What diagonal layout actually costs
The tile does not change size when you rotate it 45 degrees, and the field area does not change either. What changes is the perimeter. In a straight layout the tiles meeting a wall get a straight cut, and the piece you cut off is a rectangle with two factory edges — it very often goes straight into the opposite side of the room or the next row. In a diagonal layout every perimeter tile is cut corner to corner and yields two triangles. A triangle only fits back into the layout at a corner of the same handedness, and a rectangular room gives you four corners for a lot of triangles.
Work it through on a 10 by 8 room with 12 inch tile. The perimeter is 36 feet, which is 36 perimeter tiles, out of about 80 tiles in the room. Straight-laid, maybe half those cuts produce a usable remainder, so you throw away the equivalent of nine or ten tiles: around 11 percent, which is why 10 percent is the standard allowance. Diagonally, almost none of the triangles come back, and you throw away nearer sixteen: about 20 percent by itself, though the base allowance already covers part of that. Adding five points on top of a ten percent base is the honest middle.
The percentage climbs with two things. Small rooms, because the perimeter is a larger fraction of the total. And broken-up rooms — an L shape, an island, a hearth, three doorways — because each interruption is more perimeter without any more field.
Why the grout joint changes the tile count
People calculate tiles as area divided by tile face area, which quietly assumes the tiles butt together. They do not. A 12 by 12 tile with a 1/4 inch joint occupies a 12.25 by 12.25 square of floor, which is 1.042 square feet rather than 1.000. Over 200 square feet that is eight tiles you do not need to buy, which is more than a box.
It works the other way on small-format tile, where the joint is a bigger fraction of the module. A 2 inch mosaic chip with a 1/8 inch joint occupies 2.125 by 2.125, which is 13 percent more floor than the chip itself. That is why mosaic sheets are quoted by sheet coverage rather than by chip count.
| Tile | Joint | Face area | Floor occupied |
|---|---|---|---|
| 12 x 12 | 1/8" | 1.000 sq ft | 1.021 sq ft |
| 12 x 24 | 1/8" | 2.000 sq ft | 2.032 sq ft |
| 6 x 6 | 3/16" | 0.250 sq ft | 0.266 sq ft |
| 2 x 2 mosaic | 1/8" | 0.028 sq ft | 0.031 sq ft |
Thinset coverage is a trowel question, not a tile question
A 50 lb bag of modified thinset covers somewhere between 45 and 130 square feet, and which end you land on is set almost entirely by the notch on your trowel. The trowel is chosen by tile size, because a larger tile needs a deeper bed to accommodate the flatness variation across its face. The figures here are nominal manufacturer coverages and they run optimistic in practice for two reasons: back-buttering large tile roughly doubles the mortar under it, and any dip in the substrate that you fill with thinset comes out of the same bag.
If you are setting large format, back-buttering, or working over a substrate that is merely acceptable rather than flat, add 25 percent to the bag count. Thinset is cheap, and running out with half the floor set and a bucket going off is the worst moment on the job.
Grout volume, and why the number looks so small
The grout figure here is derived rather than looked up: the joint network volume per square foot, taken from the module geometry and the tile thickness, converted at about 105 pounds per cubic foot of cured grout. For 12 by 24 tile at a 1/8 inch joint and 3/8 inch thick, that is roughly 0.05 pounds per square foot, so a 200 square foot floor takes about 10 pounds. That is genuinely all it is, and it surprises people who have watched a tiler go through the mixing bucket.
Treat it as a floor rather than a shopping list. Grout gets wasted in the bucket, on the sponge, in the bag you opened and cannot reseal, and in the batch that stiffened while you were cleaning haze off the last section. Buy the next size bag up, and if the joints are wide or the tile has a deep bevel, expect the real consumption to run half again over the geometric figure. Also mix full bags rather than partial ones where you can: pigment does not distribute evenly through a partially used bag, and colour shift between the first section and the last is a defect you cannot sponge out.
Questions people ask
How much extra tile should I order?
Ten percent over the field area is the standard for a straight layout in a plain rectangular room, and that is the default here. Add five points for a diagonal layout and ten for herringbone or chevron. Add more, not less, for small rooms, rooms with several doorways or alcoves, and any pattern that has to align across a threshold. Then buy one box beyond whatever that comes to and store it. Tile is produced in batches with real shade and calibre variation, and a box bought later, even of the same product code, will usually be visibly different.
How many 12 x 24 tiles are in a box?
Six is the common count, which is 12 square feet, but it varies by manufacturer and by tile thickness — some ship 8 pieces for around 15.5 square feet, and thick porcelain ships fewer to keep the box liftable. The box always states both the piece count and the square footage. Enter the piece count from the actual box you are buying rather than the default here, because the box count drives the rounding and a wrong count can put you a whole box out.
Does the calculator account for the pattern layout starting point?
No, and that is a real limitation. Where you set the first tile determines the width of the cut pieces at each wall, and a bad start point can leave you with a sliver an inch wide down one side — which looks poor and breaks more often during cutting. Snap your layout lines, dry-lay a row in each direction, and shift the start so the cuts at opposite walls are roughly equal and no cut is less than about half a tile. That decision changes the appearance far more than it changes the count.
Do I need modified or unmodified thinset?
It depends on what is underneath and what the tile is, and getting it wrong causes failures that show up months later. As orientation: most uncoupling membranes and many waterproofing membranes specify unmodified mortar over them, because a modified mortar sandwiched between two non-porous layers cannot cure properly. Porcelain over cement board generally wants modified. Follow the membrane manufacturer instruction over any general advice, including this, since it is the membrane warranty that is at stake. The coverage figures here are for a standard 50 lb bag either way.