Thinset Calculator

Notch size scales with tile size and how flat the substrate is. As a working guide: tile up to 8 inches takes a 1/4" × 1/4" square notch, 8 to 16 inch tile takes 1/4" × 3/8", tile over 16 inches or any large-format plank takes 1/2" × 1/2", and heavy stone or an out-of-flat floor takes a 3/4" U-notch or a slant-notch trowel. That range is why the bag count on this page swings by roughly three times across the four options. Check the tile manufacturer spec sheet too, since some porcelain calls out a minimum notch.

ANSI A108.5 calls for at least 80 percent mortar contact on dry interior floors and 95 percent in wet areas, on exteriors, and under stone. Large-format tile rarely reaches that from combed ridges alone, so you back-butter: skim a thin flat coat onto the tile back, set it into ridges troweled all in one direction, and collapse them with a firm slide perpendicular to the ridges. Pull one tile early in the job and inspect the back — you want flattened ridges with no hollow corners. Back-buttering adds roughly 20 to 30 percent to mortar use, so raise the waste allowance accordingly.

Modified thinset (ANSI A118.4 or A118.15) has polymers added for bond strength and flexibility and is the default for porcelain, large-format tile, and most substrates. Unmodified (ANSI A118.1) is plain portland cement, sand, and a water retainer. Several sheet-membrane manufacturers specify unmodified mortar between the membrane and the tile, because sandwiching polymers between two non-porous surfaces keeps them from curing properly. That is a warranty condition, not a preference. Read the membrane installation manual before you buy mortar — the wrong choice can void coverage on the whole assembly.

Pot life is roughly 2 to 4 hours depending on brand, temperature, and humidity. Open time — how long combed ridges stay tacky enough to bond — is much shorter, often 15 to 30 minutes in warm, dry air. Mix only what you can set in about 30 minutes. Slake the batch 5 to 10 minutes, remix, then use it, and never add water to stiffen mortar that has begun to set: retempering destroys bond strength. Comb no more area than you can tile before the ridges skin over. Touch a ridge; if mortar does not transfer to your finger, scrape it off and recomb.

With the common 1/4" × 3/8" notch, a 50 lb bag covers about 75 square feet, so 100 square feet plus 10 percent waste works out to 2 bags. Step up to a 1/2" × 1/2" notch for large-format tile and coverage drops to about 45 square feet, making it 3 bags. Printed coverage figures assume a flat substrate and a trowel held near 45 degrees; a wavy floor or a worn trowel can cut them by a third. Buy one spare bag — unopened mortar stores about a year if you keep it dry and off the slab.