How Much Thinset Do I Need for Tile?

Coverage by trowel size

Coverage depends on the mortar, the trowel notch and how flat the floor and tile back are. Custom Building Products publishes these figures for PremiumPlus, a non-modified thin-set mortar:

• 1/4 x 1/4 x 1/4 inch square notch: 85 to 95 square feet per 50 lb bag.
• 1/4 x 3/8 x 1/4 inch square notch: 60 to 67 square feet per 50 lb bag.
• Larger notches and large-format tile need more mortar. We did not read those rows, so use the manufacturer sheet for your product.

Worked example: 120 square feet

With the 1/4 inch notch: 120 / 95 = 1.26 bags at best and 120 / 85 = 1.41 bags at worst. Buy 2.

With the 1/4 by 3/8 inch notch: 120 / 67 = 1.79 bags at best and 120 / 60 = 2.0 bags at worst. That is right on 2 bags with no margin, so buy 3, or 2 bags plus a smaller extra bag if available.

Flatness of the floor

Mortar cannot fix a badly uneven floor. The Ceramic Tile and Stone Consultants quote ANSI A108.02-4.2 as allowing, for tile with every edge under 15 inches, no more than 1/4 inch of variation in 10 feet and no more than 1/16 inch in 12 inches. For tile with an edge of 15 inches or more it is 1/8 inch in 10 feet and 1/16 inch in 24 inches. The standard itself is paywalled, so we are relying on their quotation.

Waste

Retail sites commonly suggest ordering about 10 percent extra tile for a straight layout and about 15 percent or more for diagonal or herringbone. These are retailer rules of thumb, not a standards-body figure. Use the flooring calculator to add waste.

Common mistakes

• Using one bag count for every trowel. A bigger notch uses a lot more mortar.
• Skipping the flatness check on large tile.
• Buying exactly the calculated amount with no margin when the result lands near a whole bag.
• Mixing more than you can use before it skins over.

How much flooring for common room sizes

Each row uses 10% waste (straight layout) and 20 sq ft boxes. Change any number in the flooring calculator.

Frequently asked questions

Sources: Custom Building Products: PremiumPlus thin-set mortar, Ceramic Tile and Stone Consultants: Large format tile and subfloor requirements, Flooring Clarity: How much overage for tile. Read 7 November 2026.