Joints, depth & bags
Grout calculator
How much grout fills the joints between your tiles, by the formula grout makers print on their technical data sheets, in kilograms or pounds and in bags.
Grout to buy
200 mm × 200 mm tiles, 3 mm joints 7 mm deep, 10 m² (107.6 ft²).
- Grout per area0.34 kg/m²0.21 L of joint per m² · 10 m of joint per m²
- Grout for the job3.7 kg3.36 kg for 10 m² (107.6 ft²) + 10% extra
- Bags to buy15 kg bags · one bag covers about 13.5 m²
The sum, in millimetres
(200 + 200) ÷ (200 × 200) × 7 × 3 × 1.6 = 0.336 kg/m²
Tile length plus width over length times width, times joint depth, joint width and the grout's density.
| Joint width | kg/m² | For 10 m² |
|---|---|---|
| 2 mm | 0.22 | 2.5 kg |
| 3 mm | 0.34 | 3.7 kg |
| 5 mm | 0.56 | 6.2 kg |
| 8 mm | 0.9 | 9.9 kg |
| 10 mm | 1.12 | 12.3 kg |
How to use the grout calculator
Enter the area you will grout, the tile's length and width, the joint depth and the joint width. The depth is normally the full thickness of the tile, as the joints are filled to the bottom. If you came from the tile calculator, its area, tile size and joint are already filled in.
The calculation is the one printed on grout makers' data sheets: tile length plus width, divided by length times width, times the joint depth, the joint width and a density factor, all in millimetres, gives kilograms per square metre. The first part is the length of joint in each square metre, so small tiles need far more grout than large ones, and doubling the joint width doubles the grout.
The density factor turns the volume of the joints into a weight of grout. Extra covers what stays in the bucket and on the float, and the uneven tile edges no formula sees; UZIN's calculator, for one, suggests adding 10%. Last, the bag size turns the weight into bags.
Worked example
Ten square metres of 200 × 200 mm wall tiles, 7 mm thick, with 3 mm joints: (200 + 200) ÷ (200 × 200) × 7 × 3 × 1.6 = 0.336 kg/m². Mapei's own table gives 0.3 kg/m² for this tile and joint, the same figure rounded. Ten square metres take 3.36 kg, or 3.7 kg with 10% extra: one 5 kg bag.
Checked against makers' tables
The formula reproduces the consumption tables printed beside it. On Mapei's Ultracolor Plus sheet, 300 × 300 × 10 mm tiles with 5 mm joints use 0.5 kg/m² (the formula gives 0.53), and 100 × 100 × 7 mm tiles with 3 mm joints use 0.7 (0.67). ARDEX's G 10 sheet lists 0.35 kg/m² for 15 × 15 cm tiles with 3 mm joints 5 mm deep; the same sum with its fresh weight of 1.70 kg per litre gives 0.34. ARDEX adds that 0.05–0.10 kg/m² is lost on the tile faces, depending on the finish and how you work.
Sources
- Mapei, Ultracolor Plus technical data sheet (2801-9-2016, GB), copy hosted by a retailer: the consumption formula (A + B) ÷ (A × B) × C × D × 1.6 = kg/m², the consumption table and pack sizes.
- ARDEX, ARDEX G 10 PREMIUM technical data sheet: fresh weight about 1.70 kg per litre, consumption by tile size for 3 mm × 5 mm joints, and losses on the tile surface.
- UZIN, Grout consumption calculator: results are estimates; add 10% as a safety buffer.
Questions
How much grout for 10 m² of 300 × 300 mm tiles?
With 8 mm thick tiles and 3 mm joints: (300 + 300) ÷ (300 × 300) × 8 × 3 × 1.6 = 0.256 kg/m², so 2.6 kg for 10 m², or about 2.8 kg with 10% extra. With 5 mm joints it is about 4.7 kg.
How many square feet does a 25 lb bag of grout cover?
It depends on the tile and the joint. For 12 × 12 in tiles 3/8 in thick with 1/8 in joints, the formula gives about 0.065 lb per ft², so a 25 lb bag covers roughly 380 ft² before any extra. With 1/4 in joints it covers half that. The coverage chart on the bag is the figure to trust.
Why do small tiles and mosaics need so much more grout?
Every tile is edged by joint, so the smaller the tiles, the more metres of joint per square metre. ARDEX's table for G 10 rises from 0.10 kg/m² for 60 × 60 cm tiles to 5.40 kg/m² for 1 × 1 cm mosaic with the same joint.
What joint depth should I enter?
Usually the tile thickness: Mapei's formula uses the thickness of the tile for the depth. If adhesive has squeezed up into the joints, or the joints will be finished with a sealant, the real depth is less and so is the grout.
Does it work for hexagonal or irregular tiles?
No. The formula assumes square or rectangular tiles laid in a grid or bond. For hexagons, octagons with insets or irregular stone, use the grout maker's own calculator or its coverage chart.