Calculating how many tiles you need comes down to three simple steps: measure the area, divide by the tile size, and add a wastage allowance. Get this right before you head to the store and you'll avoid the frustrating (and sometimes expensive) problem of running out of tiles mid-job — especially if the batch or colour run is discontinued.
Step 1 – Measure the Room Area
Start by measuring the length and width of the floor or wall surface in metres. Multiply them together to get the area in square metres (m²).
Example:
- Room: 4.2 m × 3.6 m = 15.12 m²
For an L-shaped or irregular room, break it into rectangles, calculate each section separately, then add them together.
Walls and Splashbacks
For walls, measure the height and width of each wall surface you want to tile. Subtract any large openings like windows or doors (measure those openings and deduct their area). Add all the wall sections together for your total wall area.
Step 2 – Calculate the Number of Tiles
Tile sizes are typically shown in millimetres (e.g. 300×300 mm, 600×600 mm, 300×600 mm). Convert to metres to keep the maths consistent.
Formula:
Number of tiles = Total area (m²) ÷ Area of one tile (m²)
| Tile Size | Area per Tile | Tiles per m² |
|---|---|---|
| 300 × 300 mm | 0.09 m² | ~11.1 |
| 400 × 400 mm | 0.16 m² | ~6.25 |
| 600 × 600 mm | 0.36 m² | ~2.78 |
| 300 × 600 mm | 0.18 m² | ~5.56 |
| 200 × 100 mm (subway) | 0.02 m² | ~50 |
Example using 600×600 mm tiles in a 15.12 m² room: 15.12 ÷ 0.36 = 42 tiles (before wastage)
Step 3 – Add a Wastage Allowance
Always order more tiles than your raw calculation suggests. Tiles get cut, some crack during installation, and you'll want spares for future repairs. The standard NZ trade rule of thumb is:
- Straight lay (grid pattern): add 10%
- Diagonal or herringbone pattern: add 15%
- Complex shapes, lots of cuts, or small tiles: add 15–20%
Example (straight lay, 42 tiles needed): 42 × 1.10 = 46.2 → round up to 47 tiles
Always round up to the nearest whole tile, and ideally to the nearest full box.
Account for Grout Joints
Grout joints take up space between tiles, which slightly reduces the number of tiles needed — but in practice the difference is small and the wastage allowance more than covers it. For very large-format tiles (e.g. 900×900 mm or bigger), it's worth discussing the layout with your tiler, as these require a particularly flat, well-prepared substrate to meet the requirements of good trade practice.
Don't Forget the Substrate and Adhesive
Once you know your tile quantity, you'll also need to calculate how much tile adhesive (also called tile cement or tile glue) and grout to buy. Both products have coverage rates printed on the packaging — check the manufacturer's data sheet for the specific product you choose. Brands like Ardex and Mapei are widely available in NZ and provide detailed coverage guides.
For wet areas like bathrooms and showers, the substrate and waterproofing membrane must comply with NZ Building Code clause E3 (Internal Moisture). BRANZ publishes good guidance on wet-area construction if you want to understand what's required.
Tips to Avoid Common Mistakes
- Buy from the same batch. Tiles are manufactured in batches (called a "shade lot" or "calibre"). Mixing batches can result in slight colour or size differences. Check the batch number on the box.
- Keep a few tiles as spares. Store them somewhere dry after the job — they're invaluable if a tile chips or cracks years later.
- Check your tile layout before you start. Dry-lay a row of tiles across the room to see how cuts will fall at the edges. Aim for cut tiles at edges to be at least half a tile wide.
- Factor in skirting tiles or trims separately if you're using them — measure the linear metres of perimeter and check the trim piece length.
- Large-format tiles on floors may require a thicker or more rigid substrate. Check with your tiler or refer to the tile manufacturer's installation guide.
You can pick up tiles and materials from stores like Bunnings, Mitre 10, or PlaceMakers, or from specialist tile retailers who often carry a wider range.
When to Call a Professional
While measuring and calculating tiles is something any homeowner can do, the actual installation — particularly in wet areas like showers, bathrooms, and laundries — involves waterproofing work that must meet the NZ Building Code. Poorly waterproofed wet areas are one of the leading causes of moisture damage in NZ homes.
If your project involves moving or adding plumbing, that is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed tradesperson registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB).
For a quality tile installation — especially on large areas, heated floors (in-slab heating), or tricky substrates like existing timber floors — it's well worth hiring a qualified tiler. You can find experienced local tilers on tradedirectory.co.nz.
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