What a tile calculator gets wrong
Tile quantities come down to two numbers and one judgement. The numbers
are the area you are covering and the area one tile covers. The judgement
is how much you will cut away, and that depends entirely on the pattern —
which is the part almost every calculator flattens into a single figure.
Tile area = (tile width × tile length) ÷ 144
Tiles = area × (1 + waste ÷ 100) ÷ tile area, rounded up
Boxes = tiles ÷ tiles per box, rounded up
- 144 — square inches in a square foot
- waste — 10% straight, 15% diagonal or offset, 20% herringbone
- tiles per box — printed on the box, and it varies by size and range
Worked example: 100 square feet of 12 inch tile
Tile area = (12 × 12) ÷ 144 = 1 sq ft
Straight = 100 × 1.10 ÷ 1 = 110 tiles
Diagonal = 100 × 1.15 ÷ 1 = 115 tiles
Herringbone = 100 × 1.20 ÷ 1 = 120 tiles
At 12 per box: 110 tiles = 10 boxes
Ten tiles separate the easiest layout from the hardest on a small floor.
On a large one that gap becomes a box or two, which is exactly the margin
people run out by when they use a flat ten percent for everything.
Why the allowance follows the pattern
Waste in tiling is offcuts, and offcuts are geometry. A straight lay meets
the walls square, so most cuts leave a usable piece for the opposite side.
Turn the grid forty-five degrees and every perimeter tile becomes a
triangle with nothing reusable behind it. Herringbone cuts at the border
constantly and produces awkward slivers that cannot be turned into
anything. The pattern is not a style choice with a cost attached — it is
the cost.
Common mistakes
- Buying exactly what the maths says. Order the full box,
not the tile count. Part boxes are not sold and dye lots change.
- Keeping no spares. A cracked tile in three years is
unfixable if the range has been discontinued. Keep a few back.
- Measuring the room, not the tiled area. Baths, shower
trays and kitchen units come off the floor figure.
- Applying the straight-lay allowance to a diagonal. The
commonest way to end up short by half a box.
When these numbers do not apply
- Rooms far from square. More walls means more cuts.
Bay windows and alcoves push waste above the pattern figure.
- Large format tiles. Big tiles waste more when cut,
because each offcut represents more square footage.
- Patterned and directional tiles. Anything that has to
line up across joints wastes more than a plain tile.
- Mosaic sheets. Sold by the sheet with its own coverage,
and cuts fall between the tesserae rather than through them.
Substrate and area
Tile is only as flat as what is under it. On new walls the
drywall calculator covers the
board and compound behind a tiled surface. For floors that are not simple
rectangles — a bathroom around a bath, a kitchen around an island — the
square footage
calculator subtracts the shapes you are not tiling.
Tiles for 100 ft², by size and layout
Includes each layout's own waste allowance. Buy whole boxes and keep the spares — a replacement tile from a later batch rarely matches.
Common questions
What about bullnose and trim?
Enter the exposed edge in feet and the ticket counts the pieces. Any tiled edge that is not against a wall needs a finished piece — bullnose, a pencil, a metal profile — and it is sold per piece at a length rather than by the square foot, which is why it is so often the item that holds a job up. Fourteen feet of edge in twelve inch tile is sixteen pieces once the same waste allowance is applied, because a mitred external corner costs you a piece the way a cut costs you a tile.
Can I change the waste allowance?
Yes. The pattern sets the starting figure — 10 percent laid straight, 15 diagonal, 20 herringbone — because a diagonal lay cuts every tile at the perimeter and a herringbone cuts most of them twice. It used to be locked to that, which is wrong in both directions: a long plain wall in a simple tile wastes less than any of those figures, and a room full of pipe boxings, reveals and a diagonal lay wastes more than all of them. Type over it and it stays where you put it.
Can I do a floor and a wall in one go?
Yes, and you should if they are the same tile. Switch to several surfaces and add a row each for the floor, the shower surround and the splashback. Tile rounds up hard — a box is a dozen and you cannot buy four of them — so working the surfaces out separately rounds up once per surface and can easily cost you two extra boxes on a bathroom. One order, one rounding-up.
What if the floor and the walls are laid differently?
Then do them separately. The surfaces here are assumed to share a lay pattern, because the pattern is what sets the waste allowance — a diagonal floor carries 15 percent where a straight wall carries 10. Mixing them under one allowance would be wrong for one of the two, so the calculator uses a single pattern and says so on the ticket rather than quietly averaging.
How many 12 inch tiles for 100 square feet?
110 laid straight, 115 diagonally, or 120 in herringbone. A 12 inch tile covers exactly one square foot, so the tile count is the area plus the allowance.
How much extra should I buy?
The pattern allowance covers cutting. Keeping spares for future repairs is separate — a few tiles put in the loft, bought at the same time from the same batch.
Does grout width change the count?
Barely at typical joint widths, and always in your favour since the joint replaces tile. It is well inside the waste allowance and is not modelled here.
Why do boxes matter more than tiles?
Because boxes are what you buy. Needing 111 tiles when a box holds 12 means ten boxes, and the tenth is mostly spare — worth knowing before you are at the till.
What if my pattern is not listed?
Basketweave, versailles and modular layouts have no published allowance, so the tool refuses rather than guessing. Pick the closest listed pattern or enter your own percentage.
