How it works
The calculator takes the area to tile, adds your extra allowance and divides by the area one bag covers with the chosen trowel.
Area = Length × Width
Area incl. extra = Area × (1 + Extra ÷ 100)
Bags = ⌈ Area incl. extra ÷ Coverage per bag ⌉
Total bag weight = Bags × Bag weight
⌈ ⌉ means rounding up to the next whole bag. Nothing is rounded before that step.
Where the coverage figures come from
Thinset manufacturers publish a coverage table in each product’s technical data sheet: the area one bag covers with a given trowel. The product options use those tables for two common square-notch trowels and take the low end of every range:
- CUSTOM VersaBond (TDS-119): 85–95 sq ft per 50 lb bag with a 1/4 × 1/4 × 1/4 in notch, 60–67 sq ft with a 1/4 × 3/8 × 1/4 in notch.
- TEC Full Set Plus (380/381): 95–105 sq ft and 65–75 sq ft per 50 lb bag for the same two trowels.
- LATICRETE 252 Silver (DS-676): 80–95 sq ft per 50 lb bag with a 1/4 × 1/4 in notch and 60–70 sq ft with a 1/4 × 3/8 in notch.
All three sheets state that coverage varies – with the substrate, the tile, the trowel or the jobsite conditions. Links to the data sheets are listed under Sources. For any other mortar, or another trowel, read the table on the bag and choose “Your own coverage”.
Why the trowel matters most
The notch of the trowel decides how much mortar stays on the floor. A deeper notch leaves taller ridges, and the same bag covers less area. In each of the three tables above, the 1/4 × 3/8 in notch covers 25–32 % less area per bag than the 1/4 × 1/4 in notch (low ends: 85 → 60, 95 → 65 and 80 → 60 sq ft). If you are unsure which trowel you will need, run the calculator with the deeper notch: it gives the larger number of bags.
Checking mortar coverage on the job
The purpose of the notch is to leave a full bed of mortar under the tile once it is pressed in. The TEC and LATICRETE sheets describe the check: set a tile, lift it and look at the back. CUSTOM’s sheet asks for at least 80 % contact in dry areas and 95 % in wet areas and exteriors, with all tile edges supported. For larger tiles, the CUSTOM and TEC sheets mention a larger notch or back-buttering the tile – both use more mortar than the table, which is what the extra allowance is for.
Before you buy
Buy all bags of one product at once and keep a bag in reserve if the store accepts returns of unopened bags. Check the data sheet for the floors the mortar may be used on, the mixing water per bag and how long the mixed mortar stays workable, and mix only what you can spread before it skins over. If the floor needs leveling first, estimate the underlayment with the self-leveling underlayment calculator; grout is estimated separately with the grout calculator.
Worked example
Inputs
- Measure by
- Floor length × width
- Floor length
- 12 ft
- Floor width
- 10 ft
- Mortar product
- CUSTOM VersaBond – 85 / 60 sq ft per 50 lb bag (≈ 7.9 / 5.6 m²)
- Trowel notch
- 1/4 × 3/8 × 1/4 in (6 × 9 × 6 mm) square notch
- Extra allowance
- 0 %
Result
- Area to tile
- 120 sq ft
- Area incl. extra
- 120 sq ft
- Coverage per bag used
- 60 sq ft
- Bags
- 2
- Total bag weight
- 100 lb (0.05 short tons)
Common mistakes
- Using the coverage for the wrong trowel. Going from a 1/4 × 1/4 in to a 1/4 × 3/8 in notch cuts the area per bag by a quarter to a third in the data sheets used here.
- Choosing the trowel from the tile size alone. The notch has to leave enough mortar under the tile once it is pressed down – lift a tile early on and check the coverage on its back.
- Forgetting back-buttering. Coating the back of large tiles or stone is not included in the coverage tables and needs extra mortar.
- Mixing more than you can use. Mortar that has skinned over in the bucket or on the floor has to be thrown away, not re-mixed.
- Buying different products for one job. Coverage, mixing water and open time differ between mortars; keep to one product and follow its instructions.
Assumptions and limits
- The product options use the coverage per 50 lb bag from each manufacturer’s data sheet for the trowel you pick, taking the low end of the range so the estimate errs towards more mortar.
- The coverage applies evenly to the whole area. A floor that is not flat, or tile that needs back-buttering, uses more.
- In floor mode the area is length × width. For other shapes, enter the area directly.
- Bags are rounded up to whole bags. Grout, primer, membranes and self-leveling underlayment are separate products with their own calculators or data sheets.
Frequently asked questions
How much does a 50 lb bag of thinset cover?
It depends mostly on the trowel. The data sheets used here give 80–105 sq ft per 50 lb bag with a 1/4 × 1/4 × 1/4 in square-notch trowel and 60–75 sq ft with a 1/4 × 3/8 × 1/4 in square notch, depending on the product. Each manufacturer publishes its own table – use the one for the mortar you are buying.
Why does the calculator use the low end of the range?
The tables are estimating figures and real coverage varies with the floor and the tile. Using the low end means you are more likely to have a little left over than to run out halfway through a room.
Which trowel size should I use?
The data sheets say the trowel depends on the tile size and type and the flatness of the floor. CUSTOM states that coverage under the tile has to be at least 80 % in dry areas and 95 % in wet areas and exteriors, with the tile edges supported. Follow the instructions of your mortar and tile, and check by lifting a freshly set tile.
What if my mortar is not in the list?
Choose “Your own coverage” and enter the coverage per bag for your trowel from the bag or the data sheet. Add the bag weight if you want to see the total weight.
Can I use this for wall tile?
Yes, as long as the mortar is suitable for walls. Add up the wall areas, subtract windows and doors, and enter the total in known area mode with the coverage for the trowel you use on the wall.
Sources
- VersaBond Professional Thin-Set Mortar (TDS-119) — Custom Building Products. "SQUARE FOOT COVERAGE PER 50 LB BAG": 1/4" x 1/4" x 1/4" Square-Notch "85 sq. ft." min – "95 sq. ft." max; 1/4" x 3/8" x 1/4" Square-Notch "60 sq. ft." – "67 sq. ft." (presets use 85 and 60).
- Full Set Plus Unmodified Thin Set Mortar (380/381) Product Data Sheet — TEC Specialty Products. "Approximate Coverage per 50 lbs. (22.68 kg)": 1/4" x 1/4" x 1/4" square-notched trowel "95-105 sq. ft."; 1/4" x 3/8" x 1/4" square-notch trowel "65-75 sq. ft." (presets use 95 and 65).
- 252 Silver Data Sheet (DS-676) — LATICRETE International. "Approximately 60–70 ft2/50 lb bag ... with 1/4" x 3/8" (6 mm x 9 mm) notched trowel"; "Approximately 80–95 ft2/50 lb bag ... with 1/4" x 1/4" (6 mm x 6 mm) notched trowel" (presets use 60 and 80).
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 3, 2026