Plaster Calculator
Count plaster bags for veneer finishing over board or gypsum basecoat over lath, from wall area and coat thickness.
Two plasters, two thicknesses
Veneer plaster is a skim: a 1/16 in coat over gypsum base board that gives a true plaster finish at drywall speed. A 50-lb bag covers about 150 sq ft at that skim — the figure the manufacturers publish. Gypsum basecoat is the older, thicker work over lath or masonry, built 3/8 to 1/2 in deep; it is figured by volume from the bag's published yield of about 0.57 cu ft (the Structo-Lite class of lightweight gypsum plasters).
Veneer bags = area ÷ 150 sq ft per bag · Basecoat bags = area × thickness ÷ 0.57 cu ft per bag
Example: 300 sq ft of veneer plaster is 300 ÷ 150 = 2 bags exactly — 3 bags with the 10% allowance. The same wall in 1/2 in basecoat would take 12.5 cu ft of plaster, about 7.3 bags before waste.
Working time is the real constraint
Plaster sets chemically, not by drying, and the clock is unforgiving: veneer plasters give roughly an hour of working time, basecoats a little more. The practical consequence for buying is that a wall is plastered in one continuous mix per section — count the bags per wall, stage them all before the first mix, and do not plan to stretch one bag across two days. Retarder extends the set on hot days; accelerator shortens it for patching. Neither changes the count.
Baseboard and trim lines, window returns and ceiling junctions all take plaster too — the waste allowance absorbs them on a normal room. For a ceiling, add the ceiling area to the walls or run the tool twice; plaster does not care which way up it is, but your shoulders will.
Frequently asked questions
Veneer plaster or joint compound — what is the difference?
Joint compound finishes only the seams and dries soft; veneer plaster coats the whole wall and cures hard, dense and abrasion-resistant — closer to traditional plaster. Veneer costs more per wall and needs its special gypsum base board (blue board) to bond.
Can gypsum plaster go on a bathroom or exterior wall?
No. Gypsum softens with sustained moisture. Bathrooms want cement-based or lime plasters (or tile backer and tile), and exteriors want stucco — the cement plaster in the stucco calculator.
How thick can one basecoat pass be?
About 1/2 in per pass over lath; thicker builds go in two passes with the first scratched to key the second. The calculator's thickness is the total build — split it into passes yourself if it exceeds 1/2 in.
Why do the bags weigh 50 lb but yield so little volume?
Gypsum plaster is dense and most of it stays in the wall as solid mineral: 0.57 cu ft of placed plaster per bag is the published yield for the lightweight basecoats, and plain gypsum yields less. Judge plaster by coverage, never by the bag's weight.
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