How it works
The calculator multiplies the area by the average thickness to get the volume of underlayment, then divides by the volume that one bag fills according to its data sheet.
Volume = Area × Average thickness × (1 + Extra ÷ 100)
Volume per bag = Coverage per bag × Thickness for that coverage
Bags = ⌈ Volume ÷ Volume per bag ⌉
Total bag weight = Bags × Bag weight
⌈ ⌉ means rounding up to the next whole bag. The volumes themselves are not rounded.
Where the coverage figures come from
Every self-leveling underlayment has a coverage figure in its technical data sheet: the area one bag covers at a stated thickness. The product options use these figures, at the low end of any range:
- CUSTOM LevelQuik RS (TDS-108): 46–48 sq ft per 50 lb bag at 1/8 in (3.18 mm).
- SikaLevel: 42 sq ft per 50 lb (22.7 kg) bag at 1/8 in (3.2 mm).
- ARDEX K 15: 30 sq ft (2.8 m²) per 55 lb (25 kg) bag at 1/4 in (6 mm). The metric figure gives the smaller volume per bag – 16.8 L against 17.7 L – so that one is used.
The calculator turns that pair of numbers into a volume per bag and divides your volume by it. That is the same as reading the coverage tables in the data sheets: LevelQuik RS lists 23–24 sq ft at 1/4 in and SikaLevel 21 sq ft at 1/4 in, half of their 1/8 in figures. Links to all three sheets are listed under Sources.
Finding the average thickness
The number of bags depends directly on the thickness, so it pays to measure it rather than guess. Mark the height of the finished surface, for example with a laser level or a string line stretched between two fixed points, and measure from that height down to the floor on a grid every few feet. Average the readings. A floor that needs 1/2 in at one wall and nothing at the opposite wall averages about 1/4 in – using 1/2 in for the whole room would buy twice as many bags as needed.
Check that the average stays inside the thickness range of the product. The calculator warns when it does not. For deeper pours, the SikaLevel and ARDEX K 15 sheets describe adding aggregate; follow the sheet of your product.
Before you pour
The data sheets describe the preparation in detail: a clean, sound substrate free of anything that would stop the primer bonding, and the primer specified for that substrate. Measure the mixing water exactly as printed on the bag – the ARDEX sheet warns that additional water weakens the compound and lowers its strength. The ARDEX sheet asks you to work in a continuous manner during the whole pour, and the CUSTOM sheet to keep a wet edge when pouring several mixes, so have all bags and enough helpers on site before you start.
Data-sheet coverage is a figure for estimating. SikaLevel’s sheet says it plainly: its figures do not include any allowance for surface profile, porosity or material waste. Use the extra allowance for a rough or porous floor and for the material that stays in the bucket.
Once the underlayment has cured as long as its data sheet requires for your floor covering, continue with the tile and thinset calculators for tile, or the vinyl plank calculator for resilient flooring.
Worked example
Inputs
- Measure by
- Floor length × width
- Floor length
- 12 ft
- Floor width
- 10 ft
- Average thickness
- 1/4 in
- Underlayment product
- CUSTOM LevelQuik RS – 46 sq ft per 50 lb bag at 1/8 in (≈ 4.3 m² at 3.2 mm)
- Extra allowance
- 0 %
Result
- Area to level
- 120 sq ft
- Underlayment volume
- 2.5 cu ft
- Volume per bag
- 0.479 cu ft
- Bags
- 6
- Total bag weight
- 300 lb (0.15 short tons)
Common mistakes
- Using the deepest point as the thickness. That overestimates by a lot on a floor with one low spot – use the average depth over the whole area.
- Using the coverage figure without its thickness. Coverage per bag is always stated for one thickness; at twice the thickness a bag covers half the area.
- Ignoring the thickness limits. Each product has a range for a single pour without aggregate. Thicker pours may need aggregate or several lifts, as described in its data sheet.
- Running out mid-pour. The ARDEX sheet asks you to work in a continuous manner and the CUSTOM sheet to keep a wet edge between mixes, so have every bag on site before mixing the first.
- Skipping the primer. All three data sheets call for one, chosen to suit the substrate.
Assumptions and limits
- Material use scales with thickness – a pour twice as thick uses twice the material per area. This is how one data-sheet figure is converted to your thickness, and it matches the coverage tables of the listed products.
- The thickness entered is the average over the whole area.
- In floor mode the area is length × width. For other shapes, enter the area directly.
- Bags are rounded up to whole bags. Primer, aggregate, perimeter foam and edge forms are not included.
Frequently asked questions
How much does a bag of self-leveling underlayment cover?
It depends on the product and on the thickness. At 1/8 in, the data sheets list 46–48 sq ft per 50 lb bag for CUSTOM LevelQuik RS and 42 sq ft for SikaLevel. ARDEX K 15 lists 30 sq ft (2.8 m²) per 55 lb (25 kg) bag at 1/4 in (6 mm). At 1/4 in the 1/8 in figures halve.
How thick can I pour?
Each product has its own range. The data sheets give featheredge to 1.5 in in one pour for LevelQuik RS, 1/8 to 2 in for SikaLevel and up to 1 1/2 in neat for ARDEX K 15; thicker pours need aggregate or more than one lift as described in the sheet. The calculator warns when your thickness is outside the range of the product you picked.
How do I find the average thickness?
Lay a long straightedge or a string line at the height of the finished surface and measure the gap down to the floor every few feet. Add up the readings and divide by the number of readings. Floors that slope towards one wall average about half of the depth at the low end, if the high end needs almost nothing.
Does the calculator include primer?
No. All three listed products call for a primer before pouring. Primers have their own coverage and dilution instructions on their data sheets.
What if my product is not in the list?
Choose “Your own coverage” and enter the coverage per bag together with the thickness it refers to, both from the bag or the data sheet. Add the bag weight if you want the total weight.
Sources
- LevelQuik RS (Rapid Setting) Self-Leveling Underlayment (TDS-108) — Custom Building Products. "TYPICAL SQUARE FOOT COVERAGE PER 50 LB BAG": 1/8" (3.18 mm) "46 - 48 ft²" (preset uses 46); "Can be applied from featheredge to 1.5" (3.8 cm) thick in a single pour".
- SikaLevel Self-Leveling Underlayment Product Data Sheet — Sika USA. Coverage "1/8" (3.2mm) 42 ft2" per "50 lb. (22.7 kg) bag"; "Layer Thickness 1/8” up to 2” (3 up to 50 mm)"; figures exclude "surface profile and porosity or material waste".
- ARDEX K 15 Premium Self-Leveling Underlayment Technical Data Sheet — ARDEX Americas. "Coverage: Per bag At 1/4” (6 mm): 30 sq. ft. (2.8 m2)"; "Packaging: 55 lb. (25 kg) bag"; "Installs up to 1 1/2” (4 cm) neat" (preset uses 2.8 m² at 6 mm, the smaller volume).
- 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