Thinset Mortar Calculator — Bags by Trowel Size
How much thinset you need depends almost entirely on your trowel notch, which is set by your tile size. Pick the right trowel and area, and this calculator tells you how many 50 lb bags to buy.
How to use this calculator
- Enter the area you are tiling in square feet.
- Pick the trowel notch that matches your tile size — this is the biggest factor in how much thinset you use.
- If you back-butter large tiles (recommended for 15 in and up), select Yes; it uses roughly 30% more.
- Buy modified thinset for most jobs; use unmodified under large-format porcelain and over membranes.
The formula
Thinset coverage is driven by the trowel notch, not the tile area alone. A bigger notch lays a thicker bed and covers less area per bag.
Bags = area ÷ coverage per bag. A 50 lb bag covers roughly 95 sq ft with a 3/16" V-notch, 85 sq ft with a 1/4" square notch, 60 sq ft with a 1/4"×3/8" notch, and 40 sq ft with a 1/2" notch.
Back-buttering large tiles adds about 30% to consumption.
Worked example
100 sq ft of 12-inch tile with a 1/4" square notch, no back-butter.
Coverage = 85 sq ft per 50 lb bagBags = 100 ÷ 85 = 1.18 → 2 bags
You need 2 bags of 50 lb thinset for 100 sq ft with a 1/4" square trowel.
Frequently asked questions
How many square feet does a bag of thinset cover?
A 50 lb bag covers about 95 sq ft with a small (3/16") trowel down to about 40 sq ft with a large (1/2") trowel. Bigger tiles need a bigger trowel and more thinset.
What trowel size for which tile?
Use a 1/4" square notch up to 8-inch tile, a 1/4"×3/8" notch for 8–16-inch tile, and a 1/2" notch for large-format tile 16 inches and larger.
Modified or unmodified thinset?
Use modified thinset for most installations. Use unmodified under large-format porcelain (which can trap moisture) and over uncoupling or waterproofing membranes per the membrane maker.