Area is the easy part. Your waste factor is decided by the tile format and the layout pattern, and it ranges from 10 percent to 20 depending on choices you make before you buy anything.
Length times width for a floor, or wall by wall for a splashback or a bathroom. Subtract anything fixed that the tile stops at, like a bath, a shower tray or a run of base units.
Then decide the pattern, because that is what drives the waste allowance and it needs deciding before you order rather than after.
The jump from straight to diagonal is the one people underestimate. Turning the same tile 45 degrees adds roughly half again to the waste, because instead of full tiles meeting a wall with one cut line, every edge tile becomes a triangle.
Natural stone waste is not just cutting. The fitter rejects pieces with structural flaws or veining that clashes, before any tile touches the floor. That is why the allowance is higher and why buying exactly to the calculation is a mistake with stone.
Tile is sold by nominal size, but the laid module is the tile plus one grout line.
A 12 inch tile with a 1/8 inch joint occupies 12.125 inches on the floor. Across a 12 foot run that is over an inch of accumulated difference, which is roughly a tenth of a tile.
On a small area this rounds away. On a large floor, or where you are trying to work out whether a run finishes with a full tile, it matters. Calculate on tile plus joint if the layout is tight.
This is a layout question that measuring answers.
Divide the room dimension by the tile module. If the remainder leaves a very thin sliver at one edge, that looks wrong and is difficult to cut and fix. The standard fix is to shift the starting line so both edges get an equal, larger cut instead of one full row and one sliver.
Doing that arithmetic before ordering also tells you whether a slightly different tile size would lay out better in your room, which is worth knowing while you can still change your mind.
Large format tile is unforgiving of an uneven substrate. It does not flex, so a hollow under a tile becomes a crack, and a high spot creates lippage where one tile edge stands proud of its neighbour.
Shoot the ceiling from several points around the room and compare, which gives you the floor variation indirectly. Meaningful variation means levelling compound, and it is much cheaper to budget for that before starting than to discover it with half the floor laid.
The larger the tile, the flatter the floor has to be, which is a genuine argument for smaller tiles in an old house with a bouncy floor.
Tile is batch-fired and colour varies between batches, sometimes noticeably. Order the full quantity including waste in one go, check the batch numbers match on delivery, and keep the offcuts and any spare tiles.
A cracked tile three years later is a trivial repair if you have a matching spare and a serious problem if you do not. Hard flooring generally is covered here, and a kitchen backsplash has its own guide.
You will find our top laser measure picks on the front page, ranked with the reason for each.