Grout quantity is a genuine volume calculation rather than a rule of thumb. Each square metre of tiling contains a known length of joint, and multiplying that length by the joint width and depth gives the volume of grout it will swallow. Multiplying by the grout density converts that to the kilograms you buy.
The joint length per square metre is (tile width + tile length) divided by the product of the two dimensions. Small tiles have far more joint per square metre than large ones, which is why a mosaic can need five times the grout of a large format porcelain over the same floor.
Cementitious grout has a bulk density around 1600 kilograms per cubic metre once mixed, though this varies with the sand grading and any polymer content, so it is an editable input here. Epoxy grout is heavier, typically 1700 to 1900, and is sold by the kit rather than the bag.
The formula
joint length- (w + l) ÷ (w × l) metres of joint per m² of tiling
width- Grout joint width in mm
depth- Tile thickness, or the depth the grout fills, in mm
density- Mixed grout density, around 1600 kg/m³
How it works, step by step
- Enter the tiled area and the tile dimensions.
- Enter the joint width and the tile thickness, which sets the joint depth.
- Adjust the density if your product data sheet states a different figure.
- The gauge shows kilograms; bags to buy appears in the breakdown.
Worked examples
300 × 600 tiles, 3 mm joints, 9 mm thick
Joint length is (300 + 600) ÷ (300 × 600) × 1000 = 5.00 m per m². Volume is that × 0.003 × 0.009 = 0.000135 m³/m², so at 1600 kg/m³ you need 0.216 kg per m² — 2.59 kg for 12 m².
Mosaic uses far more
A 50 mm mosaic has 40 m of joint per m² against 5.0 for the large tile. Over the same 12 m² that is 13.8 kg at 6 mm depth — many times the quantity for an identical floor.
How to read your score
Frequently asked questions
Why is my actual usage higher than the calculation?
Joints are rarely filled to a perfectly flat face, the sponge lifts grout out of the joint as you clean, and a good deal stays in the bucket and on the float. A 10% allowance covers normal practice; wide rustic joints can run higher.
Does joint depth equal tile thickness?
Usually, on a floor where the adhesive comes up to the underside of the tile. If the adhesive squeezes into the joint the effective depth is less, and on a wall you may only fill 5 to 6 mm of a 10 mm tile. Reduce the depth input if so.
Is epoxy grout calculated the same way?
The volume maths is identical, but epoxy is denser at roughly 1700 to 1900 kg/m³ and comes in fixed kits. Set the density accordingly and treat the kit weight as the bag size.
Should I mix all the grout at once?
No. Cementitious grout has a pot life of 20 to 30 minutes and stiffens after that. Mix what you can use in that window, and keep the water ratio identical between batches or the cured colour will vary between areas.
Grout quantity reference
| Tile size | Joint m/m² | 2 mm joint | 3 mm joint | 5 mm joint | 8 mm joint |
|---|---|---|---|---|---|
| 50 × 50 mm | 40.0 | 1.15 kg | 1.73 kg | 2.88 kg | 4.61 kg |
| 100 × 100 mm | 20.0 | 0.58 kg | 0.86 kg | 1.44 kg | 2.30 kg |
| 150 × 150 mm | 13.3 | 0.38 kg | 0.58 kg | 0.96 kg | 1.54 kg |
| 300 × 300 mm | 6.7 | 0.19 kg | 0.29 kg | 0.48 kg | 0.77 kg |
| 300 × 600 mm | 5.0 | 0.14 kg | 0.22 kg | 0.36 kg | 0.58 kg |
| 600 × 600 mm | 3.3 | 0.10 kg | 0.14 kg | 0.24 kg | 0.38 kg |
| 600 × 1200 mm | 2.5 | 0.07 kg | 0.11 kg | 0.18 kg | 0.29 kg |
Multiply by your area, then add a waste allowance. Deeper joints scale linearly.