Home & construction
Tile calculator
Tiles are sold in boxes, laid with grout lines, and cut at every edge. All three change the count, which is why the tile you need is never quite the area divided by the tile.
Grout joints change the count
A 12 inch tile does not cover a square foot once you lay it. With a 3 mm joint it occupies 12.12 by 12.12 inches, so a hundred square feet takes about 98 tiles rather than 100. On a small floor this hardly matters. On a large one with wide joints it is several tiles.
Joint width is a design decision as much as a technical one. Rectified porcelain can take 2 mm joints; handmade or unrectified tiles need 3–5 mm to absorb variation in tile size. Wider joints on a slightly irregular tile look deliberate; narrow joints on the same tile look like a mistake.
Waste is not padding
Every edge of the room needs a cut tile, and the offcut is usually unusable elsewhere. That is where the allowance goes.
| Layout | Allowance | Why |
|---|---|---|
| Straight, square room | 10% | Perimeter cuts only |
| Brick / offset bond | 12% | Alternate rows start with a half tile |
| Large format | 15% | Each cut wastes a big piece |
| Diagonal / herringbone | 18–20% | Every perimeter tile is cut at an angle |
Add more for a room with many corners, alcoves or fittings to cut around. Tiles are sold in boxes, so the practical figure is always rounded up to a whole box anyway — and the leftovers are worth keeping, because a cracked tile three years later is unfixable without a spare from the same batch.
Set out from the centre, not the wall
The instinct is to start tiling in a corner. The result is a full tile at one edge and a two-centimetre sliver at the other, which is the first thing anyone notices in the finished room.
Setting out from the centre line puts equal cuts at both edges. If the resulting cut is very narrow, shifting the start point by half a tile makes both edges wider and more even. Where a floor is visible from a doorway, it is usually worth arranging for a full tile at the threshold and taking the awkward cut at the far wall.
Batches, adhesive and the boring warnings
Ceramic tiles are fired in batches, and colour varies between them. Buy the whole job at once and check the batch numbers on every box match. Where they do not, mixing tiles from several boxes as you lay distributes any variation instead of leaving a visible block of slightly different tiles.
Adhesive coverage depends on trowel notch size, tile size and how flat the substrate is; the figures here are a starting point, not a substitute for the bag. Large-format tiles need back-buttering and a larger notch, which raises consumption significantly. Floors going over wooden substrates, wet areas, and anything with underfloor heating all have their own requirements worth checking before buying.
Common questions
How many 12x12 tiles do I need for 100 square feet?
About 108 once a 10% waste allowance and a 3 mm grout joint are included. The bare arithmetic says 100, the joint pushes it to roughly 98 per 100 square feet of coverage, and the waste allowance takes it to 108 — so 11 boxes of ten.
How much extra tile should I buy?
Ten percent for a simple straight layout, 15% for large format or patterned tiles, and up to 20% for diagonal or herringbone. Rooms with lots of corners or fittings need more. Since tiles come in boxes you will usually round up further, and the spares are worth keeping for repairs.
Should I include the grout joint in the calculation?
Yes, and this calculator does. With standard 3 mm joints the effect is small but real — roughly 2% fewer tiles for a 300 mm tile. It becomes significant with wide joints or small tiles, where the grout can occupy a tenth of the surface.
How much adhesive and grout will I need?
Roughly 3.5 kg of adhesive per square metre for small tiles and up to 6 kg for large format, plus a kilo or two of grout for a typical bathroom floor. Both depend heavily on trowel size, tile thickness and substrate flatness, so treat the figures here as a starting estimate and check the manufacturer’s coverage on the bag.