Buying grout is easy to get wrong. Too little and you’re mixing a second batch that may not match; too much and you’re storing half a bag that hardens in the garage. The amount depends on four things: tile size, joint width, joint depth and the area you’re covering.
Why tile size matters so much
Grout fills the gaps between tiles. Small tiles have far more joints per square foot than large ones. A floor of 2-inch mosaic tiles can need several times more grout than the same floor in 12-inch tiles, even with the same joint width.
The formula
For square or rectangular tiles:
Grout volume per unit area = ((L + W) ÷ (L × W)) × joint width × joint depth
- L and W are the tile length and width.
- Joint depth is usually the tile thickness.
- Multiply the volume by the grout’s density to get weight. Sanded cement grout is roughly 1.6 g/cm³ (about 0.058 lb per cubic inch), but check your product.
Worked example (imperial)
A 100 sq ft bathroom floor with 12 × 12 in tiles, a ⅛ in joint and ⅜ in thick tiles:
- Joint length per sq ft: (12 + 12) ÷ (12 × 12) × 144 = 24 in.
- Volume per sq ft: 24 × 0.125 × 0.375 = 1.125 cubic inches.
- For 100 sq ft: 112.5 cubic inches.
- Weight: 112.5 × 0.058 ≈ 6.5 lb.
- Add 10–15% for waste: about 7.5 lb.
One 10 lb bag is enough.
Worked example (metric)
A 10 m² wall with 300 × 300 mm tiles, a 3 mm joint and 8 mm thickness:
- (300 + 300) ÷ (300 × 300) = 0.00667 per mm.
- × 3 mm × 8 mm = 0.16 mm, which equals 0.16 litres of grout per m².
- × 1.6 kg/litre ≈ 0.26 kg per m².
- For 10 m² plus 10% waste: about 2.8 kg.
Quick reference
| Tile size | Joint | Approx. grout per 100 sq ft |
|---|---|---|
| 2 × 2 in mosaic, ¼ in thick | ⅛ in | about 26 lb |
| 4 × 4 in, ¼ in thick | ⅛ in | about 13 lb |
| 12 × 12 in, ⅜ in thick | ⅛ in | about 6.5 lb |
| 12 × 24 in, ⅜ in thick | ⅛ in | about 5 lb |
Figures exclude waste and assume a typical sanded grout density. The coverage chart on your grout bag is the final word.
Sanded or unsanded?
Most manufacturers recommend unsanded grout for joints narrower than ⅛ in and sanded grout for wider joints. Epoxy grout is heavier and more stain-resistant, so check its own coverage chart.
Tips to avoid running short
- Buy all bags from the same batch so the colour matches.
- Add 10% for floors and 15% for mosaics or irregular layouts.
- Mix in small batches; most grout has a limited working time.
Use the calculator
The Grout Calculator does this maths for you in feet or metres. Planning the whole job? Start with the Tile Calculator and read how many tiles you need for a 10×10 room.
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