What the grout calculator does
Grout fills the joints between tiles, so the amount you need depends on how much joint there is per square foot or square metre. Small tiles and wide, deep joints use far more grout than large tiles with narrow joints.
The calculator offers two methods:
- Manufacturer coverage (preferred) – many grout makers publish coverage tables for specific tile sizes and joint widths. If yours does, enter that figure and the package size.
- Joint-volume formula – when no coverage figure is available, the calculator works out the volume of the joints and multiplies by the grout density.
How to use it
- Enter the tiled area and a waste allowance.
- Choose a method.
- For the formula, enter the tile length, width and thickness, the joint width, and the grout density from the data sheet. The default density of 1.6 kg/L is an assumption.
- For manufacturer coverage, enter the area one package covers for your tile and joint size.
- Enter the package size and, optionally, its price.
The grout formula
The joint-volume formula used by many grout manufacturers is:
kg per m² = (A + B) ÷ (A × B) × C × D × ρ
- A, B = tile length and width (mm)
- C = joint depth, normally the tile thickness (mm)
- D = joint width (mm)
- ρ = grout density (kg/L, the same as g/cm³)
Why it works: in a regular grid of A × B tiles, the length of joint per unit of area is 1/A + 1/B, which is the (A + B) ÷ (A × B) term. Multiplying by the joint cross-section (C × D) gives the joint volume. With everything in millimetres, the result comes out directly in litres per square metre, and multiplying by density converts litres to kilograms.
Assumptions: square-edged tiles, joints filled to full tile depth, and a straight grid. The total is then multiplied by the area and the waste allowance.
Worked examples
300 × 300 mm tiles, 8 mm thick, 3 mm joints, density 1.6 kg/L:
- (300 + 300) ÷ (300 × 300) = 0.00667
- 0.00667 × 8 × 3 × 1.6 = 0.256 kg/m²
- For 11 m² with 10% waste: 0.256 × 11 × 1.1 = 3.10 kg → one 5 kg bag
12 × 12 in tiles, 3/8 in thick, 1/8 in joints, 120 sq ft: in millimetres that is 304.8 × 304.8, 9.5 mm thick and 3.2 mm joints, giving about 0.32 kg/m². Over 11.15 m² with 10% waste, about 3.9 kg (8.6 lb).
Compare mosaics: 50 × 50 mm tiles, 6 mm thick with 2 mm joints use about 0.77 kg/m², roughly three times as much grout per square metre as the 300 mm tiles above.
Understanding the results
- Grout needed – total weight including waste, in pounds or kilograms.
- Consumption – grout per square metre (and per square foot) for your tile and joint.
- Joint volume – the volume of the joints in litres, before density is applied.
- Packages – total weight divided by package size, rounded up.
What changes grout consumption
- Tile edges – bevelled or pillowed edges create wider joints at the surface.
- Joint depth – joints are not always filled to full tile depth; raked or partially filled joints use less.
- Grout type – cement, epoxy and urethane grouts have different densities.
- Surface – natural stone and handmade tiles with irregular edges vary joint width.
- Mixing losses – grout left in the bucket and on the float adds to waste.
Always check the product data sheet. Count tiles with the tile calculator, and work out the area first with the square footage calculator.