A tiling job fails in two ways: too few tiles, discovered mid-wall with the batch sold out, or far too many, paid for and never used. Both failures come from the same cause, which is ordering the net area and hoping the rest sorts itself out. The net area is the start of the calculation, not the end of it: cuts at edges and around obstacles consume whole tiles for partial coverage, some tiles break, the pattern wastes more than the straight lay, and a sane job keeps spares for the future. The arithmetic is simple and the allowances are conventional — 10% for a straight lay is the trade’s rule of thumb — but the conventions exist because the losses are real, and this guide states them with the numbers.

Measuring: the area, and what to subtract

Measure each wall or floor in metres, length times width, and add the areas of separate sections. For a floor with a fixed island, a bath, or a built-in unit that will not be tiled under, subtract that footprint — but only when the unit truly sits on the floor and the tile will not run beneath it; tiling under a freestanding bath or a vanity that may one day be replaced is often the right call, and the saved area is small against the cost of a future re-tile. For walls, subtract large openings (a window, a door) and keep small ones: the cuts around a window consume nearly the tiles the opening saves, and subtracting it understates the order.

Measure twice, in two directions, because rooms are rarely square: a wall that is 3.02 m at one end and 2.96 m at the other needs the larger figure, and the difference across a room accumulates into whole tiles. Record the measurements in the unit the tiles are sold in — metric tiles in metres, imperial tiles in feet — and convert before dividing, not after: dividing square metres by square feet is the commonest unit error in the trade, and it understates the order by a factor of ten.

Worked example: 4 m × 5 m floor, 300 mm tiles

Room area: 4 × 5 = 20 m². Tile area: 0.3 × 0.3 = 0.09 m². Exact tiles: 20 / 0.09 = 222.2. Apply 10% wastage to the exact count: 222.2 × 1.10 = 244.4, rounded up to 245 tiles. Boxes of 10: 245 / 10 = 24.5, rounded up to 25 boxes, or 250 tiles — the rounding at the box step adds 5 tiles, which join the spares. The rule is to round once, at the end: rounding the net count up first and then applying wastage double-counts the rounding and over-orders by a tile, while rounding down at any step produces a shortage — and the cost of the shortage (a second delivery, a different batch) dwarfs the cost of the surplus.

The same room in a diagonal lay at 15% wastage needs 222.2 × 1.15 = 255.6, rounded up to 256 tiles, or 26 boxes. In herringbone at 20%: 222.2 × 1.20 = 266.7, rounded up to 267 tiles, or 27 boxes. The pattern decision, made for appearance, moves the order by two boxes — which is why the pattern should be settled before ordering, not after the tiles arrive.

Wastage: what the allowance covers, by pattern

Cuts are the main loss: every tile that meets an edge, a corner, an obstacle or a fitting is cut, and the offcut is usually unusable at full size. Breakage is the second: tiles chip in transport, in cutting, and underfoot before the adhesive sets, and natural stone and large-format porcelain break more than small ceramic. Measurement error is the third, and it is one-directional in practice — rooms are larger than measured more often than smaller. The allowance covers all three, and the pattern sets its size: a straight grid wastes the least, because offcuts from one edge can fill another; brick bond wastes more, because the half-offset turns every other row’s end cut into waste; diagonal wastes more again, because every perimeter cut is angled and the triangular offcuts fit nowhere; herringbone wastes most, because every tile is cut at 45° and the pattern has no reusable offcut geometry at all.

The conventional allowances — 5–10% straight, 10–15% brick bond and diagonal, 15–20% herringbone — assume a competent tiler and a rectangular room. An L-shaped room, a room with many obstacles, or a first-time tiler each push the allowance toward the top of the range, and a room with a complex border or a feature panel needs its own count, tile by tile, rather than a percentage. The tile calculation on this site applies the percentage to the measured area; the pattern and the room’s complexity are the judgement it cannot make, and the allowances above are the judgement to apply.

