Why a tile order comes out short

Published 2026-09-04

A tile order is a division: the floor, divided by one tile. Orders that come out wrong have almost always got that division right. What goes wrong is the three things that are not a division — where the wastage goes, when the rounding happens, and the fact that the shop does not sell tiles.

And when it goes wrong, what runs short is usually the money rather than the tiles.

One bathroom, all the way through

A bathroom floor of 2.4 m by 1.6 m — 3.84 m². Tiles 600 × 300 mm, so 0.18 m² each, sold eight to a box. Straight lay.

The bare floor takes 22 tiles: 3.84 divided by 0.18 is 21.3, and a third of a tile is a tile. Every other number in the table is one of the three complications working on that 22.

Wastage goes on the area, before the rounding

The usual method is to work out the tiles, round up, then add ten per cent for cuts. That is 22 tiles plus ten per cent — 24.2 — rounded up again: 25 tiles, which is four boxes and 32 tiles.

Applying the same ten per cent to the area instead gives 4.22 m², which rounds to 24 tiles — three boxes.

One box, eight tiles, 1.44 m² of tile, paid for and never opened. The two roundings are the whole of the difference: the first bought a fraction of a tile, the ten per cent then charged wastage on that fraction, and the second rounding bought another fraction on top. The error only ever runs one way, and it runs upward.

The layout decides the percentage

The three figures are conventions rather than physics, and they differ because of what happens at the walls. A straight lay loses the strip the last row is cut down to. A diagonal lay loses two triangles from every perimeter tile. A herringbone perimeter is almost entirely cuts, which is why it wants twice the allowance of a straight lay.

Note what that costs here: choosing a diagonal lay adds one tile to the requirement and a whole box to the bill. Rooms with alcoves, a boxed pipe or a shower tray to cut around want more than the table figure, whatever the layout.

You do not buy tiles, you buy boxes

The box size is the shop’s decision, and on a small room it moves the bill more than the layout does. Same floor, same diagonal lay, the same 25 tiles needed:

Nothing about the room has changed, and the number of tiles paid for swings between 30 and 36. None of it depends on the size of the job either: a floor that needs 91 tiles, from a shop selling ten to a box, is ten boxes and a hundred tiles. Somebody who budgeted for 91 is short of money, not short of tiles, and finds out at the till.

Laminate is sold the same way, by the pack rather than the plank, and the laminate calculator rounds it the same way.

What is actually left over

Spare is counted against the room, not against the padded area. Three boxes is 24 tiles, the floor itself takes 22, and the spare is two. The ten per cent is not spare on top of that: it is expected to be cut up and thrown away, and counting offcuts as spare would promise pieces nobody can use.

It is the number worth reading last, because it answers the question people actually have.

Keep a box back

Two whole tiles is thin cover for a bathroom floor. Lines get discontinued, and a line that survives is a different batch by then — a slightly different colour, and on rectified stock a slightly different size. A tile cracked three years from now is either replaced from the cupboard or not replaced at all.

Where a repair would matter — a bathroom, an entrance hall, anywhere the floor cannot simply be lifted — order a whole extra box and leave it shut. On this floor that is one box, 1.44 m², against the alternative of relaying the room.

The order to do it in

Measure the floor. Multiply the area by the wastage. Divide by one tile. Round up, once. Divide by the box size and round up again — that second number is where the bill comes from. Then read the spare count and decide whether you can live with it.

The tile calculator works in that order.