For 80 sq ft of 12 × 12 in tile you need 2 bags of 50 lb thinset: a ¼ × ⅜ in square-notch trowel spreads 60 to 67 sq ft per bag of CUSTOM VersaBond, and 80 sq ft plus 10% is 88 sq ft. The notch sets the coverage, and the tile size sets the notch, so start there.
How the thinset is calculated
Every mortar data sheet lists how many square feet one bag covers with a given trowel. Divide your area plus the waste allowance by that coverage and round up. We use the low end of the published range, because a slightly wavy floor, a tile with a ribbed back or a steep trowel angle all eat mortar.
bags = area × (1 + waste %) ÷ coverage per bag (low end), rounded up
Back-buttering adds a flat skim coat on the back of each tile. No manufacturer publishes a figure for it, so the calculator treats it as a layer of the thickness you pick (1/32 in by default) and converts it to bags with the volume one bag spreads. That volume comes from the same chart: coverage times the average thickness the notch leaves. A ½ × ½ × ½ in square notch combs ridges half the time and gaps the other half, so it averages ¼ in; at 38 sq ft per bag that is 1,368 cu in of mortar per bag.
Worked example
With the calculator's defaults:
- 80 sq ft × 1.10 = 88 sq ft to cover.
- 12 × 12 in tile gets a ¼ × ⅜ in notch; VersaBond covers 60 to 67 sq ft per bag with it.
- 88 ÷ 60 = 1.47 bags, so 2 bags.
For a 12 × 10 ft floor in 12 × 24 in tile the picture changes: CUSTOM's guide calls for a ½ × ½ in notch on tile 15 in and larger, MAPEI Ultraflex LFT Rapid covers 38 to 45 sq ft per bag with it, and a 1/32 in back-butter coat adds 0.43 bag. 132 sq ft ÷ 38 = 3.47, plus 0.43 is 3.91, so 4 bags.
Which trowel notch for which tile
CUSTOM's notch trowel guide gives these pairings (width × depth × spacing):
| Tile | Trowel | Use |
|---|---|---|
| 8 in or smaller | ¼ × ¼ × ¼ in square notch | thinset |
| 8 to 16 in | ¼ × ⅜ × ¼ in square notch | thinset |
| 15 in or larger, pavers, stone | ½ × ½ × ½ in square notch | thinset or large-tile mortar |
| Large format and heavy tile | ¾ × 9/16 × ⅜ in U-notch | large-and-heavy-tile mortar |
| Wall tile 6 in or smaller | 3/16 × 5/32 in V-notch | premixed adhesive (mastic), not thinset |
The chart is a starting point. The real test is to set a few tiles, pull one up and look at its back: the mortar should cover the whole back with every edge supported. If it doesn't, go up a notch or back-butter.
Coverage per 50 lb bag by trowel
| Mortar | ¼ × ¼ in | ¼ × ⅜ in | ½ × ½ in | ¾ × 9/16 in U |
|---|---|---|---|---|
| CUSTOM VersaBond | 85 to 95 sq ft | 60 to 67 sq ft | not listed | not listed |
| MAPEI Ultraflex 2 | 75 to 90 sq ft | 55 to 65 sq ft | not listed | not listed |
| LATICRETE 253 Gold | 80 to 95 sq ft | 60 to 70 sq ft | not listed | not listed |
| MAPEI Ultraflex LFT Rapid | 75 to 90 sq ft | 55 to 65 sq ft | 38 to 45 sq ft | 25 to 30 sq ft |
Figures from each product's technical data sheet. Where a sheet lists no coverage for a notch, the calculator says so instead of guessing, and points you to a large-tile mortar.
Back-buttering and mortar contact
ANSI A108.5, as quoted on the VersaBond and Ultraflex 2 data sheets, requires at least 80% mortar contact between tile and substrate in dry interior areas and 95% in wet areas and outdoors, with all tile edges supported. MAPEI's Ultraflex 2 sheet adds that it may be necessary to back-butter the tile to get there, and its large-tile mortar asks for 85% indoors. Large porcelain, tile with a pattern pressed into its back and anything in a shower is where back-buttering pays off. That is why the calculator switches it on for tile over 15 in and for the "Shower or outdoors" setting.
Comb the ridges in one direction, parallel to the tile's short side on large tile, and move each tile back and forth across the ridges as you set it so they collapse and fill.
What comes before and after
The mortar goes onto clean backer board or a flat slab; for a whole room, the tile calculator counts tiles, boxes and backer board sheets together. In a shower, the shower tile calculator splits wall and floor tile, which usually need different notches. After 24 hours of curing, work out the joints with the grout calculator. Building a block or brick wall instead of tiling one? That takes masonry mortar, and the mortar calculator counts it.