Short definition
The size of the notches on a tiling trowel. It sets the adhesive bed depth, which in turn sets how many square metres a bag of adhesive covers.
A notched trowel has square or round teeth cut into its edge. Spreading adhesive with it leaves ridges of a known height, and those ridges collapse under the tile to form the adhesive bed. The notch size is quoted in millimetres — 3, 6, 10, 12 and 15 mm are the common ones — and it is what determines how thick the bed ends up.
Notch size is chosen from the tile, not from preference. Small tiles and mosaics bed onto a 3 or 6 mm notch. Standard floor tiles up to about 300 mm take 6 mm. Tiles up to 600 mm generally need 10 mm, and large-format tiles beyond that need 12 mm or more, partly for bed depth and partly because a bigger tile is less able to squeeze adhesive into full contact. Uneven substrates push the notch up a size.
The estimating consequence is that adhesive coverage is a function of the notch, not of the tile. A 20 kg bag that covers around 11 m² at a 6 mm notch covers roughly 7 m² at 10 mm and under 5 m² at 12 mm. Estimating a large-format job off a small-tile coverage figure can under-order the adhesive by half, which is the most common material shortfall on a tile job.
Back-buttering changes the figure again. Large tiles — generally anything over about 400 mm — are usually buttered on the back as well as trowelled onto the floor, to guarantee full contact and avoid hollow spots. That adds roughly 20% to the adhesive. Coverage figures also vary between adhesive products, so the manufacturer’s data sheet is the authority for any specific bag; the tile and grout estimator suggests a notch from the tile size and prices the bags off it.