Tile count from room and tile dimensions — with the grout line included, which most calculators quietly ignore.
Most tile calculators divide floor area by tile area and stop. That slightly overestimates the tile count, because each tile occupies its own size plus its share of the grout joint around it. This one adds the joint width to each dimension before dividing.
A 12 × 12 inch tile is exactly 1 sq ft of tile, but with a 3 mm grout line it occupies about 1.02 sq ft of floor. Over a 120 sq ft room that is about four tiles' difference — small, but it is the difference between the calculation being right and being approximately right.
The effect grows with smaller tiles and wider joints. Mosaic tiles with generous joints can have grout occupying 10% or more of the surface.
A 12 × 10 ft room is 120 sq ft. With 12 × 12 inch tiles and 3 mm joints, each tile covers 1.0198 sq ft, so you need 118 tiles for a perfect fit and 130 with a 10% allowance. At 10 tiles per box that is 13 boxes.
| Situation | Allowance |
|---|---|
| Straight lay, simple rectangular room | 10% |
| Room with alcoves, many cuts | 12–15% |
| Diagonal layout | 15% |
| Herringbone or complex pattern | 15–20% |
| Large-format tiles (600 mm and above) | 15% — offcuts are less reusable |
| Natural stone with variation | 15–20% — some will be rejected on sight |
Buy the whole quantity in one order from the same batch. Ceramic and porcelain tiles are fired in lots, and colour varies measurably between them — the batch number is printed on the box precisely because this matters. A top-up order months later frequently will not match, and on a floor the difference is obvious under raking light.
Keep the leftovers. A cracked tile years later is trivial to replace if you have spares and effectively impossible to match if you do not.
Rectified tiles — mechanically cut to a precise size after firing — can take joints as narrow as 2 mm. Standard pressed tiles vary slightly in size and need 3 mm or more so the joint can absorb the variation. Natural stone and handmade tiles typically want 3 to 6 mm. Going too narrow for the tile is a common mistake: the joints end up visibly uneven because there is no room to adjust.
Wider joints are also more forgiving of a floor that is not perfectly flat, which most floors are not.
Do not simply start against a wall. Find the centre of the room, dry-lay a row in each direction, and see where the cuts fall. If you would end up with a sliver less than about half a tile wide along one wall, shift the whole layout by half a tile so both sides get a reasonable cut instead. A narrow sliver is hard to cut, easy to break, and looks like a mistake.
Also check which wall is most visible from the doorway and put the full tiles there. Walls are rarely square to each other, and the choice of where to hide the discrepancy is the main thing separating a good tiling job from an adequate one.
About 118 for an exact fit with a 3 mm grout joint, and roughly 130 once you add a 10% waste allowance. The reason it is not a clean 120 is the grout line — each 12 inch tile actually occupies about 1.02 sq ft of floor including its share of the joint, so you need slightly fewer tiles than the raw area suggests, then more once cutting waste is included.
Ten percent for a straight lay in a simple rectangular room, 15% for a diagonal layout or a room with many cuts, and 15 to 20% for herringbone, large-format tiles or natural stone. Buy it all in one order — tiles are fired in batches and colour varies between them, so a later top-up often will not match. Keep the offcuts and spares for future repairs.
Yes, and most calculators ignore it. Each tile occupies its own dimensions plus the joint width around it, so with a 3 mm joint a 12 inch tile covers about 1.02 sq ft rather than exactly 1.00. The effect is small for large tiles and significant for small ones — with mosaics and wide joints, grout can account for 10% or more of the surface.
Rectified tiles, which are machine-cut to precise dimensions after firing, can go as narrow as 2 mm. Standard pressed tiles vary slightly in size and need 3 mm or more so the joint can absorb that variation. Natural stone and handmade tiles typically want 3 to 6 mm. Choosing a joint too narrow for the tile is a common error — the joints end up visibly uneven because there is no slack to adjust with.
From the centre, not from a wall. Find the room centre, dry-lay a row each way, and check where the cuts land. If one edge would end up with a sliver narrower than about half a tile, shift the whole layout half a tile so both sides get a decent cut. Put full tiles where they are most visible from the doorway, since walls are rarely square and something has to absorb the discrepancy.
Related: Square Footage Calculator · Area Calculator · Concrete Calculator · Roofing Calculator For walls rather than floors, the Paint Calculator works out coverage by coat, and the Colour Converter checks a colour pairing for contrast.