Self-leveling underlayment calculator: pounds and bags

Measure the deviation before you buy anything

The quantity of self-leveling underlayment is driven almost entirely by one number you have to establish on site: the average thickness of the layer. Everything else — area, consumption rate, bag size — is either known or printed on the packaging. Getting the average wrong by 1/32 in changes a 215 sq ft job by about 56 lb, which is more than a whole bag.

Work with a 6 ft straightedge and a set of measuring wedges or a tape. Lay the straightedge across the room in several directions, including the diagonals, and note the gap underneath at a grid of points — roughly one reading per 10 sq ft is enough for a domestic room. Mark the high point, because that is the point the finished surface has to clear: the layer thickness at the high point is the product's stated minimum, and every other point needs that minimum plus its own gap.

The mistake to avoid is assuming the average is half the maximum. A floor with a single deep dip and an otherwise flat surface averages far less than one that slopes steadily from one wall to the other, even when both share the same worst-case reading. Take the mean of your grid readings — that is the number the calculator above wants.

• One reading per 10 sq ft, taken under a 6 ft straightedge, in at least two directions plus the diagonals.
• Add the product's stated minimum thickness to every reading before you average: the compound has to flow, not just fill.
• Photograph the marked-up floor before you pour. Once it is level, nobody can reconstruct where the deep spots were.
• Check the edges separately — the perimeter is where screeds curl upwards and where the deepest surprises usually hide.

The substrate decides which compound you may use

Before the quantity matters, the product has to suit what is underneath it. Cementitious compounds are the default and tolerate almost any substrate; calcium-sulfate and gypsum-based ones flow beautifully and cost less, but they have to stay permanently dry, which rules them out of bathrooms, utility rooms and any floor exposed to standing water. Over an anhydrite screed the order reverses: the screed is sanded first to take off the sintered skin, because that thin weak layer is exactly what peels away when a bond fails.

Every product also carries a minimum and a maximum thickness, and both are limits rather than suggestions. A fine smoothing compound rated 1/32 to 3/8 in will not fill a 1 in dip: poured that thick it shrinks, cracks and costs three times what a deep-fill product would have. The reverse fails just as reliably — a heavily filled compound feathered out at 1/32 in never closes to a smooth surface. Where the deviation falls outside the product's range, either build up in two layers with the intermediate priming the data sheet prescribes, or move to a leveling screed and treat the floor as a screed job.

Three build-ups need a product of their own. Over underfloor heating the layer becomes part of the heated construction: use a fiber-reinforced compound, keep the cover above the pipe crown that the heating manufacturer states, and leave the edge insulation strip standing until the compound has cured so the floor can move. Over old tiles the substrate has to be sound first — tap every tile, replace the hollow ones, grind the glaze and use a grip primer, because a loose tile telegraphs through any thickness of compound. Over timber boards, screw the boards down so they stop working, then use a flexible fiber-reinforced product at the higher minimum thickness it specifies, with the decoupling fleece if the sheet asks for one.

• Gypsum-based compounds stay out of wet rooms; cementitious ones go almost anywhere.
• On anhydrite, sand the screed and use the primer the data sheet names — the sintered skin is the layer a failed bond peels off with.
• Read the minimum as carefully as the maximum: 1/32 to 3/8 in will not fill 1 in, and a deep-fill product will not close at 1/32 in.
• Underfloor heating, old tiles and timber boards each need their own product — fiber-reinforced, grip-primed, flexible — not the cheapest bag on the pallet.

The consumption rate on the bag decides the order

Leveling underlayments are quoted in pounds per 100 sq ft for every 1/8 in of thickness, and the figure differs between products. Current manufacturer data sheets give 91 lb for the lightest products, 98 lb for several common ones and 104 lb for the heaviest. The calculator's default of 104 lb is the top of that range. Across a 323 sq ft floor at 3/16 in, the difference between 91 lb and 104 lb is 63 lb — more than one 50 lb bag.

So take the rate from the bag or the data sheet of the exact product you are buying, not from a rule of thumb. If you are comparing two products on price, compare them on cost per 100 sq ft per 1/8 in rather than on cost per bag: a bag that covers less area can cost more per floor even when it is cheaper per bag.

