Skim Coat Calculator

Understanding your result

The headline is the number of pails or cartons to buy, and the line under it gives the gallons, the area, the coats and the thickness each coat assumes. The tiles show the compound for one coat, the gallons per 100 sq ft per coat, and the days, one drying day per coat plus half a day of sanding.

The figure to compare is gallons per 100 sq ft per coat. A thin Level 5 skim is about half a gallon; covering orange peel is about one gallon per coat; burying knockdown is about two. If a quote or a video gives a very different figure, the difference is almost always the thickness, which is why the table shows it.

A Level 5 skim on new board is the last pass of a finish, and the joint compound calculator includes it when you choose Level 5; use this page when you want the skim on its own, or when you are covering an old wall. Whether a room needs a skim at all is the finish level planner’s question. If the plan is new texture rather than a smooth wall, the texture calculator counts that instead.

How we calculate this

The thickness is the wet thickness averaged over the whole wall. Over a texture the first coat is thick in the valleys and almost nothing on the peaks, so the average is about half the height of the texture; orange peel stands about 1/32 in, knockdown about 1/16 in. The Level 5 film of 1/128 in gives about 0.49 gallons per 100 sq ft, which agrees with the Level 5 skim pass on the joint compound coverage table.

Ready-mixed compound shrinks a little as it dries, so the dry film is thinner than the wet one; the calculator counts what you put on the wall, which is what you buy. The waste default is 15%, higher than for taping, because rolling and pulling compound across a whole wall leaves more on the tools and in the tray.

The assumptions behind the numbers

Assumptions last reviewed October 8, 2026.

The calculator does not count primer, which a painted or damaged wall needs before skimming, or the scraping and sanding of the old texture. It assumes ready-mix; spray-applied skim products list their own coverage on the pail. If the wall is pre-1980, the texture may contain asbestos, and it must be tested before it is sanded or scraped, as the note says. How a skim fits into the levels of finish is in the levels of finish guide; which compound to thin and how much is in the joint compound guide.

Two worked examples

Where to find your inputs

Area. The walls and ceilings you will skim: length times height for each wall, length times width for the ceiling, minus large openings. The wall area calculator adds them up.

What it is for. New board with a Level 5 spec, a rough or patched wall, or a texture to cover. Run your fingers across the wall: if you can feel the texture, it is orange peel or heavier.

Coats. Leave “usual” unless you know better; a second coat on a Level 5 job only if the first left marks.

Container. Pails are cheaper per gallon; cartons are lighter to carry up stairs.

Common mistakes

• Skimming over gloss paint. The compound peels. Scuff and prime first.
• Too thick in one coat. Thick coats crack and take days to dry. Two thin coats.
• Unthinned compound. Straight from the pail it drags. Thin it to the consistency of thick yoghurt for rolling.
• No work light. A skim coat that looks flat under ceiling lights shows every ridge at sunset. Check with a light held flat to the wall.
• Sanding old texture blind. Pre-1980 texture may contain asbestos. Test first.
• Skimming a soft wall. Water-damaged board crumbles under the knife. Replace it.

Questions people ask

How much joint compound do I need to skim coat a room?

For a Level 5 skim on new board, about half a gallon per 100 sq ft per coat, so a 650 sq ft room takes about 3.2 gallons, one 4.5 gallon pail with waste. To cover an old knockdown texture, two coats averaging 1/32 in each, the same room takes about 25 gallons, six pails.

How thick is a skim coat?

A Level 5 skim is a film, about 1/128 in wet, rolled on and pulled tight with a wide knife or squeegee. A skim coat to flatten orange peel averages about 1/64 in per coat, and to bury knockdown about 1/32 in, because the first coat has to fill the valleys of the texture.

How many coats of skim coat do I need?

One for a Level 5 finish over new, well-finished board. Two to cover a texture or a patched, uneven wall: the first fills, the second flattens. A third coat is only for very heavy textures or walls that show dips under a work light after the second.

Can I skim coat over painted walls?

Yes, after priming. Compound bonds poorly to gloss or satin paint, so clean the wall, scuff it, and apply a bonding primer or a drywall primer-sealer first. Torn paper must also be sealed with primer before skimming, or it bubbles.

What compound is best for skim coating?

All-purpose or lightweight ready-mix, thinned slightly with water so it rolls on; lightweight sands more easily. Some makers sell a sprayable or rollable skim product for Level 5 work. Setting compound is too hard to sand and sets too quickly for a large skim.

Is it worth skim coating instead of replacing drywall?

Usually, for a sound wall with a dated texture or many small dents. Two coats over a 12 × 12 room's walls is two or three pails and two days, far less than tearing out and rehanging. Replace instead when the board is water-damaged, soft or loose.