The tile formula
Measure in the same units throughout. In imperial, tile sizes are in inches and the area in square feet: a 12 × 12 inch tile covers exactly 1 sq ft.
How much extra to buy
| Layout | Extra |
|---|---|
| Square room, straight layout, small tiles | 5–10% |
| Most rooms (some cuts around doors, pipes, toilet) | 10% |
| Diagonal, herringbone or chevron patterns | 15% |
| Large tiles, natural stone, many corners | 15–20% |
The extra covers tiles you cut (offcuts often can't be reused), breakages, and a few spares for future repairs. Tile colours vary slightly between production batches, so finding a matching tile years later is hard. Keep a handful.
Measuring awkward rooms
- L-shaped rooms: split them into two rectangles, work out each area and add them, or enter the full rectangle and put the missing corner in "area to leave out".
- Bathrooms: for walls, measure each wall separately (width × tiling height) and add them, then subtract the bath, shower tray, window and door.
- Fixed units: you don't need to tile under kitchen cabinets or a fitted bath, so leave those areas out.
Grout gaps
Grout joints (usually 2–5 mm) mean slightly fewer tiles cover the same area, so ignoring them, as this calculator does, gives you a small safety margin. For very small tiles like mosaics, where gaps make up a bigger share, the difference is larger.
Tiling tips
- Start from the centre of the room or the most visible wall, so cut tiles end up at the edges.
- Do a dry layout first to see where the cuts fall, and avoid thin slivers under 5 cm.
- Open boxes and mix tiles from several boxes as you go, so slight shade differences blend in.
- Check the box for coverage (m² or sq ft per box), which is a useful cross-check for this calculator.
Painting the walls too? Use the paint calculator.
Frequently asked questions
How many tiles do I need?
Divide the area by the area of one tile, then add 10% for cuts and breakage. For 8.64 m² with 30 × 30 cm tiles, that is 96 tiles, or 106 with extra.
How many 12 × 12 tiles do I need for 100 square feet?
Each covers 1 sq ft, so 100 tiles, plus 10% extra: 110 tiles.
How much extra tile should I buy?
About 10% for most rooms, 15% for diagonal or herringbone layouts, and up to 20% for complex rooms or large tiles.
How many tiles are in a box?
It depends on the tile size and brand. Check the box, which also shows the area it covers, and enter the number above.
Do I need to allow for grout lines?
Not usually. Ignoring grout gaps gives a small extra margin.
Last updated October 2026 · More home & diy