Thinset Calculator
Bags of thinset by trowel notch, bag weight and back-buttering, including 12×24 tile over 100 sq ft.
Your estimate
2 × 50 lb
Bags of thinset (incl. 10% waste)
- Mortar by weight
- 73 lb
- Coverage per 50 lb bag
- 60 sq ft
- Tiled area
- 80 sq ft
- Trowel
- ¼ × ⅜ in square-notch
Show the math
- Area = L × W − deductions = 80 sq ft
- ¼ × ⅜ in square-notch: 60 sq ft per 50 lb bag (5.6 m² per 22.7 kg)
- No back-buttering
- Bags = area ÷ coverage × (1 + 10%) = 1.47 → 2
Shopping list & cost
| Item | Spec | Qty | Cost |
|---|---|---|---|
| Thinset / tile mortar | 50 lb bags, ¼ × ⅜ in square-notch | 2 bags | — |
| Notched trowel | ¼ × ⅜ in square-notch | 1 | — |
| Mixing paddle + 5 gal bucket | low-speed drill (≈ 300 rpm) | 1 | — |
Prices are what you enter under Advanced options. Estimates only — check stock and pricing with your supplier.
Quick answer An 80 sq ft floor set with a ¼ × ⅜ in square-notch trowel (the usual size for 12 × 12 in tile) needs 2 × 50 lb bags of thinset including 10% waste — about 73 lb of mortar, at 60 sq ft per bag. A 7.5 m² floor with a 6 × 10 mm notch needs 2 × 20 kg. A 50 lb bag covers roughly 90 sq ft with a ¼ × ¼ in notch, 60 sq ft with ¼ × ⅜ in and 42 sq ft with ½ × ½ in.
Method & assumptions
Bags = Area ÷ coverage per bag (for the trowel) × (1 + waste %) Coverage per bag = chart value per 50 lb × (bag weight ÷ 50 lb) Back-butter: + Area ÷ 180 sq ft per 50 lb bag
- Coverage per bag
- from the manufacturer's chart; we use the minimum of CUSTOM's range
- Notch
- width × depth of the trowel's teeth
- Back-butter
- a flat 1/16 in skim on the back of each tile
Assumptions & limits
- Coverage figures are for a flat, primed substrate and standard polymer-modified thinset; lightweight mortars cover 20–40% more per bag.
- Square-notch coverage: ¼ × ¼ in = 90, ¼ × ⅜ in = 60, ½ × ½ in = 42, ¾ × 9/16 in U-notch = 34 sq ft per 50 lb bag (low end of the published range).
- Back-buttering estimate assumes a 1/16 in flat coat; actual use varies with the tile back pattern.
- Industry standards require at least 80% mortar contact in dry areas and 95% in wet areas — check by lifting a tile.
Rounding: purchase quantities are always rounded up to whole units as sold. See our methodology.
Diagram
How to use this calculator
- Measure the area to be tiled and subtract anything you will not tile, such as a tub or cabinet base.
- Choose the trowel notch for your tile: bigger tile needs a bigger notch to reach full coverage.
- Turn on back-buttering for large-format, natural stone and tiles with ribbed backs.
- Pick the bag size you can buy: 50 lb in the US, 20 or 25 kg in the UK and Australia.
- Keep about 10% extra — mortar skins over in the bucket and on the floor if you spread too far ahead.
Worked examples
Imperial: 12 × 10 ft kitchen, 24 × 24 in porcelain, ½ × ½ in notch, back-buttered
- 120 ÷ 42 = 2.86 bags + back-butter 120 ÷ 180 = 0.67 bag
- 3.52 × 1.10 = 3.88 → 4 bags of 50 lb
Metric: 3 × 2 m bathroom wall, 150 × 150 mm tile, 6 mm notch
- A 22.7 kg bag covers 8.4 m², so a 20 kg bag covers 7.4 m²
- 6 ÷ 7.4 × 1.10 = 0.9 → 1 × 20 kg bag
Imperial: 100 sq ft of 12 × 24 in tile, ½ × ½ in notch
- 12 × 24 in is large format. The chart uses 42 sq ft per 50 lb bag for a ½ × ½ in square notch.
- 100 ÷ 42 = 2.38 bags. With the default 10%: 2.62 bags, so buy 3 (about 131 lb).
- Back-buttering adds 100 ÷ 180 = 0.56 bag. With 10% that is 3.23 bags, so buy 4 (about 162 lb).
- CUSTOM’s large-format note discourages a ½ × ½ in notch for this size. Use the notch printed for the bag you buy.
In depth: thinset planning notes
How trowel notches turn into bags
A notched trowel lays ridges of mortar of a known size; when the tile is pressed in and moved slightly, the ridges collapse into a bed roughly half as thick as the notch depth. That is why notch size controls coverage so strongly. CUSTOM's chart for a 50 lb bag of VersaBond lists 90–100 sq ft with a ¼ × ¼ in square notch, 60–67 sq ft with ¼ × ⅜ in and 42–47 sq ft with ½ × ½ in. We use the bottom of each range because real substrates are never perfectly flat, then add 10% for mortar that skins over or is left in the bucket.
Large-format tile — any side 15 in or longer under ANSI definitions — needs a deeper bed to cope with slight warpage, so the notch goes up and coverage per bag goes down. Many installers also back-butter: a thin flat coat troweled onto the back of each tile so the ridges bond to wet mortar on both faces. It is the most reliable way to reach the 80% contact required in dry areas and 95% in showers and exteriors, and it costs roughly one extra bag per 180 sq ft. If you lift a freshly set tile and see bare patches or ridges that have not collapsed, go up a notch or back-butter.
