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

Thin-set for 1500 ft² of 24×24″ tile on an uncoupling membrane

Thin-set needed
9450 lb bags
1,500 ft² of 24×24″ with a 3/4″ × 3/4″ square notch, two layers over the membrane
Bags (setting the tile)78× 50 lb
Bags (bedding the membrane)a second, separate layer under the membrane16× 50 lb
Total weight4,700lb
Trowelchosen by the 24″ tile, not by the room3/4″ × 3/4″ square notch
Coverage per bag25 ft² less 30% for back-buttering19ft²
Back-butteringover 15″ you cannot reach 80% coverage from the trowel aloneRequired
SubstrateUncoupling membrane
Mortar typeUnmodified thin-set (ANSI A118.1) above the membrane
Area
1500 ft²
Tile size
24×24″
Substrate
an uncoupling membrane

The trowel decides the quantity

Not the room. A 3/4″ × 3/4″ square notch is what 24×24″ tile needs, and at that notch depth a 50 lb bag covers about 19 ft². Over 1500 ft² that comes to 94 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: required

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 3/4″ × 3/4″ square notch lays down enough mortar for 24″ tile; using it over 1,500 ft² takes 78 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.
  • 24×24″ needs back-buttering. ANSI A108.5 wants 80% mortar contact for interior floors and 95% for wet or exterior work, and no trowel alone gets there under a tile this size. That is the 30% extra already added above.
  • 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.

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