Why a percentage cannot answer this
Waste on a tile floor is a perimeter problem, not an area problem. Nothing gets wasted in the middle of the room — every tile there goes down whole. The waste is entirely in the ring of cut pieces around the edge, and that ring grows with the perimeter while the tile count grows with the area. Double the size of a room and the waste percentage roughly halves. A flat five percent for a diagonal is a number that was true for somebody's room once.
This page places the tiles instead. It builds both grids from the centre of the room, clips every tile that runs past a wall, and reports what survives: how many go down whole, how many get cut, how many saw cuts that takes, and how small the smallest piece ends up.
Where the diagonal actually loses
A wall parallel to a square grid only ever crosses the joints running perpendicular to it. Turn the grid 45 degrees and the same wall crosses both families of joints, so joints meet it more often and the border breaks into more pieces. That is the whole mechanism, and it is why the extra pieces show up along the walls rather than in the field.
It does not follow that you buy that many more tiles. A diagonal border piece is a triangle, and two triangles that add up to less than a tile can often come off one tile with a single cut. A square border piece is a strip the full length of the tile, and two of those only share a tile if each is under half a tile wide. The page prints both bounds — one fresh tile per cut piece, and cut pieces sharing tiles wherever the areas allow — because the honest answer is somewhere between them and depends on how patiently the cuts are planned.
The tile size on the box is not the tile
Pressed porcelain shrinks in the kiln and is sold at a nominal size. A 12 in tile that measures 11-13/16 is normal, and over a 14 ft room that is a difference of nearly a full tile in the count. Measure several out of the box, including from different boxes, and use what the tape says. The same goes for the joint: the grid steps by tile plus joint, so an eighth of an inch of joint difference moves the cut at the far wall by an inch and a half over fourteen tiles.
What it does not know
It assumes a rectangle with straight walls and square corners, which no room is. It assumes you start with a whole tile on the centre of the room, which is one of several defensible starting lines and often not the best one. It knows nothing about a doorway, an island, a hearth, a toilet flange or a step, all of which change the counts. And it has no view on whether the substrate is flat enough or stiff enough for large tile on the diagonal, which is a question for the tile instructions and the framing, not for a layout page.
Questions people ask
How much extra tile does a diagonal layout need?
For the room and tile in the defaults the page prints both counts side by side rather than a rule of thumb, because the answer swings with the room. The mechanism is that a wall at 45 degrees to the grid crosses joints of both families instead of one, so the border breaks into more pieces. Whether that turns into more tiles depends on how well the triangles pair up, and the page prints the range rather than pretending to know.
Should I start with a tile in the middle of the room or a joint?
This page starts both grids with a whole tile centred on the room so the two layouts are compared on the same footing. That is not always the right starting line: a doorway, a hearth or the front of a run of cabinets is often where a cut tile is seen from three feet away, and lining up there beats lining up on a room centre nobody looks at. The tile layout calculator works the square case from three different starting lines.
Why does my room come out with zero cut tiles?
Because the clear dimension divided exactly by tile plus joint. It is worth checking rather than celebrating, since it usually means the tile size or the joint being entered is nominal rather than measured. Feed in the tile you actually measured out of the box and the joint you intend to hold, and the far wall usually moves.
What counts as a sliver?
The threshold is a field because it is a judgement. A strip narrower than a quarter of a tile is fragile to cut, tends to float rather than bed, and runs the length of a wall where the eye follows the grout lines straight into it. On a diagonal the equivalent problem is the needle point where the grid grazes a wall, which is worse because it comes to a tip with nothing under it.
Does the page account for a doorway or an island?
No. It lays a grid over a rectangle. A doorway adds a run of cuts that the rectangle never sees, an island removes full tiles from the middle and adds a second ring of cut pieces around itself, and a hearth does both. Treat the numbers as the floor of what the room needs, and count the extra ring around any obstruction separately.