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Bathroom tile calculator: tiles, adhesive, grout, tanking and backer board
Work out the tiles, adhesive, grout, trim, backer board and tanking for a bathroom from the room size, the tile you have picked and how much of the walls you are tiling. It uses the same approach as TradeCalc in the Hard Hat Way app: tiles counted from the tile and joint size, adhesive by bed thickness, grout by the joint formula, and the wet zone set out to BS 5385 so the tanking and boards go where the water is.
What you need
Tiles
- Tiles, 300 × 300 mm
- 283 tiles23.4 m² of wall and floor at 11.0 per m² with 2 mm joints and 10% waste
- Wall area to tile
- 19.2 m²less 1.9 m² door and 0.5 m² of window
- Floor area to tile
- 4.2 m²less 0.81 m² under the tray
- Spacers
- 3 × packs of 2502 a tile
Adhesive and grout
- Wall adhesive
- 3 × 20 kg bags8 m² a bag on a 6 mm notch; flexible S1 powder in a showertradecalculator.co.uk adhesive coverage; BAL / Weber datasheets
- Floor adhesive
- 1 × 20 kg bags5.2 m² a bag on a 10 mm notch
- Grout
- 1 × 5 kg bags4.0 kg: 0.17 kg/m² for 300 × 300 × 8 mm tiles with 2 mm jointsGrout kg/m² = (L + W) ÷ (L × W) × depth × joint × 1.6 (Mapei); BAL Grout CG1 to 3 mm
- Primer (SBR or acrylic)
- 1 × 5 L tubabout 50 m² a tub diluted; the tanking kit has its own
Tanking and boards
- Tanking kits
- 1 kit3.6 m² of wet zone (two 900 mm walls to 2 m) at 4 m² a kit, two coatsMapei shower waterproofing kit (4 m²); BS 5385-1 wet zones
- Tanking tape
- 5.8 m needed, 10 m in the kits every corner, the wall-to-tray or bath junction and pipe penetrations, 50 mm overlaps
- Cement backer board, 1200 × 600 mm
- 6 sheetswet-zone walls only (3.6 m² plus waste); the rest of the room can be tiled on sound primed plaster or MR boardBS 5385-1:2018
- Tile weight on the wall
- 18.7 kg/m²8 mm ceramic plus adhesive and grout, against 50 kg/m² for cement boardBS 5385-1:2018 limits: plasterboard 32 kg/m², plaster 20 kg/m²
Finishing
- Tile trim
- 2 × 2.5 m lengths3.2 m of exposed edge: 1 window reveal, add external corners and niches yourself
- Anti-mould silicone
- 3 tubesone per 3 m of perimeter, minimum two, for the bath, tray and floor junctions
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
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Or claim your free profile link nowHow the maths works
- Wall area is the perimeter × ceiling height less 1.9 m² for the door and the window area you enter. Half height tiles to 1.2 m less the door. Wet zone only tiles the walls set out below. The floor loses the shower tray (0.9 × 0.9 m) or the bath footprint (0.7 m × its length) unless it is a wet room.
- Tiles per m² are 1,000,000 ÷ ((tile length + joint) × (tile width + joint)), so 300 × 300 mm tiles with 2 mm joints are 11.0 per m². Waste is 10% for straight or brick bond and 15% for diagonal. BS 5385-1:2018 asks for joints of at least 2 mm on tiles under 0.1 m² and 3 mm from 0.1 to 1 m².
- Adhesive is split wall and floor by bed: tiles up to 300 mm on a wall go on a 6 mm notch at about 2.5 kg/m² (8 m² per 20 kg bag); floors and 300–600 mm tiles on a 10 mm notch at about 4 kg/m² (5.2 m² a bag); 600 mm and over need a solid bed at 6–7 kg/m² (3 m² a bag). Use a flexible S1 powder in a shower.
- Grout is (L + W) ÷ (L × W) × tile thickness × joint width × 1.6 kg/m² (Mapei), so 300 × 300 × 8 mm tiles with 2 mm joints use about 0.17 kg/m², a fraction of the old 8 m² per 5 kg rule. Packs are 5 kg; CG1 grout such as BAL Grout is fine to 3 mm joints.
- The wet zone follows BS 5385-1 and the tanking guides: a shower enclosure is tanked at least 1,800 mm above the tray (2 m here) on its two walls; a bath with a shower over it three walls to 2 m along the bath; a bath alone three courses above the rim; a wet room the whole floor and every wall. Tanking kits (Mapei type) cover 4 m² in two coats and include 10 m of tape for every corner and junction.
