Thinset and grout coverage

Neither answer is a figure per square foot, which is why both are so often wrong on a materials list.

Thinset coverage belongs to the trowel: mortar is combed into ridges of a controlled depth, and the depth is set by the notch. The notch in turn is set by the tile size, so a large format floor takes about twice the mortar of the same floor in small tile with exactly the same number of tiles.

Grout belongs to the geometry: how much joint there is per square foot, which follows from the tile’s length, width, thickness and the width of the gap between pieces.

The span from top to bottom of that table is a factor of nearly four. Between the two notches most residential work actually uses — 1/4 × 1/4 in square and 1/2 × 1/2 in square — it is still a factor of 2.1: 95 square feet a bag against 45. On a four hundred square foot floor that is 5 bags or 9, for exactly the same tile count.

The industry formula, with everything in inches: ((L + W) ÷ (L × W)) × thickness × joint × 13.5, giving pounds per square foot. The table below runs it for the common sizes at each joint width.

Two things this table is for

The first is the mosaic problem. A sheet of 2 × 2 in mosaic has roughly five times the joint length per square foot of a 12 × 12 in floor, so it uses roughly five times the grout — and on a small backsplash the grout can genuinely cost more than the tile. Nobody budgets for that, because the coverage figure on the bag was printed with a different tile in mind.

The second is the joint width, which is a straight multiplier. Doubling the joint doubles the grout, exactly. It is a decision people make on the floor with the tile already open, and it changes the order.

What the numbers assume

Thinset coverage assumes a properly combed bed at the notch quoted, on a flat substrate. An uneven substrate eats mortar without improving anything, and back-buttering large format adds roughly a tenth on top. Grout figures assume the joints are filled full depth, which is what happens when the tile is set at the right height in the first place.

Neither figure includes waste. Both the tile calculator and any sensible order should apply the pattern allowance to the mortar and the grout as well as the tile — the offcuts were still buttered, and the joints around them still grouted.

Using it

The tile calculator picks the trowel from the tile size you choose, runs the grout formula on your own joint width, and prices every notch side by side so you can see what the choice costs before making it.