.

If you are pricing the job, the quote template lists the tiles, adhesive, grout and labour for the customer, and the day rate calculator shows what your labour needs to cover.

• Left out: primers, tanking for wet areas, levelling, backer boards, trims, silicone and movement joints.
• No prices. Tile prices vary too much by range and merchant to be worth guessing.
• Coverage is a guide. BAL notes its figures vary with how flat the surface is and the trowel used. An uneven wall eats adhesive.

Common questions