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Thinset Calculator

A thinset calculator works out how many bags of thinset mortar you need to set tile, based on the area and — crucially — the trowel notch size, which drives how much mortar goes down. Bigger tiles need a bigger notch, and a bigger notch eats mortar fast.

Your measurements

Result

Enter the area and your trowel, and it gives the bag count. It is the companion to the tile and grout calculators: tile for the tiles, thinset for the mortar under them, grout for the joints.

How it is calculated

Bags = tiled area × waste multiplier ÷ coverage per bag, where coverage falls with trowel notch size: roughly 90-95 sq ft per 50 lb bag at 1/4 in, ~60 at 1/4 × 3/8 in, ~45 at 1/2 in, and ~32 for 3/4 in large-format notches. Back-buttering big tiles uses more.

Steps

Enter the tiled area (length × width or total).
Pick the trowel notch — bigger notch for bigger tile.
Add a waste allowance.
Read the number of 50 lb bags.
Add extra if you are back-buttering large or heavy tiles.

Worked example

100 sq ft of tile with a common 1/4 × 3/8 in trowel needs about 2 bags of thinset (≈60 sq ft per bag).

Common questions

How much thinset do I need?

Tiled area ÷ the coverage for your trowel notch. 100 sq ft with a 1/4 × 3/8 in trowel is about 2 bags.

How does trowel size affect thinset?

A larger notch lays a thicker bed, so coverage per bag drops sharply — large-format tile can use double the mortar of small tile.

Do I need to back-butter tiles?

For large-format and heavy tiles, yes — it ensures full contact, but it uses extra thinset, so buy a bit more.

Modified or unmodified thinset?

Depends on the tile and substrate (e.g. unmodified over some membranes). Check the tile and membrane instructions; coverage is similar.