What this calculator does
Flooring is bought by the box, so the calculation is area plus wastage divided by coverage per box, rounded up. A 5 by 4 metre room at 10% wastage needs 22 m² of material and therefore 10 boxes at 2.2 m² each.
Rounding up to whole boxes often absorbs the wastage allowance entirely. At 0%, 5% and 10% wastage this room needs the same 10 boxes, because 20, 21 and 22 square metres all round to the same count. The allowance only bites once it pushes past a box boundary.
The formula
The floor area is multiplied by one plus the wastage percentage, divided by the coverage per box, and rounded up to a whole number. The spare coverage reported is the difference between what the boxes cover and the bare floor area, which is what you will actually have left over after laying.
| Term | Meaning |
|---|---|
| Wastage allowance | Extra for offcuts, breakages and pattern matching. 10% is typical. |
| Coverage per box | Square metres a box lays, printed on the packaging. |
| Dye lot or batch | Tiles from different batches can differ slightly in shade, so buy all at once. |
| Diagonal laying | Increases wastage substantially, often to 15 or 20%. |
The inputs explained
| Field | What to enter |
|---|---|
| Room length (m) | Room length in metres. |
| Room width (m) | Room width in metres. |
| Wastage allowance (%) | Wastage allowance. 10% for a simple rectangular room, 15 to 20% for diagonal laying or a complex shape. |
| Coverage per box (m²) | Coverage per box in m², from the packaging. |
| Price per box ($) | Price per box, for the material cost. |
When to use it
Ordering tiles or boards
Material is sold by the box and cannot usually be bought by the square metre.
Deciding how much spare to keep
Keeping a box for future repairs is sensible, since matching a batch later is unreliable.
Comparing products by cost
Boxes cover different areas, so the price per box is not comparable until converted.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does the wastage allowance change the order?
The same room at a range of wastage allowances.
| Wastage allowance | Boxes to buy | With wastage | Spare coverage left over |
|---|---|---|---|
| 0% | 10 | 20.00 m² | 2.00 m² |
| 5% | 10 | 21.00 m² | 2.00 m² |
| 10% | 10 | 22.00 m² | 2.00 m² |
| 15% | 11 | 23.00 m² | 4.20 m² |
Questions
How much wastage should I allow?
Around 10% for a simple rectangular room laid straight. Go to 15 or 20% for diagonal laying, a herringbone pattern, a room with many alcoves, or any tile with a pattern that has to be matched across joins.
Should I keep spare tiles?
Yes, at least a few. Matching a batch later is unreliable even with the same product code, since shade varies between production runs. Keeping a box means a future repair is invisible rather than obviously patched.
Why does rounding up matter so much?
Because boxes cannot be split. A room needing 22.1 m² and one needing 24.2 m² both take 11 boxes at 2.2 m² each, so an extra square metre of room can cost nothing while a different extra square metre costs a whole box.
Does the laying direction affect the quantity?
Yes. Laying diagonally produces angled offcuts at every wall, which are mostly unusable, and typically raises wastage from 10% to around 15 or 20%. Long boards in a narrow room also waste more than the same boards in a square one.
For painting the same room, see the paint calculator. For a tiled surface needing grout, see the grout calculator.