Updated July 18, 2026
How it works
Divide the room area in square feet by the area of one tile (width x length in inches, divided by 144). Multiply by (1 + waste percentage / 100) and round up to the next whole tile. A 10% waste buffer covers diagonal cuts, broken tiles, and future repairs.
Thinset and overage figures below come straight from Mapei's and TEC's published coverage charts, cited further down the page.
What actually justifies the overage
Buy 10 percent extra for a straight layout, 15 percent for diagonals, natural stone, or busy patterns. The overage is not optional padding. It covers the tiles you cut to fit the perimeter, the ones that snap wrong on the saw, and the spares you keep for repairs. Dye-lot color shifts between batches, so a tile you buy two years from now will rarely match. Set the spares aside the day the job finishes.
Worked example
A 12 by 10 foot floor is 120 square feet. A 12 by 12 inch tile covers exactly one square foot, so you need 120 tiles for the field. Add 10 percent and order 132. Switch to a 45 degree diagonal and bump the overage to 15 percent, which is 138 tiles. Tiles usually ship by the box, and box coverage varies, so divide your square footage by the box coverage and round up to whole boxes rather than counting individual pieces.
Overage by layout
| Layout or material | Recommended overage |
|---|---|
| Straight, square grid | 10% |
| Diagonal (45 degree) | 15% |
| Herringbone or pattern | 15% |
| Natural stone | 15% |
| Large rooms with few cuts | 10% |
Do not forget the setting materials
Tiles are only part of the order. Thinset mortar covers roughly 40 to 50 square feet per 50 pound bag, though that drops fast with large tiles, big notched trowels, or uneven substrate. Grout coverage depends on tile size and joint width, so use your grout manufacturer's chart instead of guessing. Buy backer board, a leveling system, and spacers in the same trip; running out mid-job means a tile half-set while you drive to the store. Tiling a shower floor comes with one more step before any of this: packing a sloped mortar bed under the pan. Size that mix separately with the deck mud calculator.
Measuring walls and floors
For a floor, length times width gives the area; break L-shaped rooms into rectangles and add them. For a wall, multiply height by width and subtract windows, niches, and the area behind a vanity or tub surround you will not tile. Measure the actual room, not the blueprint, because framing and finishes move the real dimensions. Then add your overage to the net figure, not before.
Common mistakes
- Applying overage to the gross area before subtracting openings, which inflates the order.
- Mixing boxes from different dye lots. Check the lot number on every box and keep them together.
- Underbuying thinset because the bag coverage assumed a small trowel and a flat floor.
- Forgetting trim pieces, bullnose, and transitions, which are sold separately and often run short.
Sources: Mapei tile and grout calculator, TEC coverage calculator.