How Many Tiles You Need
The arithmetic is area divided by area, with one refinement that most people forget: the grout gap.
Effective tile size = (Tile length + Gap) × (Tile width + Gap)
Tiles needed = Area to cover ÷ Effective tile size × (1 + Waste)
A 12″ tile with a 3 mm joint occupies 12.12″, so it covers about 2% more area than its bare size suggests. On a large floor that difference is several tiles — in your favour, which is why ignoring the gap makes you over-order slightly rather than run short.
Waste Allowance
Waste is not padding; it is the material consumed by cuts at the edges, around fixtures and in the pattern. How much depends on the layout.
| Situation | Waste allowance |
|---|---|
| Simple square or rectangular room, straight lay | 5–10% |
| Standard room with a few obstacles | 10% |
| Diagonal lay (45°) | 15% |
| Herringbone or complex pattern | 15–20% |
| Irregular room with many alcoves | 15–20% |
| Large-format tiles (over 24″) | Add 5% — each broken tile costs more area |
Buy the extra in the same purchase. Tiles are manufactured in batches and colours vary measurably between dye lots, so a top-up order months later rarely matches. Keep a full box spare for future repairs.
Grout Joint Widths
| Joint width | Suits | Grout type |
|---|---|---|
| 1.5–2 mm | Rectified porcelain, precise tiles | Unsanded |
| 3 mm | Standard floor and wall tiles | Unsanded or fine sanded |
| 5 mm | Natural stone, handmade tiles | Sanded |
| 6–12 mm | Rustic and irregular tiles | Sanded |
Unsanded grout is used below about 3 mm because sand grains will not pack into a narrower joint. Sanded grout resists shrinkage and cracking in wider joints but can scratch polished stone — test on an offcut first.
Tiles per Square Metre and Square Foot
| Tile size | Per m² | Per 100 sq ft |
|---|---|---|
| 100 × 100 mm (4″) | 100 | 929 |
| 150 × 150 mm (6″) | 44.4 | 413 |
| 200 × 200 mm (8″) | 25 | 232 |
| 300 × 300 mm (12″) | 11.1 | 103 |
| 400 × 400 mm (16″) | 6.25 | 58 |
| 600 × 600 mm (24″) | 2.78 | 26 |
| 300 × 600 mm | 5.56 | 52 |
Figures are for bare tiles without a grout gap; add roughly 1–3% for the joints.
Adhesive and Grout Quantities
| Tile size | Trowel notch | Adhesive coverage |
|---|---|---|
| Up to 150 mm | 6 mm | ~5 m² per 20 kg bag |
| 150–300 mm | 10 mm | ~3.5 m² per 20 kg bag |
| 300–600 mm | 12 mm | ~2.5 m² per 20 kg bag |
| Over 600 mm | 15 mm or back-butter | ~2 m² per 20 kg bag |
Large-format tiles usually need back-buttering — adhesive on both the substrate and the tile — to achieve the coverage building standards require, typically 95% for wet areas.
Practical Notes
- Dry-lay first. Set out a row without adhesive to see where the cuts fall. Shifting the starting line by half a tile often avoids a sliver at the wall.
- Start from the centre. Working outward from the room's centre lines keeps cuts symmetrical at opposite walls.
- Never end with a cut under 50 mm. Thin slivers are fragile and look wrong; adjust the layout instead.
- Mix boxes as you go. Even within one dye lot there is slight variation, and drawing from several boxes distributes it invisibly.
- Check flatness. Large-format tiles expose any dip in the floor; substrate flatness matters more as tiles get bigger.
Frequently Asked Questions
Should I include the grout gap in the calculation?
Yes, and this calculator does. It slightly reduces the number of tiles needed, because each tile plus its joint covers more area than the tile alone.
How much extra should I buy?
10% for a straightforward room, 15–20% for diagonal or patterned layouts. Add one full box beyond that for future repairs.
Do I deduct for kitchen units and fixtures?
Only for areas you genuinely will not tile. Many tilers deliberately tile under appliances so a future replacement of a different size still sits on finished floor.
How do I work out wall tiles?
Calculate each wall as its own area — width times height — and add them, then subtract windows and doors. Enter the total as the area here.