Subfloor & prep
How much self-leveling compound? Depth, bags and primer
A bag of leveler covers about 47 sq ft at 1/8 inch and half that at 1/4 inch. The depth is everything, and it is the number most people guess. Here is how to measure it and turn it into bags.
What self-leveler is
Self-leveling compound, or self-leveling underlayment, is a cement-based powder mixed with water to a pourable consistency. Poured on a primed floor, it flows into the low spots and finds its own level, leaving a smooth, flat surface for tile, vinyl, wood or carpet. Most products pour from about 1/8 inch to 1 inch deep in one lift, some deeper with added aggregate, and are ready for foot traffic in a few hours.
It is used to flatten a floor that fails the flatness limit for the new finish, to bring a floor up to meet a neighbouring floor, and to embed electric floor heating cables.
Coverage falls with depth
The bag tells you how much floor it covers at a given depth, usually 1/8 inch. Coverage is inversely proportional to depth: twice as deep, half as much floor. A typical 50 lb bag covers 46 to 48 sq ft at 1/8 inch, according to the coverage charts of common US products.
| Average depth | sq ft per 50 lb bag | Bags per 100 sq ft, with 10% |
|---|---|---|
| 1/8 in | 47 | 3 |
| 1/4 in | 23.5 | 5 |
| 3/8 in | 15.7 | 8 |
| 1/2 in | 11.8 | 10 |
| 1 in | 5.9 | 19 |
Measuring the depth
The depth that matters is the average depth over the area you pour. To find it, set a laser or a long straightedge with a level across the floor and measure the gap to the floor at a grid of points, about every 2 to 3 ft. The pour must reach at least the product’s minimum depth at the highest point, usually 1/8 inch, so add that to every gap. Average the results.
A shortcut that works for a floor sloping one way: measure the depth needed at the lowest point and at the highest point, and average the two.
A 200 sq ft floor
- An average depth of 1/4 inch: 47 × (0.125 ÷ 0.25) = 23.5 sq ft per bag
- Bags: 200 ÷ 23.5 × 1.10 = 9.4, so 10 bags, 500 lb
- Primer: about 220 sq ft ÷ 300 sq ft per gallon = 0.73 gal
The same floor measured as 1/2 inch at the low point and 1/8 inch at the high point averages 0.31 inch, and needs 12 bags.
Levelling part of a floor
Often only a dip or one side of a room needs leveler, feathered out to nothing at the edges. Measure the area of the dip and its average depth, not the whole room. A feathered edge averages about half the depth of the deepest point. If 40% of that 200 sq ft floor needs the 1/4 inch pour, it takes 4 bags instead of 10. The self-leveling compound calculator takes the share of the floor and the depth.
Pour in whole bags
A leveler has a working time of 10 to 20 minutes. Mix whole bags with the exact water on the bag, and pour each batch against the wet edge of the last. Too much water weakens the surface and makes it dusty. For anything larger than a small area, a second person mixing while the first pours is the difference between a flat floor and a ridged one, and large floors are pumped by contractors.
Before pouring, prime the floor with the maker’s primer, which stops the subfloor sucking water out of the leveler and helps it bond. Seal gaps, floor drains and the edges of wood subfloors so the leveler does not run out; dam doorways with foam or wood strips.
Over wood subfloors
Self-leveler over plywood needs a stiff floor, a primer suited to wood, and usually a metal lath or plastic reinforcement mesh stapled down first, with a minimum depth set by the maker, often 3/8 inch or more. A bouncy wood floor will crack the leveler; stiffen it first. The subfloor panels calculator covers new underlayment plywood if replacing the panels is simpler.
Over radiant heating
Electric floor heating cables and mats are often embedded in a thin pour of leveler, at least 1/8 inch over the top of the cable. Count the depth from the slab, not from the cable. The radiant floor heating guide covers the heater.
Curing and the next layer
Most levelers take foot traffic in 2 to 4 hours, tile the next day, and moisture-sensitive floors such as wood, vinyl and glued carpet only when the leveler has dried, which can take several days at depth. Check the bag for each.
Leveler or screed?
For depths beyond what a leveler pours in one lift, typically more than an inch, or to build a slope for a shower or a wet room, a sand-and-cement screed is cheaper. It does not self-level; it is packed and screeded by hand. The floor screed calculator counts sand and cement. Either one adds height, which the floor height calculator checks against the next room.
The short version
Measure the average depth on a grid. Read the bag’s coverage at 1/8 inch and scale it to your depth. Level only what needs it. Add 10% and prime first. The subfloor preparation guide covers the other checks before any floor, and the large-format tile guide the flattest floors.