Running out of tiles two rows from the end is a frustrating way to lose a day. The supplier may not have the same batch any more, and the client will notice a different shade. Ordering far too much costs money too. Getting quantities right is a short job, as long as you follow the same steps every time.
Step 1: Measure every surface separately
Measure each floor and wall as its own area, in metres, and write it down. Don’t measure the room once and guess the rest.
- Floors: length times width. For an L-shaped room, split it into rectangles and add them together.
- Walls: length times the height you are tiling to, which may not be the full ceiling height
- Openings: take off windows and doors, but only if they are large. Small openings create extra cuts, so it is usually safer to leave them in.
- Extras: niches, hobs, shelves, splashbacks, step-downs and window reveals. Each one has returns and edges that add up.
Note the tile size and layout against each area, because the waste allowance depends on both.
Step 2: Add a waste allowance that suits the layout
Every job loses tiles to cuts, breakages and the odd faulty piece. How much depends mostly on the layout and the tile size. These are common starting points used by many tilers, not fixed rules:
| Layout | Typical waste allowance |
|---|---|
| Straight lay, simple room, standard tiles | Around 10% |
| Brick or offset pattern | Around 10 to 15% |
| Diagonal lay | Around 15% |
| Herringbone, chevron or mixed sizes | Around 15 to 20% |
| Large format on a small, cut-heavy area | 15% or more |
Adjust up for rooms with lots of corners, pipes, niches or angled walls, and for fragile or natural stone tiles. Adjust down slightly for a big, open, square floor with few cuts.
Large-format tiles
Big tiles waste more than you would expect. An offcut from a 600 x 1200 tile may be too narrow to use anywhere else, so one cut can cost you most of a tile. On small floors with falls to a waste, large tiles often need envelope cuts as well. Draw the layout out, count the tiles you will actually use, and compare that with your percentage figure. Use whichever number is higher.
Patterns and features
For herringbone, chevron or anything with a border, set out the pattern on paper before ordering. The setting out decides how many part tiles you need at each edge.
Step 3: Turn square metres into boxes
Tiles are sold by the box, and each box covers a set area. Divide the area you need (including waste) by the coverage per box, then round up to the next full box. Always round up, never down.
Use the coverage on the box label rather than working it out from the tile size.
Step 4: Work out adhesive from the data sheet
There isn’t a single “kilos per square metre” figure for tile adhesive. The amount you use depends on:
- The notch size of the trowel. A bigger notch puts down more adhesive.
- The tile size and back profile. Large tiles and tiles with deep ribs on the back need a bigger notch.
- Back-buttering. Many large-format tiles need adhesive on the back as well as the bed, which adds to usage.
- The substrate. An uneven floor takes more adhesive than a flat one.
The manufacturer’s technical data sheet lists approximate usage for each notch size. Pick the notch size the data sheet and your local standard call for, read off the coverage, multiply by the area and add a margin for back-buttering and uneven surfaces. Then round up to whole bags.
Whatever the bag says, the aim is full contact under the tile. Lift a tile now and then to check the coverage on the back, especially in wet areas and on floors.
Step 5: Work out grout from the joint size
Grout fills the joints, so the amount you need depends on how much joint there is. Small tiles have far more joint per square metre than large ones. The joint width and the depth (usually the tile thickness) matter just as much.
Most grout manufacturers publish a calculator or a formula. The common version works like this:
Grout (kg per m²) = ((tile length + tile width) ÷ (tile length x tile width)) x joint width x joint depth x grout density
Use millimetres for the tile and joint sizes, and the density in kg per litre from the data sheet. Add 10 to 20% for mixing and cleaning losses.
If the manufacturer has its own calculator, use that instead, because densities and losses vary between products. Epoxy grouts in particular can behave quite differently from cement-based grout.
A worked example
Here is a small bathroom, worked through step by step. All the figures below are made up for the example. Use your own measurements and the real data sheets for the products you are using.
- Floor: 2.4 m x 1.8 m = 4.32 m², 600 x 600 tiles, 10 mm thick, falling to a centre waste
- Walls: 15 m² of 300 x 600 tiles, 8 mm thick, straight brick lay
- Joints: 2 mm on both
- Adhesive usage from the example data sheet: 5 kg per m² on the floor, 4 kg per m² on the walls
- Grout density from the example data sheet: 1.6 kg per litre
- Box coverage: 1.44 m² for the floor tiles, 1.08 m² for the wall tiles
| Item | Floor | Walls |
|---|---|---|
| Area | 4.32 m² | 15 m² |
| Waste allowance | 15% (large format with falls) | 10% (brick lay) |
| Area to order | 4.97 m² | 16.5 m² |
| Boxes (rounded up) | 4 boxes | 16 boxes |
| Adhesive before margin | 21.6 kg | 60 kg |
| Grout per m² | about 0.11 kg | about 0.13 kg |
| Grout for the area | about 0.5 kg | about 1.9 kg |
Adhesive in this example comes to 81.6 kg. With a 10% margin for back-buttering and uneven areas, that’s just under 90 kg, so five 20 kg bags. Grout comes to about 2.4 kg, or around 2.8 kg once losses are added, so one 5 kg bag covers both areas with some left for repairs.
Buy from one batch, and keep a box back
Tiles from different production runs can vary slightly in shade and sometimes in size. The batch or shade code is printed on each box.
- Order everything at once, and ask for it all from the same batch
- Check the codes on delivery, before a single tile goes down
- Open several boxes and mix tiles from each as you lay, so any small variation spreads out evenly
- Leave the client a few spare tiles from the same batch for future repairs, and note the product and batch number on your job sheet
Common mistakes
- Measuring the room once and guessing the walls
- Using the same waste allowance for every layout
- Forgetting niches, hobs and reveals
- Rounding down on boxes or bags
- Treating adhesive coverage as a fixed number instead of reading the data sheet
- Using the same grout figure for mosaics and large-format tiles
- Mixing batches without checking the codes
Do the sums before you quote
Work out quantities at the quoting stage, not the week you start. It makes your materials price accurate, it tells you whether the supplier has enough stock in one batch, and it gives you a list you can order from on the day the client accepts. If you’d rather not do the maths by hand on site, our free calculators cover areas and quantities for the most common jobs.