Tile jobs have three material lines that behave very differently: the tile, the bed it sits in, and the grout between. Each needs its own calculation. On top of those come the preparation materials (levelling, waterproofing, backer board) and the trims and movement joints that finish the job. Price all of them and tiling stops being the job that eats your margin.
Measure the areas
Measure floors wall to wall, including under fitted units only if the tile runs underneath. For walls, measure each tiled area separately: splashbacks, shower walls, full-height and half-height areas. Subtract windows and doors, but keep the reveals around them, which are often tiled too and need edge trims. Note the number of external corners and edges, because each needs a trim.
Tiles: work in modules
Each tile plus its share of joint covers (tile width + joint) × (tile length + joint). Divide the area by this module, not by the bare tile size. On small tiles with wide joints the difference is several percent.
Then add waste: 10% for a straight lay, 15% for diagonal, and more for large-format tiles, where a single breakage is expensive and cuts produce large offcuts you can't reuse. Buy whole boxes, all from the same batch (check the shade and caliber codes on the boxes), and leave a few spares with the client for future repairs.
Layout decides the waste
Before ordering, plan the layout. Centre the tiles so the cuts at opposite walls match and are more than half a tile wide. Avoid thin slivers at doorways and along the most visible wall. Planning the setting-out takes a few minutes and often saves a box of tiles, and it always gives a better-looking job.
- Straight lay or brick bond: the least waste. Large-format tiles in brick bond should be offset by no more than a third of their length, to avoid lipping caused by bowing.
- Diagonal: every edge piece is a triangle cut, so allow 15%.
- Herringbone and chevron: lots of edge cuts, often 15–20%.
- Mixed sizes (modular patterns): order each size separately, following the pattern's ratio.
Thinset: it's all about the notch
The trowel notch sets the bed thickness, and bigger tiles need bigger notches for full coverage.
| Tile size | Typical notch | Coverage |
|---|---|---|
| Mosaic / small wall tile | 3/16" V | about 120 sq ft per 50 lb |
| Up to 12" / 300 mm | ¼" × ¼" | about 90 sq ft per 50 lb |
| Large format | ½" × ½" | about 45 sq ft per 50 lb |
Back-buttering large tiles adds roughly 30%. In wet areas and on floors, aim for near-full coverage under every tile: lift one now and then to check. Choose the adhesive for the job: modified thinset for most porcelain, and a large-and-heavy-tile mortar for big formats.
Grout: joints, not area
Grout fills the joint volume: area × (length + width) ÷ (length × width) × joint width × depth. That's why mosaics use many times more grout per area than large tiles. Use sanded grout for joints wider than ⅛" and unsanded for narrower ones. The grout calculator applies the formula directly.
Prep
- Leveller where the floor is out of flat. Large tiles need a very flat substrate.
- Uncoupling membrane or cement backer board over wood subfloors
- Waterproofing (tanking) in showers and wet rooms
- Primer for the substrate
- Movement joints at perimeters and changes of substrate
- Edge trims at external corners, countertops and changes of floor level
- Silicone at internal corners, baths and shower trays
A worked example
A 180 sq ft kitchen floor in 12 × 24 tiles with ⅛" joints: each tile plus joints covers 12⅛ × 24⅛ inches, about 2.03 sq ft. 180 ÷ 2.03 = 89 tiles, plus 10% = 98. Thinset at about 45 sq ft per 50 lb bag (large tile, ½" notch): 4 bags. Grout: about 10 lb, so one 25 lb bag of sanded grout.
The tile calculator gives the tiles, adhesive and grout together, and the separate thinset and grout calculators handle any notch or joint.
Common questions
How many spare tiles should the client keep?
At least a few for every area, more for large floors. Replacing a cracked tile years later is easy if a matching spare is in the garage.
Can I tile over old tiles?
Sometimes, if the old tiles are firmly bonded, flat and you have the height. Use a suitable primer and adhesive, and check thresholds and door clearances.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.