Planning guide

How much extra tile should you order?

Ten real solved floors, and their measured offcut waste — from 1.9% to 46.6%. Why a flat 10% leaves some rooms short, and what to order instead.

Published 3 September 2026

Every tile calculator, every merchant and every forum answer says the same thing: add ten percent.

Ten percent of what, for what, is never explained. And on some floors it is not nearly enough — not because the estimate was careless, but because the arithmetic it came from never described your room in the first place.

What the usual calculation actually does

Almost every online tile calculator works like this: take the floor area, divide by the area of one tile, round up, add ten percent.

For a 3000 × 4200 mm room in 300 mm tile that is 12.6 ÷ 0.09 = 140 tiles, and 154 with the allowance. It is exactly right — 140 pieces is precisely what that floor takes.

Now run the same method on a 2440 × 3050 mm bathroom in 600 mm tile. Area 7.44 m², divided by 0.36 m², is 20.7 — call it 21 tiles, 24 with the allowance.

The real layout needs thirty pieces.

Not because the area calculation was wrong. Because a tile you cut in half is still a tile you bought, and the area method quietly assumes every offcut gets used somewhere else. That floor is five tile positions across by six down; laid out so the cuts are balanced, eighteen of those thirty positions are cut pieces, and almost none of the offcuts is the right shape for another. You would arrive with 24 tiles for a job that needs 30, discover it two thirds of the way through, and go back for more — from a different batch.

Ten real floors, measured

These are ten layouts solved by the planner on this site, each at its balanced start position, each with a 3 mm joint. Offcut waste is the tile area you buy that ends up in the bin, as a percentage of the area you actually lay, assuming no offcut is reused. Short by is how far the area-plus-ten-percent method falls below what the floor really needs.

The spread is not a rounding artefact. It runs from a floor that wastes nothing to a floor that throws away almost half of what it lays, and every one of these is an ordinary rectangular room with an ordinary tile.

A flat ten percent is right in the middle of that range by accident, not by design.

What actually drives it

Three things, in order of size.

How nearly the tile divides the room. This is nearly all of it. A 3620 mm wall takes exactly six 603 mm pitches with 2 mm over; that row has no cut at all. A 2440 mm wall takes four pitches and leaves 28 mm; that row needs a fifth tile to produce a strip, and the other 572 mm of it is scrap. One wall dimension moved by 30 mm can change the answer completely, which is why no rule of thumb can be right for every room.

The ratio of perimeter to area. Cuts happen at the edges. A small room has a lot of edge for its floor, so a bigger fraction of its tiles are cut ones. The worst row in the table above is the smallest room. The best two are among the largest.

Tile size relative to the room. Big tile in a small room is the expensive combination: fewer pieces overall, but a much higher proportion of them cut, and each wasted offcut is large. 600 mm tile in a 2.4 m bathroom wastes 46.6%. The same room in 300 mm tile would waste a fraction of that.

Pattern barely appears in this list, which surprises people. In the table, the same room and tile laid straight and laid at 45° produce the same piece count and the same waste figure — that diagonal layout is the one drawn at the top of this page. The reason the two come out equal is in how this waste is measured, and it is worth knowing exactly where the measurement stops being true.

The honest limit of that number

The waste figures above assume no offcut is ever reused. That is deliberately conservative, and on a real floor it is sometimes pessimistic.

An offcut is reusable when the cut you need somewhere else is the complement of the one you just made — when the two pieces together make one tile, minus a joint. That happens systematically in one specific case: when you start tight against a wall. The strip left at the far wall is exactly what the tile is short by, so cuts on opposite edges complement each other and a careful tiler gets two pieces from one tile down a whole run.

It happens much less when the layout is balanced, because balancing deliberately makes opposite edges equal rather than complementary — two 314 mm cuts do not come out of one 600 mm tile.

So there is a real trade-off, and it is not the one people expect:

• Start at the wall: better offcut reuse, worse cuts. You may save tile and end up with a 28 mm sliver down one side — and a 28 mm strip is a worse piece of tile than it sounds.
• Balanced start: worse offcut reuse, better cuts. You may buy a few more tiles and end up with a floor that looks deliberate.

Two cases break the reuse argument entirely, and both are common:

• Diagonal and herringbone layouts. Perimeter pieces are triangles. A triangle’s complement is another triangle of the opposite hand, and the two rarely both exist in the quantities you need. The drawing at the top of this page has thirty-four cut pieces in nine distinct sizes; almost none of them pair up.
• Directional or shade-variable tile. Wood-look planks, veined stone-look porcelain and anything with a printed pattern cannot be rotated freely, so half the geometrically valid reuses are visually wrong.

So what should you actually order?

Order in pieces, not in square metres, and start from a real piece count rather than an area.

1. Get the piece count for your actual layout. The planner draws it and counts every full tile and every cut piece for your room, tile and pattern. This number already includes cut pieces as whole tiles, so offcut waste is covered — you do not need to add anything for it.
2. Add for breakage and mistakes, not for waste. 5% is realistic for a simple straight lay in a plain room. 10% for a bathroom with a lot of edges, a diagonal, or a fragile tile. 15% if you have not done this before, because the first three cuts of any job are practice.
3. Add attic stock, as whole tiles. Not a percentage — a quantity. Keep at least four or five spare tiles, and more for a large floor. A cracked tile in three years’ time cannot be replaced from a different batch without it showing.
4. Round up to whole boxes, and check what a box holds. Tile is sold by the box and boxes are not all the same. A box holding 4 large-format tiles and a box holding 11 small ones round very differently — the rounding alone can be five percent.

For most rectangular rooms this lands close to the traditional ten percent, which is why the rule of thumb has survived. The point is that it now lands there for a reason, and when the room is one of the awkward ones you will see it before you order rather than halfway through laying.

Edge cases worth an extra box

Batch and shade. Tile is fired in batches, and the batch and shade codes on the box exist because colour varies between them. Order the whole job at once, from one batch, and check the codes actually match across the boxes you are handed. This is the single most common way an order goes wrong and it is invisible until the floor is down.

Discontinued and clearance lines. If the tile is end-of-line, the correct allowance is “all of it that is left”, or at least enough attic stock to repair the floor twice. There is no reorder.

Large-format tile. Higher breakage in transit and in handling, larger loss per mistake, and often only two to four tiles per box, so every rounding decision is expensive. Lean towards the top of the breakage range.

Rectified tile with a 2 mm joint. A tighter joint gives you less room to absorb variation, so more pieces get recut. Add a little. It also moves every cut on the floor, since the joint is part of every pitch — see where to start setting out a tiled floor.

A floor you cannot see all of. Under a bath, behind a vanity, inside a wardrobe. Tiling under fixtures is often the right call — but it is area you are buying and never seeing, and it is worth deciding deliberately rather than discovering on the invoice.

The short version

Ten percent is not wrong. It is just an answer to a question nobody asked, because it does not know your room. Get the piece count for the layout you are actually going to lay, add breakage on top of that, keep real spares, and round to boxes. On a plain floor you will land near ten percent anyway. On a small bathroom in large-format tile, you will avoid the trip back to the merchant.