How to use the grout calculator
Grout coverage is one of the easiest things to get wrong on a tiling order. Bags are sold by weight, but the amount you need depends on the joint volume, which changes a lot with tile size, joint width and tile thickness. This calculator works out the joint length and volume for your layout and converts it to weight and bags, so a mosaic splashback and a large-format floor are both estimated properly.
- Choose units. Metric mode takes m², millimetres and kilograms. US mode takes ft², inches and pounds.
- Enter the tiled area, net of baths, cabinets and other areas you will not tile.
- Enter the tile length and width as laid. For mosaic sheets, enter the size of one chip, not the sheet.
- Enter the joint width and depth. Use the spacer size for width and the tile thickness for depth.
- Check the density, waste and bag size against your grout's data sheet.
Grout formula
Joint length per unit area = 1 ÷ (tile length + joint) + 1 ÷ (tile width + joint)
Joint volume per unit area = joint length × joint width × joint depth
Grout weight = joint volume × density · total = per-area weight × area × (1 + waste)
Shortcut (mm, kg/m²) ≈ (L + W) ÷ (L × W) × joint width × depth × 1.6
The density factor converts the volume of the joint to the weight of dry powder needed. Manufacturers usually publish coverage formulas with a factor of about 1.6 to 1.8, which allows for the water added when mixing. The table shows how tile size changes the result for 3 mm joints at 8 mm depth.
| Tile size | Joint length per m² | Grout per m² (approx.) |
|---|---|---|
| 600 × 600 mm | 3.3 m | 0.13 kg |
| 300 × 300 mm | 6.6 m | 0.25 kg |
| 150 × 75 mm subway | 19.4 m | 0.74 kg |
| 100 × 100 mm | 19.4 m | 0.75 kg |
| 50 × 50 mm mosaic | 37.7 m | 1.45 kg |
Worked examples
A bathroom floor in metric
A 12 m² bathroom floor is tiled with 300 × 300 × 8 mm porcelain on 3 mm spacers. The joint length is 1 ÷ 0.303 + 1 ÷ 0.303 = 6.6 m per m², and the joint volume is 6.6 × 0.003 × 0.008 = 0.158 litres per m². At 1.6 kg per litre that is 0.25 kg per m², or 3.04 kg for the floor. With 10% waste you need 3.35 kg, so one 5 kg bag.
A subway tile wall
A 20 m² shower wall uses 75 × 150 × 7 mm subway tiles with 2 mm joints. There are about 19.6 m of joint per m², giving 0.27 litres and 0.44 kg of grout per m². The wall needs 8.8 kg, or 9.6 kg with waste, so two 5 kg bags.
A large-format floor in US units
A 200 ft² kitchen floor in 12 × 24 in porcelain, ⅜ in thick, with ⅛ in joints, has about 17.8 inches of joint per square foot. That is 0.84 cubic inches of joint and 0.048 lb of grout per ft², 9.7 lb in total, or 10.6 lb with 10% waste, so one 25 lb bag is plenty.
Who this calculator helps
- Interior designers and architects writing tiling specifications and checking contractor quantities.
- Tilers and contractors ordering the right number of bags and colour-matched batches.
- Kitchen and bathroom designers comparing the grout implications of mosaic versus large-format tile.
- DIY renovators buying grout for a backsplash, shower or floor.
Practical tips
- Work out the tile count and boxes first with the tile calculator, and the area of each surface with the square footage calculator.
- For worktop backsplashes, take the backsplash area from the countertop calculator.
- Buy all the grout from one batch and keep a spare bag for repairs; colours vary slightly between batches.
- Natural stone and rustic tiles absorb grout into their surface. Add 5–10% more waste and seal porous tiles before grouting.
- Pick the grout colour with the tile: a contrasting grout emphasises the grid, a matching one makes the surface read as one plane.
Not sure which tile and grout colour will work? Try the options on a photo of the room with AI Flooring before ordering.
