Bags of underlayment by area and average depth — and the average worked out properly from your high and low readings.
Self-Leveling Concrete Calculator
Inputs
Self-leveller is unforgiving of running short mid-pour — it sets before you can mix more.
Results
Underlayment needed
6 × 50 lb bags
bags = ft² × depth ÷ 12 × (1 + waste) ÷ 0.5 ft³
Calculated with BuildCalculators.com
Calculated with BuildCalculators.com · buildcalculators.com/self-leveling-concrete-calculator
Estimates are for planning only — confirm quantities with your supplier or contractor before ordering.
Self-levelling underlayment is bought by depth, and the depth people use is almost always the wrong one. The instinct is to measure the deepest hollow in the floor and order for that across the whole room, which on a floor that dishes gently from one side to the other buys close to twice what the job needs — and at $35 to $50 a bag that gets expensive fast.
What you want is the average depth: the minimum film over the high point, plus half the drop down to the low point. This calculator takes either — the average directly if you know it, or the high and low readings if you have set a level line and measured down.
Coverage is the easy part and it is remarkably consistent between brands. A 50 lb bag yields about half a cubic foot: 48 square feet at 1/8 inch, 24 at 1/4, 12 at 1/2. Ardex, Mapei and Henry all publish figures that land within a few percent of that.
high-point film + (low − film) ÷ 2ft² × depth ÷ 120.5 ft³ ÷ (depth ÷ 12)volume × (1 + waste) ÷ 0.5A floor is a solid, not a prism. If the high corner takes 1/8 inch of film and the low corner takes 3/4, the material between them tapers, and the mean depth across the room is 7/16 — not 3/4. Ordering at the deep figure buys 71% more than the floor can hold. The survey mode does this arithmetic for you because it is the single most expensive mistake on this material, and it is made by people who are otherwise estimating carefully.
Not optional, and not interchangeable between brands. An unprimed porous slab pulls the water out of the mix before it has finished flowing, and you get a floor that stopped levelling halfway across the room, with a visible arrest line you cannot sand out. Primer also stops pinholes rising through the surface as trapped air escapes the substrate. Use the primer the manufacturer sells for that underlayment.
Working time is typically 10 to 20 minutes. That governs the whole job: a 120 square foot room at 1/4 inch is five bags, which is more than one person can mix and place inside the window. Two mixers and two people is the realistic minimum for anything past about four bags, and everything — barrel, paddle, water measured out, gauge rake, spiked roller — is laid out before the first bag is opened. A seam where one batch met a set one does not level out and does not sand flat.
Self-leveller is genuinely liquid. It finds the gap under the door, the stair nosing, the duct boot, the pipe penetration and the joint between the subfloor sheets, and it does all of it while you are holding a bucket. Every doorway gets a dam, every penetration gets sealed, and on a wood subfloor the seams get taped. This is more of the job than the pouring is.
Most products go from about 1/8 inch minimum up to 1 inch neat. Below the minimum, a feathered edge cracks and delaminates because there is not enough material to develop strength. Above the maximum, you extend with pea gravel to the manufacturer's ratio, which changes the coverage per bag entirely — so if your depth comes out over an inch, read the bag before ordering to the figure here.
Underlayment levels a floor for whatever goes on top of it. It is not a slab, it will not bridge a flexing subfloor, and it will not fix a structural problem — a bouncing floor levelled today is a cracked floor next year. Most products are also not rated as a finished wearing surface unless specifically sold as one.
1/8 in film at the high point, 3/4 in at the lowest.
10 bags of 50 lb
Ordering at the 3/4 in low-spot depth would have been 17 bags — seven bags and around $300 of material that had nowhere to go.
A floor that is basically flat, taking a skim to true it up.
6 bags
Five bags is around 15 minutes of mixing for two people working steadily. This is about the largest pour one person should attempt alone.
Film 1/8 in, low spot 1½ in — past the neat limit.
30 bags — but check the product first
The average is inside the 1 in neat limit, yet the low corner is not. Deep spots usually get filled and cured in a separate first pour, or extended with aggregate; either changes this number.
