TileQuantCoverage math for tile, thinset and grout — before you buy.

Thin-set for 360 ft² of 2×2″ mosaic tile on an uncoupling membrane

Thin-set needed
850 lb bags
360 ft² of 2×2″ mosaic with a 1/4″ × 1/4″ square notch, two layers over the membrane
Bags (setting the tile)4× 50 lb
Bags (bedding the membrane)a second, separate layer under the membrane4× 50 lb
Total weight400lb
Trowelchosen by the 2″ tile, not by the room1/4″ × 1/4″ square notch
Coverage per bag95 ft² at this notch depth95ft²
Back-butteringtrowel ridges alone will collapse to full coverageNot needed
SubstrateUncoupling membrane
Mortar typeUnmodified thin-set (ANSI A118.1) above the membrane
Area
360 ft²
Tile size
2×2″ mosaic
Substrate
an uncoupling membrane

The trowel decides the quantity

Not the room. A 1/4″ × 1/4″ square notch is what 2×2″ mosaic tile needs, and at that notch depth a 50 lb bag covers about 95 ft². Over 360 ft² that comes to 8 bags.

The same floor in 2″ mosaic would take a fraction of this. Any figure quoted as "bags per hundred square feet" without asking the tile size is a guess.

Back-buttering: not needed

ANSI A108.5 wants 80% mortar contact for interior floors and 95% for wet or exterior work. Above about 15″, no trowel alone gets there — the ridges do not collapse fully under a tile that size, and the corners end up hollow. Where it is required, the extra 30% is already in the bag count above.

Pull a tile up early in the job and look at the back. You want flattened ridges across the whole face, not a stripe pattern.

Uncoupling membrane

Mortar for this substrate: Unmodified thin-set (ANSI A118.1) above the membrane.

Two habits that decide whether it holds

Let it slake. Almost every bag says mix, wait five to ten minutes, remix — that rest is what lets the polymers hydrate, and skipping it is why mortar sometimes will not hold tile on a wall.

Only spread what you can set in fifteen minutes. Mortar that has skinned over looks perfectly fine and bonds to nothing. Touch it; if your finger does not come away wet, scrape it off and start again.

Notes

  • The trowel decides the quantity, and the tile decides the trowel. A 1/4″ × 1/4″ square notch lays down enough mortar for 2″ tile; using it over 360 ft² takes 4 bags. The same floor in 2″ mosaic would need a fraction of that, and a generic "bags per square foot" figure is wrong for both.
  • At 2″ the trowel ridges collapse into full contact when you set and slide the tile. Pull one up early in the job and look at the back — you want the ridges flattened, not standing.
  • Above the membrane use UNMODIFIED mortar — modified cannot cure properly sandwiched between two non-porous layers.
  • Mix it and let it slake. Almost every bag says mix, wait 5–10 minutes, then remix — that rest is what lets the polymers hydrate. Skipping it is why mortar sometimes will not hold a tile on a wall.
  • Only spread what you can set in about 15 minutes. Mortar that has skinned over looks fine and bonds to nothing; touch it with a finger and if it does not come away wet, scrape it off.

Nearby sizes