Thinset Calculator
The trowel notch sets the mortar bed depth, and so how far a bag goes. Bigger tile needs a bigger notch, and large-format tile usually needs back-buttering as well.
How this calculator works
- Coverage figures are typical values printed on 50 lb bags of modified thinset for each trowel size; your bag chart takes priority.
- Back-buttering adds a thin skim coat to the back of each tile, roughly 15% more mortar.
- A 10% allowance covers mortar that skins over or is scraped off.
Thinset questions
What trowel size for 12×24 tile?
A 1/4 × 3/8 or 1/2 × 1/2 square notch is common for 12×24. The goal is at least 80% mortar contact on dry floors and 95% in showers and wet areas (ANSI A108). Lift a tile and check.
Modified or unmodified thinset?
Follow the tile and membrane maker. Many uncoupling membranes (e.g. Schluter-DITRA) call for unmodified thinset over the membrane; most porcelain installs on backer board use modified.
What is large-and-heavy-tile (LHT) mortar?
LHT or medium-bed mortar can be troweled thicker without slumping, for tiles 15 inches and up on any side. Coverage per bag is lower because the bed is deeper.