At 0.5 ft³ a bag — Ardex, Mapei and Henry all publish within a few percent of this.
| Depth | Coverage | Bags for 120 ft² | Bags for 200 ft² |
|---|---|---|---|
| 0.125 in | 48.0 ft² | 3 | 5 |
| 0.25 in | 24.0 ft² | 5 | 9 |
| 0.375 in | 16.0 ft² | 8 | 13 |
| 0.5 in | 12.0 ft² | 10 | 17 |
| 0.75 in | 8.0 ft² | 15 | 25 |
| 1 in | 6.0 ft² | 20 | 34 |
Self-levelling underlayment runs $35–$50 for a 50 lb bag in 2026, plus $40–$80 a gallon for primer covering 200–300 square feet. A 120 square foot room at 7/16 inch average is 10 bags — $350–$500 of material — and installed by a flooring contractor the same room runs $2–$6 per square foot including preparation.
A mixing paddle rated for the load, a barrel, a gauge rake and a spiked roller. Renting is $50–$100 for the day and buying is $150–$250. On a single small room the tools can approach the cost of the material, which is one of the few genuine arguments for hiring the pour out.
Over-ordering on the low-spot depth, and buying a second batch because the first was short. Both are avoided by surveying properly. Everything else about this material is cheap by comparison — including the primer, which people skip to save $50 and then re-pour a room over.
A single deep hollow in an otherwise flat floor does not need the whole room levelled. Patching that spot with a cementitious patch at $20–$30 a bag, then skimming the floor at 1/8 inch, is frequently a third of the cost of levelling to the depth of the worst point.
The floor tapers. Use the average — the high-point film plus half the drop. On a typical dished floor the deep-spot order is close to double what is needed.
A porous slab drinks the water out of the mix and the floor stops flowing mid-room. That arrest line is permanent, and the fix is another pour over the top.
Working time is 10 to 20 minutes. Underestimating mixing capacity leaves a seam between a wet batch and a set one, and self-leveller does not level itself into an existing set edge.
It is a liquid and it will find the stairwell, the doorway and the duct. Dam and seal everything before the first bag is mixed, not while the second is going down.
Feathering below the product minimum produces a brittle edge that cracks and lifts. Where a true feather is needed, use a product sold for feather-edging.
Underlayment is not structural. A springy subfloor has to be stiffened first, or the new surface cracks along the joists within a season.
A 50 lb bag yields about half a cubic foot — 48 square feet at 1/8 inch, 24 at 1/4, 12 at 1/2. A 120 square foot room at an average 7/16 inch takes about 10 bags. Use the average depth across the floor, not the depth of the worst spot.
Take the minimum film the product needs over the high point, then add half the drop from there to the lowest point. A floor with 1/8 inch of cover at the high point and 3/4 inch at the low averages 7/16 inch. Ordering at the 3/4 inch figure buys about 71% more than the floor can hold.
Most products go from 1/8 inch up to 1 inch neat. Beyond that you extend with pea gravel to the manufacturer's ratio, which changes the coverage per bag. Below the minimum, a feathered edge cracks and delaminates.
Yes, and with the primer made for that product. An unprimed porous slab pulls water out of the mix before it finishes flowing, so the floor stops levelling partway across the room and leaves an arrest line you cannot sand out. Primer also stops pinholes from air escaping the substrate.
Typically 10 to 20 minutes. That governs the whole job — anything past about four bags needs two people and two mixers to keep a wet edge, and everything gets laid out before the first bag is opened. A seam between a wet batch and a set one does not level out.
Usually after 4 to 6 hours, with flooring going down at 16 to 24 hours depending on the product and the depth. It is underlayment, though — most are not rated as a finished wearing surface unless sold specifically for that.
No. It is not structural and it will not bridge a subfloor that moves. A springy floor has to be stiffened first, or the new surface cracks along the joists within a season.
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It is a planning estimate. It does not replace an engineer, a site visit or a written quote — confirm quantities with your supplier before ordering.