About this calculator
Thin-set mortar is sold in 50 lb bags, but how far a bag goes depends almost entirely on the trowel. The notches comb the mortar into ridges; the bigger the notch, the more mortar sits under each square foot of tile. A bag that covers 85 sq ft with a ¼ × ¼ in notch covers only about 42 sq ft with the ½ × ½ in notch that large tile needs, so the same floor can take twice as many bags.
This thinset calculator picks the trowel from your tile size, works out the bags from the manufacturer’s coverage chart, and adds the separate bed of mortar that cement board needs when you tile over a wood subfloor. The drawing shows your floor split into the part each bag covers, and a section through the combed ridges and the bed they leave under the tile once it is pressed in.
How to use it
- Measure the length and width of the area. For an L-shaped room, a shower or a backsplash around a window, split it into rectangles and add up the results, or use the square footage calculator.
- Enter the tile size. The longest side picks the trowel: a ¼ × ¼ in notch up to 8 in, ¼ × ⅜ in up to 15 in, and ½ × ½ in for larger tile.
- Choose what you are setting on. Over plywood or OSB, the cement board goes down first in its own bed of thin-set, which takes more bags.
- Add an extra percentage if you will back-butter tile or the floor is uneven, and enter your local price to see the cost.
The formula
Bags = area ÷ coverage per bag for the trowel × (1 + extra%)
(+ area ÷ 85 sq ft for the bed under cement board)
Coverage per 50 lb bag: ¼ × ¼ in 85 sq ft · ¼ × ⅜ in 60 sq ft · ½ × ½ in 42 sq ft
Bed under the tile ≈ notch depth ÷ 2
Coverage is the low end of Custom Building Products’ charts for a 50 lb bag: VersaBond (TDS-119) gives 85–95 sq ft with a ¼ × ¼ in square notch and 60–67 sq ft with ¼ × ⅜ in, and VersaBond-LFT (TDS-132) gives 42–47 sq ft with a ½ × ½ in square notch or a ¾ in U-notch. Other brands publish similar charts; if your bag lists different figures, use them. The ridges of a square-notch trowel fill half the space between the trowel and the floor, so when the tile flattens them they leave a bed about half the notch depth: ⅛ in from a ¼ in notch, 3/16 in from ⅜ in.
Worked example
These are the calculator’s starting values and the results they produce:
| Area length | 10 ft |
|---|---|
| Area width | 10 ft |
| Tile length | 12 in |
| Tile width | 12 in |
| Trowel | Pick by tile size |
| Setting on | Concrete, or backer board already down |
| Extra | 0 % |
| Thin-set mortar, 50 lb bag | $25.00 |
Tips before you buy
- Lift a tile now and then right after setting it. Mortar must cover at least 80% of the back, and 95% in showers, wet areas and outdoors. If it doesn’t, go up a notch size or back-butter the tile.
- Comb the ridges in straight lines, all in one direction, then slide the tile across them and back. Swirled ridges trap air and leave hollow spots.
- Large tile, tile with a deep pattern on the back and stone that isn’t gauged often need back-buttering (a flat coat on the back of the tile). That adds mortar the chart doesn’t include, so add some extra.
- Mix only what you can use before it skins over, and don’t add water to mortar that has started to stiffen. Bagged thin-set usually stays workable for a few hours in the bucket.
- Buy polymer-modified thin-set for porcelain and for cement board; check the bag for the tile and substrate it is rated for.
Frequently asked questions
How many bags of thinset do I need for 100 square feet?
With 12 × 12 in tile and a ¼ × ⅜ in trowel, a 50 lb bag covers about 60 sq ft, so 100 sq ft takes 1.7 bags: buy 2. With small tile and a ¼ × ¼ in notch it is 1.2 bags (buy 2), and with 24 × 24 in tile and a ½ × ½ in notch 2.4 bags: buy 3.
How much does a 50 lb bag of thinset cover?
About 85–95 sq ft with a ¼ × ¼ in square-notch trowel, 60–67 sq ft with ¼ × ⅜ in, and 42–47 sq ft with ½ × ½ in, from Custom Building Products’ VersaBond charts. Plan on the low end; rough floors and back-buttering use more.
What size trowel should I use for tile?
A ¼ × ¼ in square notch for tile up to 8 in, ¼ × ⅜ in for tile up to about 15 in, and ½ × ½ in (or a ¾ in U-notch) for larger tile. The test that counts is coverage: lift a tile and check that mortar covers at least 80% of its back, 95% in wet areas.
How much thinset do I need under cement board?
Cement board over plywood is set in a bed of thin-set combed with a ¼ × ¼ in notch, about 85 sq ft per 50 lb bag. A 120 sq ft floor of 12 × 12 in tile takes 2 bags for the tile and 1.4 for the board: buy 4.
Sources
Areas and volumes are standard geometry. The published figures this calculator relies on come from these sources; other defaults (waste allowances, typical densities and prices) are common trade figures worth checking with your supplier.
- Custom Building Products VersaBond thin-set data sheet TDS-119: thin-set coverage per 50 lb bag with ¼ × ¼ and ¼ × ⅜ in notches; 80%/95% mortar contact; bed thickness
- Custom Building Products VersaBond-LFT large format tile mortar data sheet TDS-132: large format tile mortar coverage with a ½ × ½ in notch or ¾ in U-notch