Thinset mortar is commonly used for setting ceramic tile. It is also used to set porcelain tile and natural stone tile. The amount of thinset you need is calculated by the square footage of the floor or wall, the size of the tile, and the size of notched trowel you use to spread it. Different size tile requires that you use a specific size trowel. Using the wrong trowel can cause the tile to unevenly adhere to the floor, causing the tile to crack or come loose.

Mixing mortar
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The square feet of the project is the biggest factor in how much thinset is needed

Square Feet

Square feet is calculated by measuring the length of the longest wall, and the width of the shortest wall in a room. Multiply the two numbers together. The result will be the square footage of the room. For example, a 10-by-12-foot room equals 120 square feet. A 50-pound bag of thinset will cover a minimum of 50 square feet to a maximum of 95 square feet.

Tile Size

Tile size will determine the size of the trowel you use to spread the thinset. The larger the tiles, the more thinset it takes to properly adhere them. A 120 square foot room with 12-inch tiles will require a minimum of two bags of thinset.

Trowel Size

Not all trowels are alike. Trowels are made for small and large tile and determine how thin or thick the thinset will spread. Small tiles don't require as much thinset as larger tiles. A 2-inch tile only requires the use of a 3/16-by-5/32-inch V-notched trowel, whereas a 16-inch tile requires that you use a 1/2-by-1/2-inch U-notched trowel.

Amount of Thinset Needed

Use the square footage of your room or wall, the size of your tile, and the size of your trowel to determine the actual amount of thinset you need to complete your job. A 120 square foot room with 16-inch tiles will take a minimum of three bags of thinset.

Divide the square footage of your project by 95 (the maximum coverage of a 50-pound bag of thinset. For example, a 10-by-12-foot room is 120 square feet; divided by 95, the result is 1.26. Round the number up to the next whole number. You'll need two 50-pound bags for this project. Double the number if you are using large tiles with a 1/2-inch notched trowel. A 120-square-foot project with large tiles, such as 12-by-12-inch or natural stone such as slate, will require four 50-pound bags.