- Cement backer board goes on the wet-zone walls only, in 1200 × 600 mm sheets with waste; the rest of the room is tiled on sound primed plaster or moisture-resistant board. BS 5385-1:2018 dropped plywood as a tiling background and limits plasterboard to 32 kg/m² and skimmed plaster to 20 kg/m², so the tile weight (thickness × about 2 kg/m² per mm for ceramic, 2.4 porcelain, 2.7 stone, plus adhesive and grout) is checked against the substrate.
- Trim goes on exposed edges: the top of half-height tiling, the edges of a wet zone and 3.2 m per window reveal, in 2.5 m lengths. Silicone is one tube per 3 m of perimeter, minimum two. Spacers are two a tile in packs of 250.
Worked example
A 2.5 m × 2 m bathroom with 2.4 m ceilings, tiled floor to ceiling in 300 × 300 × 8 mm ceramic with 2 mm joints, a 900 mm shower enclosure and 0.5 m² of window has 19.2 m² of wall and 4.2 m² of floor: 283 tiles with 10% waste, 3 bags of wall adhesive and 1 of floor, 1 bag of grout (4.0 kg), 1 tanking kit for the 3.6 m² wet zone, 6 sheets of backer board on the shower walls and 3 tubes of silicone.
Questions people ask
How many tiles do I need for a bathroom?
Add the floor area to the wall area you are tiling, take off the door, window and whatever sits under the bath or tray, put 10% on for cuts (15% diagonal), and divide by the area of one tile plus its joint. At 300 × 300 mm with 2 mm joints that is 11 tiles per m². Tiles are sold by the box, so enter the tiles per box to round up to whole boxes.
How much adhesive and grout?
Adhesive depends on the bed: about 2.5 kg/m² on walls with tiles up to 300 mm (8 m² per 20 kg bag), 4–5 kg/m² on floors and 300–600 mm tiles (about 5 m² a bag), 6 kg/m² or more for large format on a solid bed (3 m² a bag). Grout is (length + width) ÷ (length × width) × thickness × joint × 1.6 kg/m²: 300 × 300 × 8 mm tiles with 2 mm joints need only 0.17 kg/m², so a 5 kg bag does a whole bathroom, while mosaics with 3 mm joints need ten times that.
What is tanking and where does it go?
Tanking is a waterproof membrane, brushed or rolled on in two coats with tape in every corner and junction, that goes under the tiles so water through the grout cannot reach the wall or floor. BS 5385-1 and the makers set the zones: a shower enclosure at least 1,800 mm above the tray (or 200 mm above the shower head), a bath surround at least three courses above the rim, and a wet room the whole floor and every wet wall. A Mapei-type kit covers about 4 m², not 8.
Can I tile onto plasterboard?
Yes outside the wet zone, primed, up to 32 kg/m² of tile and adhesive; skimmed plaster only takes 20 kg/m². In a shower BS 5385-1:2018 prefers cement backer board, and it no longer allows plywood as a tiling background at all. A 10 mm porcelain tile with adhesive is about 27 kg/m², so it is fine on board but not on skim; the calculator checks your tile against the substrate you pick.
Do I need tile backer board on every wall?
No. Board the wet-zone walls (the shower enclosure or the bath walls) and tank them; the rest of the room can go straight onto sound, primed plaster or moisture-resistant plasterboard. Boarding every wall in a 2.5 × 2 m bathroom would be about 30 sheets where 6 do the job.
How much does a bathroom fit cost?
It depends on the suite, the tiles, whether the plumbing and electrics are moving and where you live, so get a few quotes. Our bathroom fit cost guide explains what drives the price, and any electrical work in a bathroom is notifiable under Part P and must be done by a registered electrician.
Related calculators
How many tiles, boxes, bags of adhesive and grout an area needs by tile size, joint width, layout and where it is going, from the BAL, Weber and Mapei datasheets.
Plasterboard calculator: boards, screws, tape, Easi-Fill and studsHow many sheets of plasterboard a wall, partition or ceiling needs by board size, with the screws, joint tape, Easi-Fill and metal or timber studs to go with them.
Radiator BTU calculator: heat loss for a room and the rad size to fitHow many watts and BTU a room needs on a −3 °C day, and which catalogue rad (K1, K2 or K3) covers it at your flow temperature.
Pipe calculator: copper or plastic pipe, clips, fittings, solder, lagging and a flow checkHow many 3 m lengths, clips, fittings, inserts, solder and lagging a run needs, and whether the bore will carry the outlets on it without running too fast.
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