Thinset: coverage by trowel
The trowel’s notch decides how thick the mortar bed is, and the bed thickness decides how far a bag goes. The figures below are the typical range printed on 50 lb bags of modified thinset; your bag’s own chart governs, and it is on the back.
| Trowel notch | Used for | 50 lb bag covers | Bags per 100 sq ft |
|---|---|---|---|
| 3/16 inch V-notch | Mosaic sheets, tile under 4 inches | About 100 to 120 sq ft | 1 |
| 1/4 x 1/4 inch square | 4 to 8 inch tile, most wall tile, subway | About 90 to 100 sq ft | 1 to 1.5 |
| 1/4 x 3/8 inch square | 8 to 13 inch tile, 12 x 12 floors | About 60 to 67 sq ft | 1.5 to 2 |
| 1/2 x 1/2 inch square | Any side 15 inches or longer: 12 x 24, 24 x 24, planks | About 40 to 45 sq ft | 2.5 |
| 1/2 x 1/2 plus back-buttering | Large format on floors, any natural stone that needs full coverage | About 30 to 38 sq ft | 3 |
The trowel is proven, not picked. Set a tile, pull it up, and look at the back. The industry standard is at least 80% mortar contact on a dry floor and 95% in a shower, on exterior work and under natural stone. Bare spots mean a bigger notch or back-buttering, which means more bags than the table above. Large-format tile also wants a large-and-heavy-tile mortar, not a standard thinset.
Grout: the formula
Grout fills the joints, so the amount is the joint volume. For tile of length L and width W, thickness T and joint width J, all in inches:
- Joint volume per square foot = 144 × (L + W) ÷ (L × W) × J × T cubic inches.
- Pounds of dry grout per square foot ≈ that volume × 0.06. Cement grout runs close to 0.06 lb of powder per cubic inch of filled joint.
- Add 10 to 15% for what stays on the float, in the bucket and in the haze sponge, then round up to whole bags.
Worked: 12 x 12 inch tile, 3/8 inch thick, 1/8 inch joints. 144 × 24 ÷ 144 × 0.125 × 0.375 = 1.125 cubic inches per square foot, × 0.06 = about 0.07 lb per square foot. A 25 lb bag covers about 370 square feet before the extra, about 320 after. The formula agrees with most makers’ charts to within about 15%, which the extra covers.
Grout by tile size: one 25 lb bag covers about
| Tile | Thickness | Joint | Grout type | 25 lb bag covers |
|---|---|---|---|---|
| 2 x 2 mosaic (shower floor) | 1/4 inch | 1/8 inch | Sanded | About 90 sq ft |
| 4 x 4 | 1/4 inch | 1/8 inch | Sanded | About 185 sq ft |
| 3 x 6 subway | 5/16 inch | 1/16 inch | Unsanded | About 300 sq ft |
| 6 x 24 wood-look plank | 3/8 inch | 1/8 inch | Sanded | About 300 sq ft |
| 12 x 12 | 3/8 inch | 1/8 inch | Sanded | About 370 sq ft |
| 12 x 24 | 3/8 inch | 3/16 inch | Sanded | About 330 sq ft |
| 12 x 24 | 3/8 inch | 1/8 inch | Sanded | About 490 sq ft |
| 24 x 24 | 3/8 inch | 1/8 inch | Sanded | About 740 sq ft |
The pattern is the point: halving the tile size roughly doubles the grout, and widening the joint raises it in proportion. A mosaic shower floor of 15 square feet can take as much grout as 60 square feet of large tile on the bathroom floor.
Which grout
| Grout | Joints | Comes as | Notes |
|---|---|---|---|
| Unsanded cement | Up to 1/8 inch | 10 or 25 lb bags | For glass, polished marble and anything that scratches; shrinks and cracks in wide joints |
| Sanded cement | 1/8 inch and wider | 10 or 25 lb bags | The default for floors; needs sealing |
| Premixed urethane or acrylic | Per the pail | 1 and 2 gallon pails | No mixing, no sealing; check the pail’s own chart, coverage is lower per dollar |
| Epoxy | Per the kit | Kits by the quart or gallon | Stain-proof and waterproof; short working time; 3 to 5 times the cost |
What else goes on the setting-materials order
| Item | How much | Why it is on the list |
|---|---|---|
| Thinset for the layer underneath | About 1 more 50 lb bag per 100 sq ft | Uncoupling membranes and cement board seams are set or taped in mortar too; use the mortar the membrane maker names |
| Caulk, color-matched to the grout | One 10 oz tube per 25 to 30 ft of joint | Inside corners, where tile meets the tub, and the floor-to-wall joint are caulked, never grouted |
| Grout sealer | One quart per 150 to 300 sq ft of tile, by tile size | Cement grout only; premix and epoxy do not need it |
| Spacers and leveling clips | One bag per 50 to 100 sq ft | Clips matter on any tile 15 inches or longer |
| Mixing paddle, two buckets, a margin trowel | One set | Mix half bags; thinset skins over in 20 to 30 minutes |
Worked example: one bathroom
A 5 x 8 foot bathroom: 40 square feet of floor in 12 x 24 inch tile, 80 square feet of shower walls in 3 x 6 inch subway, and a 3 x 5 foot shower floor (15 square feet) in 2 x 2 inch mosaic.
| Surface | Thinset | Grout |
|---|---|---|
| Floor, 40 sq ft, 12 x 24, 1/2 x 1/2 trowel plus back-buttering | 40 ÷ 35 = 1.2 bags | 40 ÷ 490 × 25 = 2 lb sanded |
| Shower walls, 80 sq ft, subway, 1/4 x 1/4 trowel | 80 ÷ 95 = 0.9 bag | 80 ÷ 300 × 25 = 7 lb unsanded |
| Shower floor, 15 sq ft, mosaic, 3/16 V-notch | 15 ÷ 110 = 0.2 bag | 15 ÷ 90 × 25 = 4 lb sanded |
| Membrane and board seams | About 1 bag | None |
| Total with 10 to 15% extra | 4 bags of 50 lb (one of them large-and-heavy-tile mortar for the floor) | One 10 lb bag of unsanded, one 10 lb bag of sanded |
Grout for a whole bathroom is usually two small bags; thinset is usually three to five big ones. If the two areas use the same grout color and joint size, one 25 lb bag covers it with room to spare, and it keeps the color consistent. The tile itself, with its waste factor, is in how much tile for a shower and how much flooring do I need; a kitchen backsplash has its own short list in how much backsplash tile do I need.
Three things that change the number
- Back-buttering and bigger notches. They cost a bag or two and they are cheaper than a cracked large-format tile. Plan for them on any floor tile 15 inches or longer.
- An uneven floor. Thinset is not a leveler. A floor more than 1/4 inch out over 10 feet needs a self-leveling underlayment first, ordered by the bag on its own chart.
- Buying all the grout at once. Grout color varies by batch like tile does. Buy every bag in one trip and check the batch codes match, for the same reason as matching the tile’s lot.
Pick the trowel from the tile, prove it with one pulled tile, and let the joint math decide the grout. Then buy the setting materials with the tile, not on the morning of the install. Where tile falls in the whole job is in the order of a bathroom remodel.