Understanding your result
The headline is the number of boxes to buy. Under it sit the floor area, the tiles to order (waste included) and either the spare tiles that whole boxes leave you or the cost if you entered a price. The breakdown table walks from the area to the boxes in the order the arithmetic happens, so if the final figure surprises you, run down the rows until you find the one that does not match your expectation.
The row to check first is area each tile covers with its joint. It is the tile face plus half a grout joint on every side, and it is what the floor is really divided into. The second row to check is the waste allowance: it is a percentage of the tiles, not a guess at how many you will break, and it comes from the layout you chose. If your room has a bay window or several alcoves, pick “custom” and give it a few more points.
The spare tiles row is not waste. It is the difference between whole boxes and the tiles you need, and it is the stock you keep for a cracked tile in five years. If the spare count is large relative to the order, as it often is in a small bathroom, a shop that sells by the piece will save you a box. Before you order, run the tile layout calculator with the same room and tile: it tells you whether the last row against a wall will be a sliver, which is the usual reason real jobs use more than the 10% allowance.
How we calculate this
The one step most calculators skip is the pitch. A tile does not cover its own face only; with its share of the joint it covers slightly more, and the floor is divided into pitches, not faces. The difference is small for large tiles and wide for small ones, and it always reduces the count. The waste percentages come from the layout table on the waste factors reference page: 10% for straight and brick layouts, 15% for a 45-degree diagonal, 20% for herringbone and chevron. The guide to waste factors explains where those numbers come from and when to go above them.
In metric mode the calculator takes metres, centimetres and millimetres and converts them with exact factors, so a 60 × 30 cm tile with a 3 mm joint is handled the same way as a 24 × 12 in tile with a 1/8 in joint. Boxes are converted from the coverage on the label whichever unit it is printed in. For the mortar under the tile and the grout between them, the thinset calculator and the grout calculator take the same tile size and joint.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Tile size | 12 × 24 in (30 × 60 cm) | The most common floor format in US home centres and European DIY stores; see the standard sizes reference |
| Grout joint | 1/8 in (3 mm) | TCNA Handbook and ANSI A108.02 recommend a joint of at least 1/8 in for non-rectified tile, and no less than 1/16 in for rectified tile |
| Waste by layout | 10% straight, 15% diagonal, 20% herringbone | Industry rule of thumb used by tile manufacturers’ own calculators (Daltile, MSI) and installer guidance; detailed on the waste factors page |
| Coverage per box | 16 sq ft (1.44 m²) | Typical for 12 × 24 in porcelain, 8 pieces per box, from manufacturer specification sheets (MSI, Daltile, 2025). Boxes range from 12 to 20 sq ft, so read the label |
| Room | 12 × 10 ft (3.6 × 3 m) | Chosen only so the example result is a typical bathroom or kitchen |
Assumptions last reviewed October 7, 2026.
The calculator treats the floor as a rectangle or an area you have already worked out. It does not know where the walls fall relative to the tile grid, so it cannot tell you the size of the cut tiles; the tile layout calculator does that. It does not model thresholds, floor vents or a toilet flange, which are small and covered by the waste. And it assumes one tile size over the whole floor; a border or an inset of a different tile is a second run of the calculator. The complete method for working out how much flooring you need shows how to measure a room so the area you type here is right.
Two worked examples
A 14 × 12 ft kitchen in 12 × 24 porcelain
Straight layout, 1/8 in joint, 16 sq ft boxes at $38 each.
- Floor area: 14 × 12 = 168 sq ft
- Pitch per tile: 24.125 × 12.125 ÷ 144 = 2.031 sq ft
- Tiles before waste: 168 ÷ 2.031 = 82.7
- Tiles to order: 82.7 × 1.10 = 91
- Tiles per box: 16 ÷ 2 = 8, so 91 ÷ 8 = 11.4, rounded up to 12 boxes
- Spare tiles: 96 − 91 = 5; cost 12 × $38 = $456
Twelve boxes is 192 sq ft of tile for a 168 sq ft floor, 14% more than the room. That is the normal outcome of 10% waste plus rounding to whole boxes, and the five spare tiles are the repair stock. A 12 × 24 tile is large-format, so the result also flags the flatter floor, the one-third offset and the larger trowel the TCNA Handbook asks for.
An 85 sq ft hallway in 13 × 13 ceramic on the diagonal
Area typed directly, 3/16 in joint, boxes of 15.5 sq ft.
