How much grout do I need?
Grout fills the joints between tiles, so the quantity depends on how much joint there is per square metre and how big each joint is. Small tiles have many more joints than large ones: a square metre of 100 × 100 mm mosaic has 20 metres of joint, while a square metre of 600 × 600 mm porcelain has just over 3 metres. That is why mosaic can need five or six times as much grout per square metre.
The joint volume is the joint length times its width times its depth — and the depth is the tile thickness. Multiplying by the density of the grout gives kilograms per square metre.
Joint width
Typical joint widths are:
- 1.5–2 mm for rectified porcelain with precise, straight edges.
- 3 mm for most standard floor and wall tiles.
- 5–10 mm for rustic, handmade and natural stone tiles with irregular edges.
The grout formula
G = (L + W) ÷ (L × W) × j × d × ρ [kg/m²]
L and W are the tile length and width in mm, j the joint width in mm, d the joint depth (tile thickness) in mm and ρ the grout density (1.6 for cement grout, 1.5 for epoxy). Total grout = A × G × 1.15.
Worked example
- Room 4.0 × 3.0 m = 12 m², tiles 600 × 600 × 10 mm, 2 mm joint, cement grout.
- G = (600 + 600) ÷ (600 × 600) × 2 × 10 × 1.6 = 0.107 kg/m².
- Net 12 × 0.107 = 1.28 kg; with 15% waste 1.47 kg → one 2 kg bag.
- Mosaic 100 × 100 × 6 mm, 3 mm joint, 5 m²: G = 0.576 kg/m² → 3.31 kg → 2 × 2 kg bags.
Cement or epoxy grout
Cement-based grout is the usual choice for most floors. It has a density of about 1.6 kg/dm³ and comes in 2 kg and 5 kg bags. Epoxy grout is stain-resistant and waterproof, ideal for kitchens, showers and commercial floors, but more expensive and harder to work; its density is about 1.5 kg/dm³ and it is commonly sold in 2.5 kg tubs. The calculator chooses the combination of package sizes that covers the need with the least leftover, or the lowest cost if you enter prices.
Why add 15% waste?
Some grout is always lost: it stays in the bucket, on the float and on the sponge, and some joints are deeper than the tile where the adhesive did not reach the edge. Fifteen percent covers this for most jobs. Mix grout in small batches so it does not start to set before you finish.
Frequently asked questions
How much grout per m²?
Anything from about 0.1 kg/m² for large rectified tiles with 2 mm joints to over 0.5 kg/m² for mosaic. Use the formula or the calculator with your tile size.
Does joint depth equal tile thickness?
For calculation, yes. In practice the joint is slightly shallower where adhesive has squeezed up, which the 15% allowance more than covers.
Can I use the same grout for walls and floors?
Most cement grouts are suitable for both. Use a flexible grout on underfloor heating and timber bases, and epoxy where stains or water are a concern.
Do I need to seal cement grout?
Sealing is recommended in kitchens, bathrooms and hallways, because unsealed cement grout absorbs dirt and water. Apply a penetrating sealer once the grout has fully cured. Epoxy grout does not need sealing.
How long does grout take to cure?
Cement grout is usually walkable after 24 hours and fully cured after about a week. Epoxy cures faster but has a shorter working time. Check the product data sheet.