Tile size, grout joint and what they do to your quantities

A tile is bigger than its face

A tile on a floor does not cover only its own face. Each tile also owns half the joint on every side, so the area it occupies is the tile plus one joint in each direction. A 12 × 12 inch tile with a 1/8 inch joint occupies 12.125 × 12.125 inches, about 2% more than its face. A 4 × 4 inch tile with the same joint occupies 4.125 × 4.125, about 6% more. The floor is divided into these pitches, not into faces, so counting by the face over-counts tiles, and the smaller the tile, the more it over-counts.

The floor tile calculator counts by the pitch. For a 12 × 10 ft room in 12 × 24 inch tile with a 1/8 inch joint, the pitch is 2.031 sq ft and the room takes 59.1 tiles before waste, 65 with 10%: 9 boxes of 8.

How wide should the joint be?

The joint is not decoration. It absorbs the small differences in size between tiles so the joints stay straight, and it gives the floor a little room to move. ANSI A108.02 and the TCNA Handbook set the minimums; a joint narrower than the tile’s size variation produces wandering lines.

Grout: where the joint really shows

The joint changes the tile count by a few percent, but it changes the grout by much more. Grout fills the joint to the full depth of the tile, so the grout volume depends on three things together: the joint width, the tile thickness, and the length of joint per square foot, which is much greater for small tiles.

The figures are from the grout calculator, without a mixing allowance, and the full table is on the grout and thinset coverage reference. A 4 × 4 floor with quarter-inch joints takes nine times the grout of a 24 × 24 floor. The joint width also decides the grout type: unsanded for joints up to 1/8 inch, sanded above it.

Mortar follows the tile size

Bigger tiles need a deeper bed of thinset to reach full contact, so the trowel notch grows with the tile: a 1/4 inch square notch for tile up to about 8 inches, 1/4 × 3/8 for 12 to 16 inches, 1/2 inch for large format. A deeper notch covers fewer square feet per bag, so a floor of large tile takes roughly twice the mortar of the same floor in small tile. The guide to thinset coverage has the figures, and the thinset calculator does the arithmetic.

Layout follows the tile size too

Large tiles make the cuts at the walls more visible and more expensive. A 24 inch tile that ends in a 2 inch sliver along a wall wastes most of a tile and looks wrong. Before you order, check where the cuts fall: centring a tile or centring a joint on the room changes every cut by half a tile. The tile layout calculator compares the two.

Nominal size and real size

The size on the box is often a nominal size, a rounded name for the tile, not its measured face. Many tiles sold as 12 × 12 inches are made to a metric size of 300 × 300 mm, which is 11.81 inches, and many sold as 12 × 24 are 300 × 600 mm. Tiles are also made slightly undersize on purpose so that tile plus joint lands on the nominal pitch. The difference is small per tile but it adds up across a room: on a 14 ft wall, 0.19 inches per tile is more than two inches by the far wall, enough to move every cut.

Measure two or three tiles from the box with a tape before you plan the layout, and use the measured size in the calculators. For the count it rarely changes the boxes; for the layout it decides whether the last row is a clean half tile or a sliver. Check the box for a calibre or shade code too, and buy all the tile from one run, because sizes can differ slightly between runs of the same product.

Small tile versus large tile, side by side

Neither is cheaper in every way. Large tiles save grout and joints and look clean; they cost more in mortar, preparation and handling. The large-format tile guide covers what big tiles need, and the shower guide covers the small tiles a sloped floor demands.