Batches and spares: the decision that outlasts the job

Tiles vary between production batches — shade (tone) and calibre (size) — by amounts invisible box to box and obvious wall to wall. Every box carries a batch code, and the whole order should carry one code: check it on delivery, before the adhesive is mixed, because a mixed-batch wall cannot be fixed by reordering the first batch once it is sold out. The same logic makes spares non-optional: keep 5–10% of the order, a full box at minimum, stored flat and dry, for the cracked tile, the dropped pan, the future repair. A spare tile from the original batch is a ten-minute repair; a tile from a later batch is a visible patch.

Buy the spares as part of the original order, not as a later thought: the batch will not be available later, and the surplus from box rounding is rarely a full box of usable spares on its own. On a 25-box order, the 4 surplus tiles from rounding plus a dedicated spare box is the shape of a sensible purchase — the surplus covers the last cuts, the spare box covers the future.

Adhesive and grout: sized from coverage rates

Adhesive and grout are not sized from the tile count but from coverage rates printed on the bag. Tile adhesive coverage depends on the trowel notch and the tile size: a 10 mm notched trowel lays roughly 4–5 m² per 20 kg bag on a flat floor, less on an uneven substrate or with large-format tiles that need back-buttering. Grout consumption depends on the joint width, the joint depth, and the tile size — small tiles have far more joint length per square metre than large ones, so a 100 mm mosaic can take three times the grout per square metre of a 600 mm tile. The bag’s coverage table, applied to the measured area with the actual joint width and tile size, is the estimate; buy one bag beyond it, because running out of grout mid-wall leaves a visible colour difference between batches mixed on different days.

Neither adhesive nor grout is included in the tile calculator’s tile count, and the calculator’s page states that exclusion openly, along with trim and labour. The exclusion is a scope decision, not an oversight: the rates vary by product and substrate, and the bag’s table is the authority. What the calculator gives is the tile order; the adhesive and grout orders are a second calculation from the same measurements, and doing both from the same measured area is how the two orders stay consistent.

Frequently asked questions

How many tiles do I need for a 4 m × 5 m floor?

With 300 mm × 300 mm tiles: 20 m² of floor divided by 0.09 m² per tile is 222.2 tiles exact, and with a 10% wastage allowance applied to the exact count that is 245 tiles, in 25 boxes of 10 (250 tiles). A diagonal lay at 15% needs 256 tiles (26 boxes); herringbone at 20% needs 267 tiles (27 boxes).

What wastage allowance should I add for tiles?

Five to 10% for a straight grid, 10 to 15% for brick bond and diagonal lays, and 15 to 20% for herringbone and complex patterns. Go to the top of the range for L-shaped rooms, rooms with many obstacles, or a first-time tiler. The allowance covers cuts, breakage and measurement error — all three are real, and the percentage is cheaper than a second order.

Why do tiles need to be from the same batch?

Tiles vary between production batches in shade (tone) and calibre (size). The variation is invisible box to box and obvious wall to wall, and a mixed-batch wall cannot be fixed once the first batch is sold out. Check the batch code on every box on delivery, before the adhesive is mixed, and keep spares from the same batch for future repairs.

How many spare tiles should I keep?

Five to 10% of the order, a full box at minimum, stored flat and dry. Spares cover the cracked tile, the dropped pan, and the future repair — and a spare from the original batch is a ten-minute invisible repair, while a later batch is a visible patch. Buy spares as part of the original order.

How much tile adhesive and grout do I need?

Size them from the coverage rates on the bag, not from the tile count. Adhesive coverage depends on the trowel notch and tile size (roughly 4–5 m² per 20 kg bag with a 10 mm notch on a flat floor). Grout depends on joint width, depth and tile size — small tiles have far more joint length per square metre than large ones. Buy one bag beyond the estimate for grout, to avoid a batch colour difference.

Should I subtract doors and windows from the wall area?

Subtract large openings such as windows and doors, but keep small ones in the area. The cuts around a window consume nearly the tiles the opening saves, so subtracting a small opening understates the order. For floors, subtract only fixed units the tile genuinely will not run beneath.