Standard advice is to add 10%. That figure is right for one specific case and wrong for several common ones.
What decides it
Waste comes almost entirely from cut tiles whose offcut cannot be used elsewhere.
In a straight lay against a rectangular room, the cut row along one wall produces offcuts of a consistent width — and if that width matches the cut needed on the opposite wall, half of them get used. Waste stays low.
Change the pattern and that stops working:
- Diagonal produces triangular offcuts. A triangle cut from a corner has no second home.
- Herringbone and chevron cut at angles that vary along the perimeter, so almost nothing repeats.
- Rooms with alcoves, doorways and fixtures produce one-off cuts, each with a unique offcut.
| Situation | Add |
|---|---|
| Straight lay, simple rectangle | 10% |
| Straight lay, several corners or obstacles | 12–15% |
| Diagonal or brick-offset | 15% |
| Herringbone, chevron | 20%+ |
| Small room, many cuts relative to area | 20% |
| Large-format tile | 15% — breakage matters more |
Small rooms need a larger percentage
Waste scales with the perimeter, not the area, because cuts happen at edges.
A 4 × 6 ft bathroom is 24 square feet with 20 feet of perimeter. A 20 × 20 ft room is 400 square feet with 80 feet of perimeter — sixteen times the area for four times the edge.
Ten percent of 24 square feet is 2.4 square feet, which is roughly two tiles. That will not cover the cuts around a toilet flange, a vanity and a doorway. In small rooms, work from the cuts you can actually foresee rather than from a percentage.
Large format breaks
Big tiles are heavy, thin relative to their span, and unforgiving. They break during handling, during cutting, and occasionally while being set.
The waste allowance for large format is less about offcuts and more about breakage, which is why 15% is reasonable even for a simple layout.
The two costs of getting it wrong
Too little is the expensive error. Running short means a second purchase, and a second purchase means a different batch — see the shade-variation problem in how many tiles per 100 square feet. At worst the line is discontinued and the floor cannot be finished as designed.
Too much costs the price of a few boxes, most of which should be kept as spares anyway. Some suppliers accept returns on unopened boxes, some do not.
The asymmetry is stark, and it is the whole argument for rounding up rather than down.
Do the arithmetic properly
- Measure each area separately — do not use one figure for a whole floor with several rooms.
- Subtract genuinely tiled-around fixtures only if they are large. A toilet footprint is not worth subtracting; a kitchen island is.
- Add the waste percentage from the table.
- Round up to whole boxes, since that is how tile is sold.
- Check every box shares a lot number.
Step 4 usually adds a few percent on its own, which is a free part of the margin.
FAQ
How much extra tile should I buy?
10% for a simple straight lay, 15% for diagonal or offset patterns, 20% or more for herringbone or a small room with many cuts.
Why do small rooms need more waste allowance?
Waste comes from cuts at the perimeter, and small rooms have a high perimeter relative to their area. A percentage of a small area is not enough tiles.
Can I return unused tile?
Sometimes, on unopened boxes. Keep at least one box regardless — tile lines are discontinued and spares make future repairs invisible.
Does large-format tile need more waste allowance?
Yes, around 15%, mostly for breakage during handling and cutting rather than for offcuts.
What is the most expensive mistake?
Buying too little. The second purchase comes from a different batch with a different shade, and the difference shows across a finished floor.