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How many tiles do I need for the bathroom — floor and walls?

Work out the floor area plus the wall area (perimeter × tile height), add 10% wastage for a straight lay and 15% for a diagonal one, then round up to whole boxes.

Floor tiles: the room's internal area

The floor area of a bathroom is measured internally from wall to wall at floor level. The tiles need to cover the whole floor, including under a freestanding toilet and a vanity unit — so only deduct where something built-in and permanent stands, typically a tiled shower niche or a bath on legs with a closed front panel.

Example: The bathroom measures 2.40 m × 3.00 m internally. Floor area = 7.20 m². With 10% wastage: 7.20 × 1.10 = 7.92 m² of floor tiles to order.

If the bathroom has a fall towards a floor drain, it doesn't change the area to any meaningful degree — the fall is typically 1-2 cm per metre and disappears in the rounding. What it does cost, though, is extra cuts around the drain, which is one of the reasons 10% is a safer starting point in a bathroom than the 5-7% you can get away with in a dry living room.

Wall tiles: perimeter times height

The wall area is worked out as the room's internal perimeter multiplied by the height the tiles are to reach. This is a far more reliable method than measuring each wall separately, because the perimeter automatically accounts for all four walls at once.

Example: The room is 2.40 × 3.00 m. Perimeter = 2 × (2.40 + 3.00) = 10.80 m. The tiles are to reach 2.20 m. Wall area = 10.80 × 2.20 = 23.76 m².

If the tiles only need to go up behind the shower and the basin, work those walls out separately: a shower wall 1.00 m wide at 2.20 m high is 2.20 m², and a splashback behind the basin 1.20 m wide at 60 cm high is 0.72 m².

Deductions for doors and windows: only deduct if the opening is large. An ordinary door at 0.90 × 2.05 m = 1.85 m² is worth deducting. A bathroom window at 0.60 × 0.80 m = 0.48 m² can be left in — half of it goes to off-cuts around the reveal anyway. Rule of thumb: deduct openings over 1 m², leave smaller ones counted into the area.

With the door deducted: 23.76 − 1.85 = 21.91 m². With 10% wastage: 21.91 × 1.10 = 24.10 m² of wall tiles. If you'd rather keep the door in the sum as extra margin, you land on 23.76 × 1.10 = 26.14 m².

How much wastage on tiles?

Tiles snap when they're cut, and they snap crooked more often than boards do. That's why the wastage percentage on tiles typically sits higher than on a wood floor:

• Straight lay, format up to 30 × 60 cm: 10%. The standard starting point for a bathroom.
• Offset (running bond) or diagonal lay: 15%. Every row starts with a cut piece.
• Large formats, 60 × 60 cm and up: 15-20%. One bad cut costs a whole large tile, and they can rarely be used elsewhere.
• Mosaic on mesh sheets: 5-10%. The mesh can be trimmed between the chips, so wastage is small — but calculate in whole sheets.
• Small rooms with lots of corners, niches and pipe penetrations: add 3-5 percentage points on top. A 3 m² bathroom can easily need more cuts than a 20 m² living room.
• Tiles with directional pattern (wood-grain, marble look): add 5 percentage points on top, because you can't freely flip an off-cut.

The supplier's own guidance always overrides these figures. Manufacturers of large formats and of tiles with a calibration note often specify a concrete allowance in their instructions — and that's the one to follow if it says something different.

From square metres to boxes

Tiles are sold in boxes with a fixed number of tiles, and the label tells you how many square metres the box covers. A typical box covers 0.9-1.5 m² for wall tiles and 1.0-1.8 m² for floor tiles — always check the label.

Finishing the sum: You need 7.92 m² of floor tiles, and the box covers 1.44 m². 7.92 ÷ 1.44 = 5.5 boxes → 6 boxes = 8.64 m². For the walls: 24.10 m² ÷ 1.08 m² per box = 22.3 boxes → 23 boxes = 24.84 m².

Always order the whole tile order in one go. Tiles are fired in batches, and the colour varies between batches — it shows up clearly in a wet shower niche under raking light. The box label carries both a batch number and a shade code; note them down, and keep one unopened box as a spare.

What people forget to count

Tiled skirting. If an 8-10 cm tile skirting is to run along the floor, it's the perimeter again: 10.80 m × 0.10 m = 1.08 m², but it's typically cut from whole tiles, so work it out in running metres and ask the supplier how many metres come out of one box.

Niches and shelves. A shower niche measuring 30 × 60 cm has a base, top, two sides and a back wall — that's easily 0.4-0.5 m² of tiles on an opening that looks like it removes tiles. A niche adds to the total; it doesn't subtract from it.

Grout and adhesive. Grout consumption is small — typically under 1 kg per m² for ordinary wall tiles, more for small tiles with a lot of grout joint. The adhesive is the heavy one: allow 4-6 kg per m² with a notched trowel. Both figures are on the bag, and they depend on the tile size and how flat the substrate is. Large tiles need more adhesive per m² than small ones.

Waterproof membrane. This typically covers the whole floor and the walls up to at least shower height, and it must be laid according to the manufacturer's instructions and the applicable wet-room regulations. That work is worth getting a professional to stand behind — a leaking wet room is the most expensive mistake you can make in a bathroom.

What decides what the tiling ends up costing

The price of a tiling job comes down to five things, and you can work out four of them yourself before asking for a quote:

• How many square metres of wall need tiling. This is the single biggest item — and the one people most often guess wrong on.
• Tile format and type. Mosaic and large-format tiles are both more expensive to install than a standard format: one has many grout joints, the other needs two people to handle it.
• The pattern. A straight lay is fastest; offset, diagonal and herringbone patterns cost both more time and more wastage.
• How much the substrate needs levelling before tiling can start. An out-of-true wall costs more than a tile does.
• Niches, skirtings, pipe penetrations and corners. Every single cut is time.

Ask for a written quote based on your own figures — and give the contractor the floor area, the wall area and the tile height, not just "a 7 m² bathroom". A bathroom with 7.2 m² of floor can have anywhere from 5 to 30 m² of tiles on the walls, and that's where the difference lies.

If you don't have the measurements

Both the floor area and the perimeter come from the same four internal wall measurements. Measure each wall corner to corner at floor level, and measure the diagonal — if it doesn't equal the square root of (length² + width²), the room is out of square, and the tile rows then need to be set out from a chosen main line rather than from the wall.

If you have a measured floor plan, both the area and the wall lengths are already there. HouseSense produces that plan by scanning the room with the iPhone's LiDAR — or by you drawing the room with your finger and typing in the measured centimetres. The app also gives you the walls as an elevation, the wall seen from the front with height and openings, which is exactly the figure you need to multiply the perimeter by.