How to calculate thinset
Thinset is bought by coverage, not weight. Every manufacturer publishes how many square feet one bag covers with each trowel notch. Divide your area by that number, add about 10% for mortar left in the bucket, and round up. If the tile is over 15 in on a side, add the mortar for back-buttering, which the charts leave out.
Worked example — 100 sq ft of 12×24 porcelain: with a 3/4 in U-notch held at 30°, VersaBond covers 42 sq ft per 50-lb bag. 100 × 1.10 ÷ 42 = 2.62 bags. A 1/32 in back-butter skim on 100 sq ft is 450 cubic inches of mortar, about 0.4 of a bag. Total 3.02, so buy 4 bags. With a 1/4 × 3/8 in notch and VersaBond LHT the same floor takes 2 bags — the trowel matters more than the brand.
Coverage per bag by trowel (minimum published)
| Mortar | ¼ × ¼ | ¼ × ⅜ | ½ × ½ | ¾ U · 45° | ¾ U · 30° |
|---|---|---|---|---|---|
| Store-brand thinset (50 lb) | 75–75 | 55–55 | 35–35 | 25–25 | 25–25 |
| VersaBond (50 lb) | 85–95 | 60–67 | 42–47 | 34–38 | 42–47 |
| VersaBond LHT (50 lb) | 90–100 | 60–67 | 42–47 | 34–38 | 42–47 |
| VersaBond LITE (35 lb) | 90–100 | 63–70 | — | 36–40 | 36–40 |
| ProLite (30 lb) | 90–100 | 63–70 | 45–50 | 36–40 | 45–50 |
| MegaLite (30 lb) | 90–100 | 63–70 | 45–50 | 36–40 | 45–50 |
| Ultraflex 2 (50 lb) | 75–90 | 55–65 | — | — | — |
| Ultraflex LFT (50 lb) | 85–95 | 65–75 | 40–50 | 35–40 | 35–40 |
| Ultraflex LHT SG (50 lb) | 75–90 | 55–65 | 38–45 | 25–30 | 25–30 |
| Keraflex Super (44 lb) | 100–115 | 74–83 | 50–58 | 34–38 | 34–38 |
| 253 Gold (50 lb) | 80–95 | 60–70 | — | — | — |
| 254 Platinum (50 lb) | 80–95 | 60–70 | 35–40 | — | — |
| MultiMax Lite (25 lb) | 85–95 | 60–70 | 40–50 | — | — |
| TEC 3N1 (30 lb) | 80–90 | 55–65 | 40–50 | 30–40 | 30–40 |
Square feet per bag, from each maker's technical data sheet. "—" means the maker does not list that trowel; the calculator then scales the 1/4 in rate by bed depth and labels the result an estimate. The 30-, 35- and 25-lb lightweight bags cover what a 50-lb bag does, so compare bags, not pounds.
Which trowel for which tile
- Mosaics and thin wall tile: 3/16 in V-notch.
- Tile up to about 8 in (3×6 subway, 4×4, 6×6): 1/4 × 1/4 in square notch.
- Tile 8–15 in (12×12, 4×12): 1/4 × 3/8 in square notch.
- Tile over 15 in (12×24, 24×24, planks): 3/4 in U-notch held at 30° or a 1/2 in notch, with an LHT mortar and back-buttering. Custom and TEC no longer recommend the 1/2 × 1/2 square notch with some of their mortars.
The notch is only a starting point. Lift a tile after setting it: you want 80% of the back covered in dry areas and 95% in showers and outdoors. If you see bare ridges, go up a notch.
LHT, LFT and lightweight mortars
Large and heavy tile mortars (ANSI A118.4H or A118.15H, sold as "LHT" and formerly "LFT") can be built up to 1/2–3/4 in without slumping, which long porcelain needs. Standard modified thinsets like MAPEI Ultraflex 2 are rated for beds up to 1/4 in and are not meant for tile over 15 in. Keraflex Super is sold in 44-lb bags, so it looks pricier per pound but covers more per bag.
Common mistakes
- Quoting the high end of a coverage range. Real floors are not flat; the calculator uses the low end.
- Forgetting back-buttering on large tile — about 0.4 bag per 100 sq ft.
- Adding too much water. Follow the bag: most 50-lb bags take about 5–6 quarts.
- Using a modified thinset under KERDI. Schluter calls for its own ALL-SET or an unmodified mortar.
Count the tile itself with the tile calculator, then the grout calculator. Setting cement board first? The backer board calculator includes the thinset under the board.