Grout Calculator

How Grout Quantity Is Calculated

Grout fills the joints between tiles. The amount needed depends on tile size, joint width, joint depth, and tile thickness.

Grout Volume Formula: Grout (kg) = [(L + W) / (L × W)] × J × D × A × ρ

Where:

• L = tile length (mm)
• W = tile width (mm)
• J = joint width (mm)
• D = joint depth (mm, usually = tile thickness)
• A = total tiled area (m²)
• ρ = 2.33 kg per litre of joint

That last figure is not a density in the ordinary sense. It is how much dry grout, the powder you buy in a bag, fills a litre of finished joint, and it is set where the manufacturer coverage charts put it. Quoting the density of the cured material instead understates what you need to buy, because the mixing water is not in the bag.

Simplified Practical Formula: Most tile manufacturers provide a coverage rate in kg/m² for their grout. Multiply that by your area and add 10% waste.

Worked Example: 300mm × 300mm tiles, 3mm joints, 10mm thick tiles, covering 15 m²:

• Joint factor = (300 + 300) / (300 × 300) = 600 / 90,000 = 0.00667
• Volume per m² = 0.00667 × 3 × 10 = 0.2 liters/m²
• Total volume = 0.2 × 15 = 3.0 liters
• Weight = 3.0 × 2.33 = 7.0 kg + 10% waste = 7.7 kg

⚠️ The boxes above want centimetres, not millimetres. The example is written in mm because that is how tile is sold, but the metric setting reads cm. That same job is 30 × 30 cm tiles, a 0.3 cm joint, 1 cm thick. Type 300 where it wants 30 and the answer comes back a hundred times too big, which is the one mistake worth watching for on this page.

The same job in inches comes to 264 square feet from a 25 lb bag for a 12-inch tile at a 1/8-inch joint, which is what the tile grout amount, tile grout bag and tile grout quantity calculators all return. The four pages run one figure between them.

Reference Values:

• Small tiles (100×100mm), 2mm joints: ~0.8 kg/m²
• Medium tiles (300×300mm), 3mm joints: ~0.5 kg/m²
• Large tiles (600×600mm), 5mm joints: ~0.4 kg/m²
• Always add 10–15% for waste, cuts, and uneven surfaces
• Sanded grout: joints wider than 3mm; Unsanded: joints 3mm or narrower

A note on tile size and joint depth. Small mosaic tiles have far more joint length per square foot than large-format tiles, so they eat much more grout for the same area. Joint depth usually equals the tile thickness, so thicker tiles pull more grout into every joint. If you are setting a mix of sizes, run the calculator once for each tile size and add the results rather than averaging, since the joint math is not linear.