- Pitch per tile: 13.1875 × 13.1875 ÷ 144 = 1.208 sq ft
- Tiles before waste: 85 ÷ 1.208 = 70.4
- Tiles to order at 15% for the diagonal: 70.4 × 1.15 = 80.9, rounded up to 81
- Tiles per box: 15.5 ÷ 1.17 = 13.2, rounded down to 13
- Boxes: 81 ÷ 13 = 6.2, rounded up to 7 boxes, with 10 tiles spare
The diagonal costs this hallway about four more tiles than a straight layout would, and the rounding to whole boxes adds ten spare. For a long narrow hall the diagonal is also the layout most likely to end in slivers along both long walls, so the layout calculator is worth a look before the first tile goes down.
Where to find your inputs
Room length and width. Measure at floor level, wall to wall, at the widest points, and include the floor inside closets and under the door swing to the far side of the threshold. For an L-shaped or odd room, use the floor area calculator and switch this page to “The floor area”.
Tile size. The nominal size on the box (12 × 24) is what to enter; the actual tile is a few millimetres smaller and the difference is absorbed by the joint. For a tile sold in centimetres, switch to metric and type the centimetres.
Grout joint. Decide it before you count. Non-rectified tile needs at least 1/8 in; rectified (ground-edge) tile can take 1/16 in; handmade-look tile is often set at 3/16 in. Spacers come in exactly these sizes.
Coverage per box. It is printed on the box and on the product page, usually as square feet per carton with the piece count. If you only have the piece count, multiply it by the tile face area in square feet.
Common mistakes
- Dividing the area by the tile face. The floor is divided into tile-plus-joint pitches. For small tiles the face-only count is several percent too high.
- Using 10% for a diagonal. Every wall gets a run of triangular offcuts. Use 15%, and 20% for herringbone.
- Forgetting the closet. A closet floor is tiled too, and it adds a box in most bedrooms.
- Reading the box as “sq ft per tile”. The coverage is per box. Eight tiles per box is normal for 12 × 24; six to ten for other sizes.
- Buying exactly the count and no more. Dye lots change. The spare tiles from whole boxes are your only matching repair stock.
- Ignoring the last row. A layout that ends in a 1 in sliver is relaid with a half tile at each side, which uses more tile. Check the layout before you order.
- Mixing units. A 60 × 30 cm tile is not 60 × 30 in. Use the units toggle and enter the figure as printed.
Questions people ask
- How many 12 × 24 tiles do I need for a 10 × 12 room?
- A 10 × 12 ft room is 120 sq ft. With a 1/8 inch joint each tile covers 2.03 sq ft, so the floor takes 59.1 tiles before waste. At 10% for a straight layout that is 65 tiles, which at 8 tiles per 16 sq ft box is 9 boxes with 7 tiles spare.
- Why does the calculator count the grout joint?
- Because the joint is floor you do not have to cover with tile. A 12 × 12 inch tile with a 1/8 inch joint occupies 12.125 × 12.125 inches, about 2% more than its face; a 4 × 4 mosaic piece with the same joint occupies 6% more. Over a large floor the difference is a box or two, in your favour.
- How much waste should I add for floor tile?
- 10% for a straight or brick layout in a simple rectangular room, 15% on the diagonal, 20% for herringbone or chevron. Rooms with alcoves, several doorways or a bay window are at the top of each range. Under 10% leaves nothing for breakage, bad cuts or the repair you will want in a few years.
- How do I turn tiles into boxes?
- Divide the box coverage by the face area of one tile and round down to get the tiles per box, then divide the tiles you need by that figure and round up. A 16 sq ft box of 12 × 24 tile holds 8 tiles; 91 tiles is 11.4 boxes, so you buy 12.
- Should I buy the spare tiles or return them?
- Keep at least a handful, ideally a full box, from the same batch. Tile is made in dye lots that differ slightly in colour and size, and a replacement bought later rarely matches. The leftover shown by the calculator is from rounding up to whole boxes; it is your repair stock, not an error.
- Does tile size change the waste?
- Yes, in two opposite ways. Large tiles produce fewer cuts but each cut wastes a bigger piece, and offcuts of large-format tile are rarely reusable, so 10% is a minimum for 12 × 24 and larger. Small tiles and mosaics produce many cuts but the offcuts are small and often reusable. The layout matters more than the size.