How Much Extra Tile to Buy

What the waste factor is actually covering

Three things. Cuts at the edges of the field, where a tile is trimmed and the offcut is too small to use elsewhere. Breakage, both during transport and during cutting — large-format and thin porcelain tiles snap more often than small ceramic ones. And the attic stock you should keep after the job, because a tile discontinued in two years cannot be matched when something cracks.

The proportion each of those takes depends on the room, not on the tile. A small bathroom floor is almost all edge, so the cut waste is high relative to the area. A large open floor is mostly full tiles with a border of cuts, so the same percentage is generous. This is why a flat rule works better on small rooms and gets wasteful on large ones.

The layout multiplies the waste

A straight set parallel to the walls produces one cut per tile at the perimeter. A 45-degree diagonal produces two cuts on every perimeter tile and turns each corner into a triangle offcut that fits nowhere — 15 percent is the working minimum and 20 is safer on a complicated floor.

Herringbone and chevron are worse again, because every course ends in an angled cut and the offcuts alternate direction. Twenty percent is standard and some installers order 25 on a small, busy floor.

Patterned or veined tile deserves its own allowance. If the tile has a directional grain or a marble-effect vein, some tiles will be rejected during layout for not matching their neighbours, and that rejection rate is on top of the cut waste.

Round up twice

Take the area, add the waste factor, then divide by the coverage per box and round that up. A 40 square foot backsplash at 10 percent is 44 square feet. If the tile comes in boxes covering 10.7 square feet, that is 4.11 boxes, so you buy 5 and you actually own 53.5 square feet.

The second rounding is why buying an extra box "just in case" is often free — you may already be paying for most of it. It is also why ordering by area rather than by box makes shortages common: the supplier ships whole boxes, and a calculation that stops at square feet has not finished.

Check the dye lot on every box before the first tile goes on the wall. Boxes from different lots vary in shade, and discovering that halfway across a floor is a full tear-out rather than an inconvenience.

What else the estimate is missing

Tile is the visible half of the material list. The rest is thinset, grout, backer board or waterproofing membrane, edge trim, spacers and sealant, and on a shower that supporting material can approach the cost of the tile itself. Waterproofing in particular is sold by the roll or the bucket in fixed sizes, so it has its own rounding.

Run the room through a tile calculator that includes those lines rather than pricing tile alone — the shortfall between "tile cost" and "tiling cost" is where budgets break.

Common questions

How much extra tile should I buy?

10 percent for a straight layout, 15 for diagonal or a cut-heavy room, 20 for herringbone, chevron or patterned tile. Then round up to whole boxes.

Should I keep leftover tile?

Yes — at least a few square feet, stored flat and dry with the dye lot noted. Tile lines change constantly, and a cracked tile two years from now cannot be matched from stock that no longer exists.

Does tile size change the waste factor?

Large-format tile wastes more on a small or irregular room, because each cut discards a bigger piece and each break loses more area. On a large open floor the effect largely disappears.

How much thinset and grout do I need?

Both depend on tile size and trowel notch rather than on area alone: bigger tiles need a deeper notch and more thinset per foot, while wider grout joints and thicker tiles take more grout. Use the coverage figures on the bag for your tile size rather than a flat rate.

In this topic

• Tile Calculators
• Guides