The job
Modelled, not recorded. Every figure here is the answer the Grout Calculator gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
The job is the tiled walls of a family bathroom: 8.5 m² (91.49 sq ft) of tiling around the bath and the shower, in a rectified porcelain tile 600 mm (23.62 in) by 300 mm (11.81 in). Rectified tiles are ground square at the factory, which is what lets them be laid with a narrow joint; the joint here is 2 mm, a narrow joint suited to a rectified tile.
The joint depth is 9 mm, the thickness of the tile, because grout fills the joint down to the adhesive bed. The density is left at the calculator's figure for a mixed cementitious grout, 1,600 kg/m³; an epoxy grout is a different product with its own density on the data sheet.
The allowance stays at the calculator's 10 per cent, for grout left on the float, washed off the face of the tiles and mixed in a batch that stiffens before it is used. On a job this small the allowance is a few spoonfuls; the bag size decides the purchase.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Area to grout | 8.5 m² (91.49 sq ft) | The tiled area of the walls only, measured after tiling is set out, with the window reveal and the bath panel left out. |
| Tile length | 600 mm (23.62 in) | Left at the calculator's own default. |
| Tile width | 300 mm (11.81 in) | The short side of the rectangular tile, as the box states it. |
| Joint width | 2 mm (0.08 in) | A narrow joint for a rectified tile, from the tile maker's minimum and the spacers used. |
| Joint depth | 9 mm (0.35 in) | The tile's thickness, which is how deep the grout goes. |
| Grout density (mixed) | 1,600 kg/m³ (99.88 pcf) | Left at the calculator's own default. |
| Waste allowance | 10 | Left at the calculator's own default. |
The working, step by step
1. Share of the surface that is joint
0.99 %
Each tile with its joint makes a module a little bigger than the tile, and the joint is the difference: here 0.99 % of the wall. The calculator works this from the geometry rather than from a table, and corrects for the crossings where two joints meet, which the formula on most packaging leaves out.
2. Joint volume
0.76 L (0.2 gal)
That share of the area, times the depth of the joint, is the space the grout fills: 0.76 L. It is easier to picture than a mass, and on this wall it is about a jug's worth.
3. Before the allowance
1.21 kg (2.68 lb)
The volume times the mixed grout's density gives the mass, 1.21 kg, which is how grout is sold. The density entered is for mixed grout, so this is a mass of mixed grout; the powder in the bag is a little less once the water is added, which keeps the figure on the safe side.
4. Allowance at 10%
0.12 kg (0.27 lb)
The allowance adds 0.12 kg for what is lost on the float and in the wash-down, which brings the total to the headline. The calculator shows it as its own row so the margin is visible rather than hidden in a rounded figure.
5. 5 kg bags
1 bag
Rounded up to the bag size the calculator counts for a metric reader, the job takes 1 bag, with most of it left over. That is normal for a fine joint on large tiles: the bag is the smallest step a merchant sells, and the job uses a fraction of it.
Grout needed
1.336 kg
2.945 lb
The grout this wall needs is 1.336 kg, which is 2.945 lb, bought as the smallest bag the range comes in. Buy it in the colour chosen against the tile under daylight, and keep the rest of the bag sealed for repairs; a colour mixed from a different batch rarely matches. If the walls are being sealed with silicone at the bath and in the internal corners, those joints take sealant rather than grout and are left out of the area by the time this figure is used.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Grout needed | 1.336 kg (2.945 lb) | Cementitious wall grout in the chosen colour, allowance included. |
| 5 kg bags | 1 bag | Whole bags at the bag size counted, one colour and batch. |
The allowance, and what it does here
The 10 per cent is applied to the mass once, after the joint volume is worked out. It covers grout left on the float and in the bucket, grout washed off the tile faces, and a mix that goes off before it is used. It does not cover joints that are deeper than the tile because the adhesive was not cleaned out of them, which is a fault to fix before grouting, not a percentage to add.
The common mistake
Estimating grout from the area alone
A rule of so many kilograms per square metre works for one tile and one joint and no other. Grout fills the joints, so the same wall in a small tile has several times the joint length of the same wall in a large tile, and a wider joint fills more for every metre of it. Using a rule meant for small tiles on large ones buys several times the grout needed; the reverse runs out halfway across a mosaic. Enter the actual tile, joint and depth, as the calculator asks, or use the formula on the grout's own bag.
Change the job, and see what moves
The same wall, with a wider joint
Joint width: 3 mm (0.12 in) → 1.996 kg (4.401 lb)
Lay the same tiles with a 3 mm joint, a common joint for a tile that is not rectified, and the grout rises to 1.996 kg. The joint share grows almost in step with the joint width, so half as wide again is close to half as much again of grout.
The same wall, in a small square tile
Tile length: 100 mm (3.94 in); Tile width: 100 mm (3.94 in) → 5.228 kg (11.53 lb)
Tile the same wall in a 100 mm square tile and the grout jumps to 5.228 kg, now 2 bags. A small tile has far more joint per square metre: here 3.88 % of the wall is joint, against 0.99 % with the large tile.
What to run next
- Tile Adhesive Calculator
The tiles are bedded in adhesive first, counted from the same area and the trowel the tile size calls for.
- Floor & Wall Tile Calculator
If the bathroom floor is being tiled too, the floor's tiles and boxes are counted on their own.
- Caulk & Sealant Calculator
The joints at the bath, the shower tray and the internal corners take flexible sealant, counted by length.
Sources
The method and every constant behind these figures belong to the Grout Calculator, version 1.0.0, last verified 2026-09-17. It rests on:
- Derived rather than tabulated: tiles on a grid form modules of (length + joint) × (width + joint), so the share of the surface that is joint is 1 − length × width ÷ ((length + joint)(width + joint)); joint volume is that share × area × joint depth, and mass is volume × the mixed grout's density
- The consumption formula printed on cementitious grout packaging, (length + width) ÷ (length × width) × joint width × joint depth × density, is the same geometry without the joint-crossing correction and reads within 2% of it at common tile sizes
Frequently asked questions
- How much grout for 8.5 m² of wall tiles?
- In a 600 mm by 300 mm tile with a 2 mm joint 9 mm deep, the calculator gives 1.336 kg including its 10 per cent allowance.
- Why does a small tile need so much more grout?
- Because grout fills the joints, and a small tile has many more of them. In a 100 mm tile the same wall takes 5.228 kg against 1.336 kg.
- Is the calculator's figure the same as the one on the grout bag?
- Close to it. Its sources state that the formula printed on cementitious grout packaging is the same geometry without the correction for joint crossings, and reads within a couple of per cent at common tile sizes.