Tile layout and centering calculator

Why the starting point matters

Tiles repeat every tile-plus-joint. Once you pick where one tile sits, every other tile's position is fixed, and so are the pieces left at the walls. Start with a full tile against the left wall of a room that is 5⅓ tiles wide, and the right wall gets a strip one third of a tile wide. Shift the whole grid by a few inches and both walls get a piece two thirds wide instead. Same tile count, far better result.

Installers settle this with a dry layout: tiles laid loose along a chalk line before any adhesive is mixed. This page does the same arithmetic on paper so you can check the room, and choose the tile orientation, before buying anything.

How to use it

Measure the room wall to wall in both directions at floor level (or, for a wall, from corner to corner and from the bottom of the tiled area to the top). Enter the tile dimension that will run along each direction: for a 12 × 24 in tile laid with its long side down the room, 24 along the length and 12 along the width. Enter your grout joint and the movement gap you will leave at each wall.

The result card gives the best of the three starting points in each direction. The tables show all three, so you can see how much better the best one is, and whether a full tile at a particular wall is worth giving up a little symmetry.

The method

Along one direction, subtract the perimeter gap from each end to get the usable span. Tile k then occupies the stretch from origin + k × (tile + joint) to that plus one tile. The three starting points differ only in the origin:

• Full tile against one wall: the first tile starts right after the gap. Every bit of cutting happens at the far wall.
• Grout joint on the centre line: a joint straddles the middle of the room, so both walls get the same cut.
• Tile centred on the centre line: a tile straddles the middle, again giving equal cuts.

For each, the first and last tiles that fall inside the usable span are found, and the visible width of each is measured. The best option is the one whose smaller edge piece is largest; ties go to a centred layout. There is a useful guarantee: the two centred layouts are exactly half a module apart, so one of them always leaves edge pieces of at least half a tile. You never need to accept slivers on both sides of a room.

A worked example

The default room is 11 ft 3 in by 8 ft 7 in, with 12 × 24 in tiles, 1/8 in joints and a 1/4 in gap at each wall. Along the 11 ft 3 in length, the usable span is 134.5 in, or 67.25 in each side of the centre. With a joint on the centre line, a half joint plus two tiles and their joints take 48.31 in, leaving an edge piece of 18.94 in (18 15/16 in) at each wall: most of a tile. Centring a tile instead would leave 6 7/8 in slivers.

Across the 8 ft 7 in width the answer flips. With a tile centred, 6 in of it plus three more tile-and-joint modules take 42.5 in of the 51.25 in half span, leaving 8 3/4 in pieces at each wall. A joint on the centre would leave 2 11/16 in slivers. So this room is laid out with a joint running down the middle lengthways and a tile centred across it, which is not something most people would guess.

Where this stops being reliable

• Out-of-square rooms. Walls that are not parallel make the edge pieces taper. The calculation is exact for the span you enter and nothing else; check both ends of the room.
• Doorways and focal points. A threshold, a tub front or a kitchen island may matter more than symmetric walls. The "full tile against one wall" row shows what starting from a visible edge costs at the other end.
• Nominal tile sizes. Measure a real tile. A tile that is 23 1/2 in rather than 24 in shifts every edge by half an inch per tile, which across a room is several inches.
• Rooms joined by an opening. If the tile runs through into a hallway, the layout has to work in both spaces; set it from the line that joins them.

Once the layout is set, the floor tile calculator gives the quantity to order.