Formula
Tiles are the room area times one plus waste, divided by the area of one tile, rounded up. A 12 × 12 tile is one square foot, so a 10 × 10 room at 10% waste is 110 tiles. At 10 per box that is 11 boxes. Diagonal at 15% is 115 tiles. Herringbone starts at 20%.
joint, in³ per tile = depth × joint × (tile length + tile width + joint)
per ft² = that volume × 144 / ((L + joint) × (W + joint))
A 12 inch tile with a 1/8 inch joint and 1/4 inch of grout depth is about 0.74 in³ of joint per square foot. A bag’s square-foot rating will not match that volume times 100 lb/ft³, so the bags come from the coverage you type. Thinset defaults to 50 ft² per bag as a planning figure for a common notch, and the chart on the bag wins. The drywall calculator and the fence calculator are the other finish takeoffs.
Worked examples
10 × 10 ft, 12 × 12 tile, straight, 10% waste, 10 per box. 110 tiles, 11 boxes. Thinset at 50 ft² per bag is 2 bags. Grout at 100 ft² per bag is 1 bag.
The same floor on a diagonal. 15% waste is 115 tiles, still 12 boxes only if you change the box size. At 10 per box it is 12 boxes.
Joint check. 12 × 12, 1/8 inch joint, 1/4 inch deep, is 0.738 in³ per square foot. Over 100 ft² that is about 0.043 ft³ of joint, which is not a bag count.
Reference
Planning waste. Not a TCNA allowance.
| Pattern | Starting waste |
|---|---|
| Straight | 10% |
| Diagonal | 15% |
| Herringbone | 20% |
The 12 × 12 joint at 1/8 by 1/4 is 0.738 in³/ft².
Questions
Are 10, 15, and 20 percent a standard?
No. They are planning allowances for a straight set, a diagonal, and a herringbone. A careful layout on a square room can beat them. An out-of-square room can miss them. Change the percent.
Why doesn’t the grout volume match the bag chart?
The volume is the empty joint. A powder’s coverage also depends on how it is mixed and how much it shrinks. The square feet printed on the bag is what you order from. The volume is there so you can see the joint.
Does thinset cover the waste tiles?
No. Thinset and grout bags are figured on the floor area, not on the extra tile you cut and throw away.
Sources
- Tile count is area × (1 + waste) / tile area, rounded up. Waste of 10 / 15 / 20 percent is a planning allowance for this site, not a TCNA standard.
- Joint volume is depth × width × (tile length + tile width + joint), per tile, converted to a square foot. Bag coverage is an input because powder charts do not match that volume times a fixed density.