Thinset Calculator

Thinset formula

Area to cover = Length (ft) x Width (ft) x (1 + waste %)
Coverage per bag = trowel coverage / 1.25 if back-buttering
Bags = Area to cover / Coverage per bag, rounded up
Trowel coverage per 50 lb bag: 3/16 V = 130, 1/4 x 1/4 = 95, 1/4 x 3/8 = 65, 1/2 x 1/2 = 42 sq ft

Worked example

A 12 x 15 ft floor in 12 x 24 in porcelain, set with a 1/2 x 1/2 in trowel and back-buttered: 180 sq ft plus 10% waste = 198 sq ft. A 1/2 in notch covers 42 sq ft per bag, and back-buttering cuts that to 42 / 1.25 = 33.6 sq ft. 198 / 33.6 = 5.9, so buy 6 bags of 50 lb thinset, about 300 lb of mortar.

Quick reference

How to calculate thinset

Thinset coverage depends almost entirely on the trowel notch, because the notch sets how thick the mortar bed is. Bigger tile needs a bigger notch to get full contact, so a 50 lb bag that covers 95 square feet under 6 inch tile covers only about 42 square feet under 24 inch tile. Pick the trowel for your tile size, look up the coverage, add 10 percent waste and divide.

Back-buttering means skimming a thin layer of mortar onto the back of each tile before setting it. It is required for large format tile and natural stone and is a good idea on any tile with deep ridges on the back. It uses roughly 25 percent more thinset. A subfloor that is out of flat also eats mortar, so buy an extra bag if the floor needs any filling.

Choosing the right mortar

Use a polymer-modified thinset (ANSI A118.4) for porcelain and for tile over cement board, plywood or concrete. Use unmodified thinset (A118.1) where the membrane manufacturer requires it, which includes most uncoupling membranes. For tile 15 inches or longer on any side, buy a large format or medium-bed mortar, which resists slumping in a thicker bed.

Mix only what you can use in about two hours, let it slake for 10 minutes and remix, and never add water to stiffening mortar. Spread with the flat side first to key into the substrate, then comb in one direction so air can escape when the tile is pressed in. Lift a tile now and then to confirm you are getting full coverage.

Key facts

• A 50 lb bag of thinset covers about 95 sq ft with a 1/4 x 1/4 in trowel, 65 sq ft with a 1/4 x 3/8 in trowel and 42 sq ft with a 1/2 x 1/2 in trowel.
• Tile with any side longer than 15 in is large format and needs a 1/2 in notch, a medium-bed or LFT mortar and back-buttering.
• Industry standards call for at least 80% mortar contact under tile in dry areas and 95% in wet areas and outdoors.
• Modified thinset (ANSI A118.4) is for most tile over cement board and concrete; unmodified (A118.1) is required over many uncoupling membranes.
• Mixed thinset has a pot life of about 2 to 3 hours and an open time of 20 to 30 minutes once spread.

Frequently asked questions

Sources and references

• •VersaBond Professional Tile Mortar coverage per 50 lb bag by trowel size (Custom Building Products)
• •VersaBond Professional Thin-Set Mortar technical data sheet TDS-119 (Custom Building Products)
• •Ultraflex 2 polymer-modified mortar product page and coverage (Mapei)