About the Grout Calculator
Grout is the material that fills the joints between tiles. It is bought in bags, and like most finishing materials it is easy to buy far too much or, worse, too little and have to match the colour from a different bag halfway through the job.
The quantity depends on the volume of the joints, and that is surprisingly sensitive to the tile size. Small tiles have much more joint per square metre than large ones, so the same floor area can take several times as much grout with mosaics as with large-format tiles.
This grout calculator uses the formula tile and grout manufacturers publish, built from the tile's length, width and thickness and the joint width, to give the grout needed in kilograms or pounds and the number of bags.
How to Use the Grout Calculator
Choose metric or imperial units.
Enter the tiled area.
Enter the tile's length, width and thickness, and the joint width. The joint is usually filled to the full thickness of the tile, so the thickness stands in for the joint depth.
Enter the bag size in kilograms and the waste allowance.
How the Grout Is Worked Out
joint length per m² = (L + W) ÷ (L × W) (L, W in metres)
grout (kg per m²) = (L + W) ÷ (L × W) × joint width × joint depth × 1.6
(L, W, width, depth in mm)
total = kg per m² × area × (1 + waste %)The 1.6 is the density of cement grout in kilograms per litre.
Step-by-Step Example
10 m² of 300 × 300 mm tiles, 8 mm thick, with 3 mm joints.
Joint length: (300 + 300) ÷ (300 × 300) × 1000 = 6.67 m per m²
Grout per m²: (600 ÷ 90,000) × 3 × 8 × 1.6 = 0.256 kg
Total: 0.256 × 10 × 1.10 = 2.8 kg
Bags: one 5 kg bagWhere the Formula Comes From
Picture a square metre of tiling. Every tile has a joint along one side and one end. For tiles of length L and width W, a metre of tiling contains 1 ÷ W rows of joints running one way and 1 ÷ L running the other, each a metre long. Added together, that is (L + W) ÷ (L × W) metres of joint in every square metre.
Multiply by the joint's width and depth and you have the volume; multiply by the grout's density and you have the weight. The formula is simple geometry, which is why every manufacturer's coverage chart follows it.
Tile Size Changes Everything
Because joint length per square metre depends on 1 ÷ L + 1 ÷ W, halving the tile size doubles the joint:
| Tile size | Joint per m² | Grout per m² (3 mm × 8 mm) |
|---|---|---|
| 100 × 100 mm | 20 m | about 0.77 kg |
| 300 × 300 mm | 6.7 m | about 0.26 kg |
| 600 × 600 mm | 3.3 m | about 0.13 kg |
So a mosaic floor can use six times the grout of the same floor in large-format tiles. Joint width matters in direct proportion too: a 5 mm joint takes two and a half times the grout of a 2 mm joint.
Choosing Joint Width
Joint width is partly practical and partly a matter of looks.
Wall tiles are usually laid with 2 to 3 mm joints.
Floor tiles typically use 3 to 5 mm, giving a little more tolerance for variation in the tiles and the floor.
Rectified tiles, cut to precise sizes, can be laid with narrower joints.
Handmade or rustic tiles vary in size and need wider joints, sometimes 6 to 10 mm, to absorb the differences.
Cement or Epoxy Grout
Cement-based grout is the everyday choice: easy to work, affordable and available in many colours. It needs sealing in wet areas to resist staining.
Epoxy grout is waterproof, very stain-resistant and does not need sealing, which makes it ideal for kitchen worktops, showers and heavy-use floors. It is denser, more expensive and has a short working time. Because its density differs, use the manufacturer's own coverage chart for epoxy.
Grouting Well
Wait for the adhesive. Grout only once the tile adhesive has set, usually after 24 hours, or the tiles can shift.
Rake out the joints. Clear any adhesive that has squeezed up between the tiles so the grout can fill the full depth.
Work diagonally. Press grout into the joints with a rubber float held at an angle, moving across the joints rather than along them so it is not dragged out.
Clean in two stages. Wipe the surplus off with a barely damp sponge once the grout has firmed slightly, then polish off the remaining haze with a dry cloth later.
Seal if needed. Cement grout in kitchens and bathrooms benefits from a sealer once cured, which makes it much easier to keep clean.
Understanding Your Result
Grout needed is the total weight, including waste.
Bags gives the number of bags of the size you entered.
Grout per area shows kilograms per square metre and pounds per square foot.
Joint length shows the metres of joint in each square metre.
Worth knowing covers small tiles and epoxy.
When Should You Use This Calculator?
Alongside the tile calculator, once the tiles are chosen.
For mosaics, where grout use is much higher than people expect.
When comparing joint widths for a design.
Before buying coloured grout, so all bags come from one batch.
Common Mistakes
Assuming grout use is the same for every tile. Small tiles take far more.
Underestimating joint depth. Grout fills the tile's full thickness.
Mixing too much at once. Mix what you can use within the working time.
Leaving grout haze to harden. Wipe the faces within the time on the bag.
Buying coloured grout in separate trips. Colours can vary between batches.