Thinset Calculator

Thinset calculator

A thinset calculator divides the tile area by the area one bag covers. At 50 sq ft per 50-lb bag with a 1/4 inch notch, an 80 sq ft floor needs 80 ÷ 50 = 1.6 bags. You cannot buy 0.6 of a bag, so the result rounds up to 2 bags, and at $18 a bag that is $36.

The formula in plain words

1. Area = length × width.
2. Bags = area ÷ 50, rounded up.
3. Cost = bags × price per bag.

If your label gives another coverage figure for your trowel, replace the 50 with that number.

Worked examples

A bathroom floor, 10 × 8 ft:

• Area: 80 sq ft
• Bags: 80 ÷ 50 = 1.6, rounded up to 2
• Cost: 2 × $18 = $36

A kitchen, 15 × 12 ft:

• Area: 180 sq ft
• Bags: 180 ÷ 50 = 3.6, rounded up to 4
• Cost: 4 × $18 = $72

A hallway, 12 × 10 ft:

• Area: 120 sq ft
• Bags: 120 ÷ 50 = 2.4, rounded up to 3
• Cost: 3 × $18 = $54

Notice that 3.6 bags and 2.4 bags both leave part of a bag unused. That leftover is your cushion for mixing errors and spills.

Bags by area and coverage

The calculator uses 50 sq ft per bag. If your label lists a different figure for your trowel, check the bag count here.

Tiled area	At 50 sq ft per bag	At 40 sq ft per bag	At 60 sq ft per bag
100 sq ft	2	3	2
200 sq ft	4	5	4
300 sq ft	6	8	5
500 sq ft	10	13	9

The last two columns are what-if figures. They show how fast the bag count moves if your coverage is lower or higher.

Why coverage depends on the trowel

Thinset is combed into ridges, then the tile is pressed down to flatten them. The notch depth sets how much mortar sits on the floor. A deeper notch uses more thinset per square foot. Tile makers also recommend larger notches for larger tiles, so that the mortar can fill the gap under the tile and give full contact.

Rather than guess, check the label on your bag for a coverage chart, and use the one that matches your trowel.

Preparing the surface

• The floor should be clean, flat and stable. Fix loose boards and cracks first.
• Follow your tile manufacturer’s flatness limits. Large tiles are less forgiving.
• Use the right underlayment or backer board for the floor or wall.
• Mix thinset to the consistency on the label, let it slake if required, and use it within the working time.
• Spread only as much as you can tile before it skins over.

How to measure

Measure the length and width of the room and multiply. For rooms with alcoves, break the floor into rectangles and add them. Subtract large areas that will not be tiled, like a tub. For walls, multiply length by height.

Use the tile calculator for the number of tiles, and the grout calculator for the grout bags. If you are pricing the whole job, the tile installation cost calculator adds labor.

Common mistakes

• Using the thinset coverage for small tile on large-format tile.
• Mixing a whole bag at once and letting it harden before use.
• Using premixed adhesive where a cement-based mortar is needed, such as in a shower.
• Forgetting back-buttering on large tile, which uses extra thinset.
• Buying exactly the calculated number of bags and running short on the last row.

For a shower or other wet area, the manufacturer’s instructions for membranes, backer and mortar take priority. The tile shower cost calculator prices that job.