Materials
Tile and Grout Calculator
Get tile counts, adhesive bags, and grout volume for any tile size, joint width, and area.
Your answer
Buy every tile from the same batch number. Shade varies noticeably between production runs of the same product.
How tile quantities are calculated
Tile counts are not area divided by tile area — they are area divided by the tile plus its share of grout joint. A 300 by 600 mm tile with a 3 mm joint effectively occupies 303 by 603 mm. On a small bathroom that difference is a tile or two; across a large floor it is a whole box.
Waste is then added on top. Ten per cent is the standard for a straight lay, rising to fifteen for diagonal or brick bond, where every perimeter tile is cut and the offcuts rarely fit anywhere else.
Adhesive: the notch decides the bag count
Adhesive consumption is driven entirely by trowel notch size, because the notch sets the bed thickness. Doubling the notch roughly halves the coverage per bag, which is why large-format tiles cost far more in adhesive than mosaics on the same floor.
| Notch size | Typical tile | Coverage per 20 kg bag |
|---|---|---|
| 6 mm | Up to 150 mm tiles, mosaics | 4.5 m² |
| 8 mm | Up to 300 mm tiles | 3.5 m² |
| 10 mm | 300-600 mm tiles | 3.0 m² |
| 12 mm+ | Large format, back-buttered | 2.2 m² |
Grout is a volume calculation, not an area one
Grout fills the joints, so the volume is total joint length multiplied by joint width and tile thickness. Small tiles have vastly more joint length per square metre than large ones — a mosaic sheet can use five times the grout of a 600 mm porcelain tile over the same floor.
Before you buy anything, check flatness
Large tiles cannot flex over a hump in the substrate, so they tilt and the edges stand proud — lippage. Anything over 400 mm needs the base flat to roughly 3 mm over 3 m. Levelling compound is far cheaper than relaying a floor.
Common mistakes
- Ordering tiles from different batch numbers and getting a visible shade change mid-wall.
- Ignoring the joint width in the count, then running one box short.
- Under-buying adhesive because the bag quotes coverage at the smallest notch.
- Grouting before the adhesive has fully cured, trapping moisture and causing patchy colour.
- Starting from a wall instead of a set-out line, so the pattern drifts out of square.
How many spare tiles should I buy?
Ten per cent on top of the measured area for a straight lay, fifteen for a diagonal or brick bond, plus a few whole tiles kept aside for future repairs.
How much adhesive does a bag cover?
A 20 kg bag of cement-based adhesive covers roughly 4-5 m² at 6 mm notch, or 2.5-3 m² at 10 mm notch for large-format tiles.
What grout joint width should I use?
Two to three millimetres for rectified porcelain, three to five for standard ceramic, and eight or more for handmade or rustic tiles where size varies.
Why does grout need more than I expect?
Grout volume is joint width multiplied by tile depth multiplied by total joint length. Smaller tiles have far more joint length per square metre, so they use more grout.
Last reviewed February 2026