A safety margin on top is normal practice rather than padding. Substrate that is more absorbent than expected, a room that turns out to slope more than the grid suggested, material left in the bucket and in the pump line — five to ten percent covers all of it, and the field above lets you add it explicitly instead of quietly inflating the thickness.

Primer changes the job, not the quantity

Priming is the step most often skipped and most often responsible for a failed pour. On an absorbent substrate — cement screed, anhydrite, old concrete — the bare surface pulls the mixing water out of the compound within minutes. The compound stops flowing, cures short of full strength and leaves pinholes where escaping air could not close behind it. Primer regulates that absorption so the compound keeps its water for the few minutes it needs to self-level.

On a non-absorbent substrate the problem is the opposite: a tiled or sealed surface offers nothing to bond to, so the primer's job is to provide grip. These are different products, or at least different dilutions of the same product, and the data sheet says which. Dilution matters: an over-diluted primer on a thirsty screed does almost nothing, and an undiluted primer on a dense surface can leave a glossy film that the compound slides on.

Whatever the substrate, it has to be clean, sound and free of anything that breaks a bond — dust, curing agents, adhesive residues, paint, gypsum from plastering. Vacuum rather than sweep, and let the primer dry to the state the data sheet describes before mixing the first bag. None of this changes how many pounds of compound the floor needs; all of it changes whether those pounds stay put.

Mixing, pot life and the one-pour rule

Self-leveling underlayment is unusually sensitive to water. A quart too much per bag and the mix separates, loses strength and dries with a weak, dusty surface; a quart too little and it will not flow and stays under the straightedge as a ridge. Measure the water rather than judging it, mix with a forced-action paddle for the full time the bag states, and let the mix stand for its maturing time if one is specified.

Working time is short — typically 15 to 30 minutes from mixing, less in a warm room. That is what makes the pour a planning problem rather than a mixing problem: every batch has to reach the previous one while the previous one is still wet, or the two do not merge and a ridge stays. Pour in strips, keep a wet edge, and pass a spike roller over the fresh surface to release trapped air.

This is also why the calculation should be generous by exactly one bag. Running out at three quarters of a room is not an inconvenience, it is a cold joint that has to be ground back and a surface that is no longer flat. An unopened bag returns to the merchant; a ground joint costs an afternoon and a hire of a grinder.

• Weigh or measure the water; do not judge it by consistency in the bucket.
• Mix full bags. A part bag mixed by eye is the usual cause of a patch that cures a different color and hardness.
• Keep the room between roughly 60 and 75 °F, out of direct sun and drafts, during the pour and the first day.
• Spike-roll to release air, and work in strips wide enough that the next strip meets a wet edge.

Drying, residual moisture and when the covering can go down

Two different times are printed on every data sheet and they get confused constantly. 'Walkable' is when the surface carries foot traffic without marking — about 2 to 3 hours for current cementitious compounds (current data sheets give 2, 2 to 2.5 or 2 to 3 hours). 'Ready for covering' depends on the layer and the covering: tiles go down as soon as the floor is walkable; vinyl, linoleum and carpet after about 6 hours up to 3/16 in (5 mm), 12 hours up to 3/8 in (10 mm) and 24 to 48 hours above that; parquet takes longer (one data sheet: 12 hours up to 5 mm, 24 hours above).

The old screed rule of about a week per 1/4 in does not apply to leveling compound — it describes sand-cement screed, which dries by evaporation, while modern compounds bind most of their water chemically. What does stretch the times: a cold room, high humidity, poor ventilation, thick layers and products with slower binders (one data sheet states 12 hours, another 48 hours). The times on the data sheets are measured at 68 to 73 °F (20 to 23 °C) and 50 to 65 % relative humidity. The screed or slab underneath must itself be dry before you pour; for parquet and vapor-tight floors also check the covering manufacturer's moisture limit.

Under a heated floor there is a further step: the heating has to be brought back up under a documented heat-up protocol before the covering goes down, and that protocol takes its own days. Build them into the schedule at the start; discovering them the week the flooring is due is how a job loses two weeks.

Once the subfloor is level and dry, the covering becomes a separate calculation — plank size, pattern, cut list and waste — and that is exactly the job the Autolay planner does from a photo of the plan, an IFC building model or a 3D scan.