Counts grout lines, applies wastage separately, and rounds to whole boxes.
Tiles to buy
120
109 for the area, plus 10% wastage — 20 boxes
Area to tile
10 m²
Each tile with its joint covers 0.091809 m²
Spare after the job
11
Keep a few for future repairs
Counting the 3 mm grout joint saves 3 tiles against the 112 a bare area-divided-by-tile-area sum would give. The smaller the tile, the larger that gap becomes.
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About the Tile Calculator
Working out how many tiles a room needs looks like dividing the floor area by the area of one tile. That sum is wrong in two directions at once, and the errors do not cancel.
It is too high because a tile does not sit against its neighbour. Every tile carries a grout joint, so the shape that actually repeats across the floor is the tile plus one joint. A 300 mm tile with a 3 mm joint repeats every 303 mm.
And it is too low because tiles break, cuts go wrong, and the last row is never a whole tile. Trade practice is to order roughly ten per cent above the measured quantity, and more when the lay pattern produces offcuts that cannot be reused.
This calculator does both separately, so you can see what each is worth, and then rounds to whole boxes — because almost no range is sold loose.
Mathematical Formula & Logic
The repeating unit:
Unit area = (tile length + joint) × (tile width + joint)
Each tile carries half a joint on all four sides, so across a field the repeat is one full joint in each direction. With a 300 mm square tile and a 3 mm joint the unit is 303 × 303 mm, or 0.091809 m², against 0.09 m² for the bare tile.
The tile count:
Tiles = ceil( ceil(area / unit area) × (1 + wastage) )
Rounded up twice, deliberately. Once to a whole tile, because you cannot lay part of one. Then again after wastage, and again to whole boxes at the point of purchase. Every rounding goes the same way, which is the direction that does not leave you stranded.
Why the joint matters more on small tiles
The joint is a fixed width, so it is a larger fraction of a small tile than a large one. Over 10 m²:
300 mm tile, 3 mm joint — 109 tiles, against 112 ignoring the joint. Three tiles.
100 mm tile, 2 mm joint — 962 tiles, against 1,000 ignoring the joint. Thirty-eight tiles.
That is close to four boxes on a mosaic-scale format, from a step most calculators skip entirely.
Step-by-Step Example
A bathroom floor 4 m by 2.5 m, tiled in 300 mm square porcelain with a 3 mm joint, boxes of six.
1. Area: 4 × 2.5 = 10 m².
2. Repeating unit: 0.303 × 0.303 = 0.091809 m².
3. Tiles for the area: 10 / 0.091809 = 108.9, rounded up to 109.
4. Wastage at 10%: 109 × 1.10 = 119.9, rounded up to 120.
5. Boxes: 120 / 6 = 20 exactly, giving 120 tiles.
So twenty boxes, with eleven tiles spare after the 109 the floor consumes.
What the joint was worth here
Ignoring it gives 10 / 0.09 = 111.1, rounded to 112 tiles, and 124 after wastage — 21 boxes rather than 20. One extra box for a step that takes no measuring at all, only the number already printed on your spacers.
How the lay pattern changes the allowance
Ten per cent suits a straight or brick lay, where a tile cut to fill the last 180 mm of a row often leaves a 120 mm strip that fits somewhere else. The offcut is usable.
A diagonal lay does not behave that way. Every perimeter tile is cut at 45 degrees, and the offcut is a triangle whose hypotenuse rarely matches any other gap on the floor. Herringbone and chevron are worse again, because the pattern meets the wall at alternating angles and almost every edge piece is unique.
That is reasoning from the geometry of the cuts rather than a published figure — the merchants quote 10% and generally leave it there. The suggestions in the pattern selector reflect it, and the field stays editable because your room, your tile size and your confidence with a wet saw all move the number.
One thing worth doing regardless of the arithmetic: keep several spare tiles from the same batch once the job is finished. Tile colour drifts between production runs, so a replacement bought two years later to fix a cracked tile will rarely match — and a single mismatched tile in a floor is far more visible than people expect.
Reference Data & Values
tile size
joint
unit area
per 10 m²
saved by counting joints
100 × 100 mm
2 mm
0.010404 m²
962 tiles
38
200 × 200 mm
3 mm
0.041209 m²
243 tiles
7
300 × 300 mm
3 mm
0.091809 m²
109 tiles
3
600 × 300 mm
5 mm
0.184525 m²
55 tiles
1
600 × 600 mm
3 mm
0.363609 m²
28 tiles
0 (rounding absorbs it)
Frequently Asked Questions
Divide the area by the area of one tile plus its grout joint, round up, add a wastage allowance, then round up again to whole boxes. A 4 by 2.5 metre floor in 300 mm tile with a 3 mm joint needs 109 tiles for the area itself and 120 once 10% wastage is added — twenty boxes of six.
Yes, and it is the step most calculators skip. A 300 mm tile with a 3 mm joint repeats every 303 mm, so each unit covers 0.0918 m² rather than 0.09. Over 10 m² that is three fewer tiles. On 100 mm tile with a 2 mm joint it is thirty-eight fewer, because the joint is a much larger share of a small tile.
Ten per cent is the figure tile merchants commonly quote, covering breakage, cutting errors and obstacles. Allow more where the lay produces unusable offcuts. A straight lay often leaves a reusable strip from a perimeter cut; a diagonal lay cuts 45-degree triangles that rarely fit anywhere else, and herringbone meets the wall at alternating angles so almost every edge piece is unique.
Because you cannot lay part of a tile and most ranges are not sold loose. The count rounds up to a whole tile, wastage is applied, and the result rounds up again to whole boxes. Every step increases the number, which is the only safe direction — running short mid-floor means a second order and a possible batch mismatch.
Slightly fewer than the 2.78 the bare arithmetic suggests, because of the joint. With a 3 mm joint each tile occupies 0.363609 m², so a square metre takes about 2.75 and 10 m² takes 28 tiles once rounded up.
Yes, several, from the same batch. Tile colour drifts between production runs, so a replacement bought later to fix a cracked tile will rarely match. A single mismatched tile in an otherwise uniform floor draws the eye far more than people expect, and the cost of keeping a few is trivial against re-tiling.
Yes — the geometry is identical and only the area changes. For a wall, measure the area actually being tiled and subtract anything that will not be covered, such as a window reveal or the footprint of a fitted unit, before entering it.
Enter the sheet dimensions and the joint between sheets, not the individual tesserae. Mosaics are sold and laid by the sheet, so treating each small piece as a tile produces a wildly inflated count and a meaningless box figure.
No. Adhesive depends on the trowel notch and how flat the substrate is, and grout depends on joint width and depth as well as tile size. Neither follows reliably from tile count, so both need their own calculation rather than a rule of thumb applied to this answer.