About the Thinset Calculator
This thinset calculator estimates how many bags of tile mortar (thin-set) to buy from the area you are tiling and the trowel you will use. The trowel notch sets how thick the mortar bed is, so it drives coverage far more than the tile itself: a 50 lb bag that covers around 110 sq ft with a 3/16-inch V-notch covers only about 40 sq ft with a 1/2-inch square notch.
Use it when planning a floor, shower wall or backsplash, after you have picked the tile and checked the manufacturer’s recommended trowel. Large-format tile usually needs a bigger notch plus back-buttering to reach full contact, and the calculator adds a skim coat on the back of each tile when you switch that on.
Coverage figures are typical midpoints of the ranges printed on dry-set and polymer-modified thinset bags; rough or uneven substrates use more. If your bag lists a different coverage for your notch, choose "Custom" and enter it.
With the default inputs, the bags of thinset is 2. Change any value above to recalculate instantly.
How to use the thinset calculator
- 1Measure the area to tile in square feet.
- 2Pick the trowel notch the tile manufacturer recommends, or enter the coverage from your bag.
- 3Choose the bag size you will buy.
- 4Turn on back-buttering for large-format tile or wet areas where full coverage is required.
- 5Buy the number of bags shown, plus one if the substrate is uneven.
Formula and method
Thinset is sold by weight but used by volume: the trowel notch decides how thick the combed ridges are, so each notch size has a fairly stable coverage per bag. The calculator takes the coverage for your notch (scaled for 25 lb bags), and if you back-butter it adds the mortar for a 1/16-inch skim coat, about 180 sq ft per 50 lb bag — roughly half what a 1/4-inch square notch leaves.
Because two coats share one bag, their coverages combine like resistors in parallel. The tile area plus a waste allowance is divided by that effective coverage and rounded up to whole bags.
- A
- Tile area (sq ft)
- w
- Waste allowance (decimal)
- Cnotch
- Coverage per bag for the trowel notch (sq ft)
- Cbutter
- Coverage per bag for a back-butter skim (sq ft)
Worked examples
100 sq ft of 12 × 12 tile with a 1/4 × 3/8 notch
With 10% waste you cover 110 sq ft. At about 65 sq ft per 50 lb bag that is 1.69 bags (about 85 lb), so buy 2 bags.
250 sq ft of 24 × 48 porcelain, 1/2 in notch, back-buttered
A 1/2-inch notch covers about 42 sq ft per bag, and the back-butter skim adds mortar that would cover 180 sq ft per bag, giving an effective 34 sq ft per bag. 275 sq ft ÷ 34.05 = 8.08 bags, so buy 9.
60 sq ft mosaic backsplash, 3/16 V-notch, 25 lb bags
A 25 lb bag with a 3/16-inch V-notch covers about 55 sq ft. 66 sq ft including waste needs 1.2 bags (30 lb), so buy two 25 lb bags.
Frequently asked questions
How many square feet does a 50 lb bag of thinset cover?+
Roughly 100–120 sq ft with a 3/16-inch V-notch, 85–95 sq ft with a 1/4 × 1/4-inch square notch, 60–70 sq ft with 1/4 × 3/8 inch and 40–45 sq ft with a 1/2 × 1/2-inch notch. Always check the chart on your bag.
What trowel size should I use for 12 × 24 tile?+
Most tile and mortar makers start with a 1/2-inch square or slant notch for tile with a side of 15 inches or more, often with back-buttering. Pull a tile after setting and check that mortar contact meets the required coverage.
Do I need to back-butter tile?+
Back-buttering is commonly recommended for large-format tile, natural stone and wet areas like showers, where mortar coverage requirements are higher. It flattens out voids on the tile back and adds roughly a quarter to a third more mortar.
Is modified or unmodified thinset better?+
Polymer-modified thinset has better bond and flexibility and is the default for most floors and walls. Some membranes and certain stone or glass tiles call for unmodified mortar, so follow the membrane and tile manufacturer’s instructions.
Does tile size change how much thinset I need?+
Only indirectly. Coverage depends on the trowel notch, but larger tiles need bigger notches and back-buttering, so a large-format floor can use more than twice the thinset of the same area of small wall tile.