Thinset mortar bonds tile to the floor or wall. How much you need depends mainly on the trowel notch: larger tiles need a deeper notch to fill the gap under the tile, and back-buttering the tiles adds more. This calculator turns the area and the notch into pounds and bags.
Coverage per 50 lb bag
| Trowel | Typical use | Coverage |
|---|---|---|
| 3/16 in V-notch | mosaics, small wall tile | 90 to 100 sq ft |
| 1/4 × 1/4 in square notch | tile up to about 8 in | 40 to 50 sq ft |
| 1/4 × 3/8 in square notch | tile 8 to 16 in | 30 to 35 sq ft |
| 1/2 × 1/2 in square notch | large-format tile and stone | 20 to 25 sq ft |
These are typical values; the exact coverage is printed on each bag and varies with the product and the substrate.
How the calculation works
- Thinset = area ÷ coverage per 50 lb × 50 lb
- Back-buttering adds about 30%
- Waste is added, and the total is divided by the bag weight and rounded up
Example
Tiling 100 sq ft of floor with 1/4 × 1/4 in square notch trowel takes 100 ÷ 45 × 50 = 111 lb of thinset. With 10% waste, that is 122 lb, so 3 bags of 50 lb.
Choosing the trowel
The goal is full contact: industry standards (ANSI A108) call for at least 80% coverage under tile in dry areas and 95% in wet areas and outdoors, with all corners supported. Lift a tile after setting it to check. If the back is not covered, use a larger notch or back-butter.
Large-format tiles (any side 15 in or more) usually need a large notch, back-buttering and a medium-bed or large-and-heavy-tile mortar.
Tips
- Mix only what you can use within the pot life, usually 2 to 4 hours, and do not add water to thinset that has started to stiffen.
- Comb the ridges in one direction, not in swirls, so air can escape when the tile is pressed down.
- Use modified thinset where the substrate or tile calls for it, and unmodified thinset where the membrane manufacturer requires it.
- Keep a spare bag for cuts, repairs and mistakes; an unopened bag can often be returned.