Choosing the right mortar
Thinset comes unmodified (cement, sand and water retention additives) and modified with polymers for better bond and flexibility. Most porcelain and most installations over plywood or backer board call for a modified mortar meeting ANSI A118.4 or A118.15; some uncoupling membranes ask for unmodified thinset underneath and on top — follow the membrane maker. Large-and-heavy-tile (LHT) mortars resist slumping in thicker beds, and lightweight mortars cover 20–40% more area per bag, which the calculator can't know about, so check the bag's own coverage chart and adjust.
Mix with a low-speed drill and paddle to the consistency the bag specifies, let it slake for five to ten minutes and remix without adding more water. Spread only what you can tile before it skins over — touch it with a fingertip; if mortar doesn't transfer, scrape it up and re-spread. In the UK and Australia the same product is sold as cement-based tile adhesive in 20 kg bags; coverage scales with weight, so a 20 kg bag goes about 12% less far than a US 50 lb bag with the same trowel.
Reference table
| Trowel notch | Typical tile · sq ft per 50 lb bag |
|---|---|
| 3/16 in V-notch with premixed mastic | small dry-area wall tile · about 40–48 sq ft per gallon |
| ¼ × ¼ in square | up to ~8 in tile · 90–100 |
| ¼ × ⅜ in square | 8–15 in tile · 60–67 |
| ½ × ½ in square | 15 in and larger · 42–47 |
| ¾ × 9/16 in U-notch | large & heavy tile · 34–38 (45°) / 42–47 (30°) |
Key facts
- Minimum mortar contact
- 80% dry areas, 95% wet areas and exteriors
- Large-format tile
- any side 15 in (38 cm) or longer
- Mixing
- let it slake ~10 minutes, then remix
- Pot life
- typically 2–4 hours (check the bag)
Tips & common mistakes
- Comb the ridges in one straight direction, not swirls — it lets air escape and the ridges collapse.
- Pull up a tile now and then to check coverage; if the back is not mostly covered, go up a notch size or back-butter.
- Use large-and-heavy-tile (LHT) mortar for big porcelain and stone; it resists slumping in thick beds.
- Premixed mastic is fine for small dry-area wall tile but not for floors, showers or large tile.
Frequently asked questions
How many square feet does a 50 lb bag of thinset cover?
About 90–100 sq ft with a ¼ × ¼ in notch, 60–67 sq ft with ¼ × ⅜ in, and 42–47 sq ft with ½ × ½ in (CUSTOM VersaBond chart). Back-buttering reduces this.
How much thinset do I need for 100 square feet?
With a ¼ × ⅜ in trowel, about 1.7 bags of 50 lb, so buy 2. With a ½ × ½ in trowel, about 2.4 bags — buy 3, or 4 if you back-butter large tile.
How much thinset for 12×24 tile?
12×24 in is large format. This page’s chart lists a ½×½ in square notch at 42 sq ft per 50 lb bag, and large format is also back-buttered. For 100 sq ft with the default 10% extra and 50 lb bags: the notch alone is 2.62 bags, so buy 3 (about 131 lb). With back-buttering it is 3.23 bags, so buy 4 (about 162 lb). CUSTOM’s large-format note discourages a ½×½ in notch for that size — follow the bag you buy if it specifies a different notch. Count the tiles on the tile calculator first.
What size trowel should I use for 12 × 24 tile?
Usually ½ × ½ in square or a ¾ × 9/16 in U-notch at 30°, plus back-buttering, because 12 × 24 is large-format. Lift a tile to confirm 80–95% coverage.
What is the difference between thinset and mortar?
Thinset is a tile mortar — Portland cement, fine sand and (for modified thinset) polymers — applied in a thin bed. Masonry mortar for bricks and blocks is a different, coarser product.
Should I back-butter tile?
Yes for large-format tile, natural stone, tiles with deep back patterns and in showers. It improves coverage and bond, at a cost of roughly one extra bag per 180 sq ft.
Sources
- CUSTOM Building Products – VersaBond Professional Tile Mortar (coverage chart) — accessed : “Square foot coverage per 50 lb bag (min): 1/4" x 1/4" square-notch 90 sq. ft.; 1/4" x 3/8" 60; 1/2" x 1/2" 42; 3/4" x 9/16" U-notch @45° 34”
- MAPEI – How many bags do I need for my installation? — general reference — not cited for a specific figure
- Tile Council of North America – TCNA Handbook (ANSI A108 mortar coverage) — general reference — not cited for a specific figure
Default values (yields, coverage rates and densities) are typical figures from manufacturer literature. Always check the label on the product you buy.
Related calculators
- Tile CalculatorCount floor or wall tiles and boxes, including a 100 sq ft bathroom, with joints and overage for straight, offset, diagonal or herringbone.
- Grout CalculatorPounds or kilograms and bags of grout from tile size, thickness and joint width, including 12×24 tile over 100 sq ft.
- Backsplash CalculatorKitchen backsplash tile estimate: subway tiles or mosaic sheets for the counter run and the taller section behind the range, plus grout, adhesive and trim.
Learn more in our guides
Method, waste and worked examples: Tile quantities: layout waste, grout and thinset.
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DIYCalcLab. “Thinset Calculator.” Nov 6, 2026. https://diycalclab.com/tile/thinset-calculator/
Estimates only. Verify quantities with your supplier and local building codes. For structural, electrical or safety-critical work, consult a licensed professional. Full disclaimer.