Self-levelling calculator
Self leveling
concrete calculator
Underlayment is poured to fix a floor that is not flat, so the depth is never the same across the room. Enter the thin end and the deep end and this works from the average — plus the primer, which the bag count on its own always forgets.
Enter measurements ↓Optional
What will it cost?
Enter what you actually pay per bag and the quantities above are priced up. The field starts at a typical US price so you have something to work from — change it to your own.
Typical US range $22–$48 per bag, checked 2026-08-29. Prices move — use your own supplier's figure.
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Put these figures to work
Sizing one part of a pour is easier when the slab itself is settled. Start with the main estimate, then come back for the detail.
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Based on your result
What you'll need
Quantities update after every calculation. Buy every bag before you start — underlayment sets in about fifteen minutes, and a run to the store mid-pour leaves a cold joint.
- Levelling compound
- Bags to reach the average depth
- Calculate above
- Primer
- Seals the slab so the pour does not flash off
- —
- Mixing gear
- Paddle mixer, two buckets, gauge rake
- As needed
- Perimeter foam
- Stops the pour bonding to walls and door frames
- —
The maths
How this self leveling concrete calculator works
The bag tells you coverage at one depth, usually an eighth of an inch. Your floor is not at one depth — that is why you are levelling it. Averaging the thin end and the deep end turns the slope into a single figure the bag count can use.
volume = area × (average depth ÷ 12)
bags = ⌈ volume × waste ÷ bag yield ⌉
coverage = bag yield ÷ (depth ÷ 12)
Coverage per 50 lb bag by depth
| Depth | Area per bag | Bags per 100 sq ft |
|---|---|---|
| 1/8 in | 43.2 sq ft | 3 |
| 1/4 in | 21.6 sq ft | 5 |
| 3/8 in | 14.4 sq ft | 7 |
| 1/2 in | 10.8 sq ft | 10 |
| 3/4 in | 7.2 sq ft | 14 |
| 1 in | 5.4 sq ft | 19 |
Figures for a 50 lb bag yielding 0.45 cubic feet. Brands vary between about 0.42 and 0.50 — the selector above follows yours.
-
01
Find the low point first
Lay a long straightedge across the floor and measure the biggest gap under it. That gap is the depth in the dip; the high point is where the straightedge touches.
-
02
Average the two ends
A floor running from an eighth of an inch to three quarters averages 0.44 inches — nearly four times the coverage a bag quotes at its headline depth.
-
03
Prime, always
Bare concrete drinks the water out of the mix and it flashes off before it flows. Primer is the step people skip and then repour.
-
04
Buy it all up front
Underlayment stays workable for roughly fifteen minutes. Mixing the last batch late leaves a visible cold joint you cannot sand out.
Good to know
Self-levelling questions, answered
A 50 lb bag yields about 0.45 cubic feet, which covers 43 square feet at an eighth of an inch or 11 square feet at half an inch. A 200 square foot floor averaging a quarter inch takes about ten bags.
Lay a straightedge across the worst run and measure the largest gap beneath it. That is the depth in the dip. The high point is wherever the straightedge touches, which is usually an eighth of an inch of minimum coverage rather than zero.
Most products go to about an inch neat, and one to two inches when gauged with pea gravel. Deeper than that needs a second lift after the first has set, or a sand and cement screed instead.
Yes on concrete, and yes on plywood. Unprimed slabs pull the water out of the mix so it stops flowing and sets rough, and trapped air rises through as pinholes. Primer costs a fraction of a repour.
Over a sound, well-fastened subfloor, with the right primer and usually a reinforcing mesh, yes. Deflection is the enemy — if the floor moves underfoot, the underlayment will crack no matter how it is primed.
Light foot traffic in two to four hours for most rapid-set products, and tile or vinyl over it after 16 to 24 hours. Check the bag, since the range across brands is wide.
It flows and finds its own level, but it will not travel far on its own. Pour in ribbons across the floor and help it out with a gauge rake, then break the surface tension with a spiked roller.