How this calculation works
Calculate the share of a repeating tile grid occupied by joints, multiply by filled depth and area, then use the selected product’s mass per litre. Add an explicit allowance and round purchases to whole bags. The thin-joint model is an approximation with different intersection treatment.
Module joint fraction = 1 − tile area / ((length + joint) × (width + joint)); litres = m² × filled mm × joint fractionA worked example
For 10 m² of 300 mm square tiles, 3 mm joints and 6 mm fill, the full-module model gives about 1.182 L of joint volume. At the illustrative 1.6 kg/L and 10% allowance, the estimate needs two 2 kg bags.
Good to know
Ideal repeating rectangular grid; boundaries, movement joints, bevels, uneven beds and product tooling are excluded. Filled depth and mixed density must match the intended product and installation.
Reference: ARDEX: joint width, depth and grout coverage (opens in a new tab) ↗
Grout occupies joints rather than whole tiles
Tile purchase quantity and grout demand are different calculations. Grout fills the spaces between tiles to a selected depth. A wider joint or deeper fill can increase grout demand without changing the tiled area.
The full-module model uses one tile-plus-joint repeating footprint, counting joint intersections once. It allocates part of that footprint to tile and the remainder to grout. Finite floor edges and movement joints are not individually drawn or quantified.
Know when the narrow-joint approximation differs
The thin-joint approximation uses joint width multiplied by the sum of reciprocal tile dimensions. It is convenient when joints are small relative to the tiles, but it approximates the intersecting joint geometry.
The alternative module model keeps the tile-plus-joint area in the denominator. Different manufacturer coverage formulas may use their own approximation and product factor. Choose a stated method rather than treating a small disagreement as a rounding bug.
Convert volume with the actual product yield
One square metre covered to a depth of one millimetre has a volume of one litre. Multiplying the joint share by area and depth therefore gives joint litres directly with the units used on this form.
Converting those litres into kilograms requires the product’s mixed density or equivalent coverage factor. Loose powder bulk density and water density are different quantities. The illustrated factor is an input example, not a recommendation for every grout product.
Separate theoretical fill, allowance and purchase rounding
The calculator first finds geometric mass, then adds the explicit allowance and finally rounds to whole bags. These stages answer different questions. The last step creates a purchase surplus; it does not mean all purchased material will necessarily be used.
The tile quantity calculator helps plan the tiles themselves. The brick wall face quantity calculator instead counts masonry units, illustrating why a material count and a joint-volume estimate are different tasks. Verify the selected grout’s installation instructions, joint suitability and coverage data before ordering or mixing.
A few questions, answered
Is filled depth always the full tile thickness?
No. Enter the intended filled joint depth for the chosen installation.
Can I use loose powder density?
Use the product’s mixed yield or equivalent coverage factor, not a loose powder bulk density.
Does the tile count determine grout bags by itself?
No. Joint width, fill depth, tile dimensions, product yield and allowance also matter.