The usual answer to how much extra tile to buy is 15 percent, or 20 for a diagonal layout. The more useful answer is that it often does not matter, because tile comes in whole boxes. On an 80 square foot floor, buying 0, 5 or 10 percent extra all comes to the same 6 boxes.
How much extra tile to buy is not a percentage
You buy boxes. The percentage only matters when it pushes you over a box boundary, and on any given room most of the range does not. An 80 square foot floor with boxes covering 15 square feet each:
- 0% extra: 80 sq ft to cover, 6 boxes
- 5%: 84 sq ft, 6 boxes
- 10%: 88 sq ft, 6 boxes
- 15%: 92 sq ft, 7 boxes
- 20%: 96 sq ft, 7 boxes
- 25%: 100 sq ft, 7 boxes
Six boxes or seven. That is the entire decision, and everything between 15 and 25 percent buys you the identical order. Agonizing over whether the right allowance is 12 or 18 percent is agonizing over nothing.
Boxes needed
boxes = area × ( 1 + extra ) ÷ coverage per box, rounded up
80 sq ft at 15% is 92, and 92 ÷ 15 rounds up to 7 boxes
So buy the extra box
Since the choice is usually one box either way, the question becomes whether that box is worth having. It nearly always is, for reasons that have nothing to do with the percentage.
Dye lots do not come back
Tile is manufactured in batches, and color varies measurably between them. Boxes bought together share a lot number; a box bought three months later usually does not, and the difference is visible in daylight. The industry grades this up front: the Ceramic Tile Education Foundation explains the V0 to V4 shade variation ratings printed on the box, and anything at V3 or V4 varies enough that a later lot will not blend. Whatever you are going to need, you need it now.
Breakage happens after delivery
Tiles crack when they are cut, and a cut that goes wrong on the last row is a tile you cannot replace from the pile you have already used. Some arrive damaged. A spare box absorbs both without a second trip.
Future repairs are the real argument
A cracked tile in year three is a twenty minute job if you have a matching spare in the garage, and a whole-floor problem if you do not. Half a box put away is the cheapest insurance in the project.
When the percentage genuinely matters
On a larger floor the boundaries come thick and fast, and the allowance stops rounding away.
- 8 x 6, 55 sq ft: 4 boxes
- 10 x 10, 115 sq ft: 8 boxes
- 12 x 14, 193 sq ft: 13 boxes
- 20 x 16, 368 sq ft: 25 boxes
At 25 boxes, a five point change in the allowance is a box or two of real money. Small rooms are where the percentage disappears into rounding; large ones are where it is worth thinking about.
What actually drives the waste
The allowance is not a superstition, it is an estimate of offcuts, and the layout decides how big it should be.
- A straight grid in a square room: 10 percent is usually plenty. Cuts happen on two edges and many offcuts are reusable at the opposite wall.
- A diagonal layout: 20 percent or more. Every tile at every edge is a diagonal cut, and the triangles left over fit nothing.
- Herringbone or a patterned lay: more again, because the pattern dictates where cuts fall rather than the room doing it.
- A cut-up room: alcoves, a kitchen island, doorways and anything curved all add cuts, and each one produces an offcut nobody can use.
Measuring the floor, not the room
Tile goes on the floor, so the measurement is length times width, and unlike paint there is a case for subtracting: anything permanently fixed that the tile genuinely will not go under.
- Subtract a built-in island or a bath panel that is staying, since the floor under it will not be tiled.
- Do not subtract a freestanding appliance, or anything you might replace. Tiling under a dishwasher costs a few tiles and saves an ugly step when the next one is a different height.
- Add any doorway threshold and the strip inside a closet, which people forget and then have to cut into after the main floor is set.
For an L-shaped room, split it into rectangles and add them, which is easier to get right than trying to measure the whole thing at once.
What else goes on the order
The box count is the visible half of the shopping list. These are bought by coverage too, and running out of any of them stops the job just as hard.
- Adhesive or thinset, where coverage depends on the trowel notch and the tile size rather than on the floor area alone.
- Grout, which depends on joint width and tile thickness. Wide joints on large tiles use far more than the bag suggests.
- Backer board if the substrate needs it, plus screws and tape.
- Trim and edging, counted in linear feet along exposed edges rather than by area.
- Spacers and a decent cutter, which are cheap and are the difference between neat joints and obvious ones.
Box coverage matters more than the allowance
Coverage per box varies enormously by tile size, and it moves the order far more than any waste percentage. On the same 80 square foot floor at 15 percent:
- 10 sq ft a box: 10 boxes
- 15 sq ft a box: 7 boxes
- 20 sq ft a box: 5 boxes
- 25 sq ft a box: 4 boxes
So check the box before doing any arithmetic, and be careful comparing two quotes: a price per box means nothing until you know what the box covers.
The tile calculator takes the room, the box coverage and the allowance and returns boxes rather than square feet, which is what you actually order. The flooring calculator does the same for boards and planks, and the paint calculator covers the walls above.
Work out your own floor
Frequently asked
Fifteen percent for a straight layout and twenty or more for a diagonal one. On a small floor the choice often makes no difference: an 80 square foot room needs 6 boxes at 0, 5 or 10 percent, and 7 boxes at anything from 15 to 25.
Because tile is sold in whole boxes. The allowance only matters when it crosses a box boundary, so on an 80 square foot floor every value from 15 to 25 percent buys exactly the same 7 boxes.
Twenty percent or more. Every tile meeting a wall is cut at an angle and the offcut triangles fit nowhere else, so waste is much higher than a straight grid where many offcuts are reusable.
Almost always. Dye lots vary between production batches, so a box bought later will not match, and a spare makes a future cracked tile a twenty minute repair rather than a whole floor.
About 8 boxes at 15 percent extra, with boxes covering 15 square feet each. Check your own box coverage first, since it ranges widely and changes the count more than the waste allowance does.