How the thinset estimate works
A notched trowel leaves ridges of mortar of a set size. The bigger the notch, the more mortar each square foot takes, so coverage per bag is tied to the trowel. The calculator divides your area by the typical coverage for the notch you choose.
bags = area × (1 + waste) × back-butter factor ÷ coverage per bag
Worked example
A 100 sq ft floor laid with a 1/4 × 3/8 in square-notch trowel: 100 sq ft plus 10 percent is 110 sq ft. At about 65 sq ft per 50 lb bag that is 1.69 bags, roughly 85 lb, so buy 2 bags. In metric, that is about 38.4 kg, or two 20 kg bags.
Switch to 12 × 24 in porcelain with a 1/2 × 1/2 in notch and back-buttering, and the same floor works out to 138 sq ft of effective coverage, 3.06 bags. Buy 4: the jump from 3.06 is why it pays to check before you start rather than run out mid-floor.
Typical coverage per 50 lb bag
| Trowel notch | Metric equivalent | Typical coverage | Common use |
|---|---|---|---|
| 1/4 × 1/4 in square | 6 × 6 mm | ≈ 90 sq ft (85–100) | Tiles up to about 8 in |
| 1/4 × 3/8 in square | 6 × 10 mm | ≈ 65 sq ft (60–70) | Tiles about 8 to 16 in |
| 1/2 × 1/2 in square | 12 × 12 mm | ≈ 45 sq ft (40–50) | Large-format tile and uneven floors |
Ranges vary between products and between lightweight and standard mortars, so treat these as rules of thumb. Lightweight mortars often cover more per bag by weight.
Back-buttering adds mortar
Large-format tiles, usually defined as any tile with a side of 15 in or more, and most natural stone need a thin layer of mortar spread on the back of each tile as well as the notched layer on the floor. That skim coat helps get full contact. The calculator adds 25 percent when you select it; this is a working rule of thumb, and heavily cupped tiles or lug backs can need more.
Coverage you need under the tile
| Location | Mortar contact usually required |
|---|---|
| Dry interior floors and walls | At least 80 percent |
| Showers, wet areas and exteriors | At least 95 percent |
| Natural stone | At least 95 percent |
Lift a freshly set tile now and then to check. If the ridges have not collapsed into a solid bed, use a bigger notch, back-butter, or flatten the floor first.
Choosing the mortar
Thinset bags in North America are labeled with the ANSI standard they meet, which tells you what the mortar is for:
| Label | What it means | Typical use |
|---|---|---|
| ANSI A118.1 | Unmodified (dry-set) mortar | Ceramic tile, and over some membranes that require it |
| ANSI A118.4 | Modified with polymers for better bond | Porcelain and general floor and wall tile |
| ANSI A118.15 | Improved modified mortar | Demanding jobs, exterior tile, large porcelain |
Large and heavy tile mortars are made to hold a thicker bed without the tile sinking. The tile and membrane makers say which mortar their warranty needs, so check both before you buy. Coverage per bag differs between these products, which is another reason to read the label.
Tips for mixing and spreading
- Mix with the amount of water on the bag, let it slake (rest) as directed, then remix. Do not add water later to revive stiff mortar.
- Comb the ridges in straight lines, not swirls, so air can escape when you press the tile down.
- Use the mortar type the tile and setting surface call for. Porcelain, large tiles and tile over membranes often need a specific modified or large-tile mortar.
- Wear a dust mask when mixing powder, plus gloves and eye protection. Wet cement is caustic.
Once the tile is down, estimate the joints with the grout calculator.
Frequently asked questions
How many square feet does a 50 lb bag of thinset cover?
Roughly 90 sq ft with a 1/4 × 1/4 in square notch, 65 sq ft with 1/4 × 3/8 in and 45 sq ft with 1/2 × 1/2 in. These are rules of thumb; the bag label gives the product's own figures.
How much thinset do I need for 100 square feet?
With a 1/4 × 3/8 in trowel and 10 percent extra, about 1.69 bags, roughly 85 lb, so buy two 50 lb bags.
When should I back-butter tile?
For large-format tiles with any side of 15 in or more, natural stone, and wherever you need 95 percent contact, such as showers and exteriors.