Larger tiles need a deeper trowel notch to get full contact, so they use much more thinset per square foot. Manufacturers publish coverage for each notch size; tiles over 15 in on any side also need back-buttering, which adds about a quarter more mortar.
How to use the Thinset Calculator
- Tile area
- Tile size → trowel notch
- Back-butter tiles
- Waste
- Bag size
- Price per bag
- Every result field is editable: type into Bags of thinset and Tile area plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the tile area, choose the tile size (which sets the trowel), whether you will back-butter, waste, bag size and price. The calculator divides the area by the coverage of one bag.
Tips for an accurate estimate
- Measure the formwork, not the drawing – forms bulge and subgrades are rarely perfectly flat, so add 5–10%.
- Below about 1 cubic yard, premixed bags are usually cheaper; above that, ready-mix saves time and gives a more consistent pour.
- Order ready-mix in quarter- or half-yard steps and ask about short-load fees for small deliveries.
- Keep fresh concrete damp for at least a week – curing matters as much as the mix for final strength.
Common mistakes to avoid
- Running short mid-pour, which leaves a weak cold joint.
- Adding extra water on site to make concrete easier to place – it lowers strength and increases cracking.
- Ignoring the volume of rebar, posts or pipes in the pour, or the gravel base under it.
Inputs explained
| Field | What to enter |
|---|---|
| Tile area | Units: ft², m², yd², acre |
| Tile size → trowel notch | Choose one: Mosaic up to 2 in → 3/16 in V-notch, Up to 8 in → ¼ × ¼ in square, 8–15 in → ¼ × ⅜ in square, 16 in and larger → ½ × ½ in square |
| Back-butter tiles | Choose one: No, Yes |
| Waste | In % |
| Bag size | Choose one: 50 lb, 25 lb |
| Price per bag | Units: /piece |
What each result means
- Bags of thinset = [Tile area]×(1 + [Waste]÷100)×(1 + 0.25×[Back-butter tiles])÷[Coverage per bag]
- Bags to buy = ceil([Bags of thinset] – 0.000001)
- Cost = [Bags to buy]×[Price per bag]
Example values
How the main result changes when Tile area changes and everything else stays at the example values above:
| Tile area (ft²) | Bags of thinset |
|---|---|
| 60 | 0.69 |
| 90 | 1.04 |
| 120 | 1.39 |
| 150 | 1.74 |
| 180 | 2.08 |
| 240 | 2.78 |
Formula
Bags = area × (1 + waste) × (1 + back-butter) ÷ coverage per bagCoverage per 50 lb bag: ≈ 95 ft² (¼ × ¼ in notch), 60 ft² (¼ × ⅜ in), 40 ft² (½ × ½ in)Worked example
Inputs — Tile area: 120 ft²; Tile size → trowel notch: Up to 8 in → ¼ × ¼ in square; Back-butter tiles: No; Waste: 10 %; Bag size: 50 lb; Price per bag: $22 /piece.
Results — Coverage per bag = 95 ft²; Bags of thinset = 1.39; Bags to buy = 2; Cost = $44.00.
Frequently asked questions
How much does a bag of thinset cover?
A 50 lb bag covers about 95 ft² with a ¼ × ¼ in trowel, 60 ft² with ¼ × ⅜ in and 40 ft² with ½ × ½ in.
What trowel size should I use?
Roughly ¼ × ¼ in for tile up to 8 in, ¼ × ⅜ in for 8–15 in and ½ × ½ in for large-format tile; check coverage by lifting a tile.
When should I back-butter?
For tiles larger than 15 in on any side, natural stone and any time you need 95% coverage (wet areas, exteriors).
What formula does the Thinset Calculator use?
Bags = area × (1 + waste) × (1 + back-butter) ÷ coverage per bag.