How much thinset do I need?
Thinset is sold in bags — usually 50 lb — so the goal is to turn your tiling area into a whole number of bags. The formula this calculator uses is:
Bags = area ÷ coverage per bag, rounded up
The catch is that coverage isn't fixed. It's set by the size of the notched trowel you use, and the trowel is chosen to match your tile. Small tiles use a small notch and a bag goes a long way; big tiles need a deeper notch that lays down far more mortar, so a bag covers much less. Pick your tile size above and the calculator sets a typical coverage — then adjust it to your exact trowel or product.
Thinset coverage by trowel and tile size
These are typical figures for a 50 lb bag. Coverage drops on uneven substrates and when you back-butter, so treat them as a starting point and round up.
| Tile size | Trowel notch | Coverage / 50 lb bag |
|---|---|---|
| Mosaic (≤ 4″) | 3/16″ V-notch | ~95 sq ft |
| Small (4–8″) | 1/4″ × 1/4″ square | ~85–90 sq ft |
| Medium (8–12″) | 1/4″ × 3/8″ square | ~70 sq ft |
| Large (12–16″) | 1/2″ × 1/2″ square | ~45–50 sq ft |
| Large format (16″+) | 3/4″ U or square | ~30–40 sq ft |
Modified vs. unmodified thinset
Both are cement-based, but they're not interchangeable:
- Modified thinset has polymers for a stronger, more flexible bond. It's the everyday choice for most floor and wall tile, large-format tile, and tiling over plywood or heated floor systems.
- Unmodified thinset is required under and over many waterproofing and uncoupling membranes (like Schluter DITRA), and is specified for some porcelain and glass. Check the tile and membrane instructions before you buy.
Tips for a solid tile job
- Mix to a peanut-butter consistency. It should hold a ridge from the trowel without slumping. Let it slake (rest) for ~10 minutes, then remix before use.
- Only mix what you can use. Thinset has a limited pot life — about 1–2 hours in the bucket and 15–30 minutes of open time once it's combed out. Work in small sections.
- Match the trowel to the tile. The right notch is what gives full coverage — at least 80% for dry areas and 95% for wet or exterior. Back-butter big tiles and stone.
- Don't tile over the bag rating. Coverage assumes a flat substrate. Wavy floors and walls eat extra mortar, so buy a spare bag for anything but a small job.