Plaster Calculator
Estimate bags of veneer plaster or gypsum basecoat, blueboard or lath, and mixing water for a wall, with coverage for skim coat, basecoat and two-coat plaster.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- 50 lb bags of veneer plaster
- 2 bags
- About 100 sq ft per bag; 198 sq ft including 10% waste
- Wall area
- 180 sq ft
- 20 ft x 9 ft minus openings
- Blueboard sheets
- 7 sheets
- 4 ft x 8 ft, 32 sq ft each, including 10% for cuts. 4x8 sheets fall 12 in short on this wall; order 4x9 or 4x10 sheets and hang them vertically
- Mixing water
- 6 gal
- About 3 gal per 50 lb of dry plaster. Mix one bag at a time; working time is about an hour and a set batch is trash
Plaster formula
Wall area = Length x Height - openings; Order area = Wall area x (1 + waste %)
Bags = Order area / coverage per bag (veneer 100 sq ft per 50 lb, basecoat 50 sq ft per 80 lb, two-coat over lath 12 sq ft per 80 lb)
Sheets or lath pieces = Order area / 32 sq ft (blueboard) or / 18 sq ft (lath)
Mixing water (gal) = Bags x bag weight (lb) x 3 / 50Worked example
A 15 x 8 ft wall with one 20 sq ft door, veneer plaster over new blueboard: 15 x 8 - 20 = 100 sq ft, or 110 sq ft with 10% waste. Veneer plaster: 110 / 100 = 1.1, so 2 bags of 50 lb. Blueboard: 110 / 32 = 3.4, so 4 sheets. Water: 2 x 50 x 3 / 50 = 6 gallons.
If the same wall gets a 1/8 in gypsum basecoat first, that is 110 / 50 = 2.2, so 3 bags of 80 lb basecoat and 3 x 80 x 3 / 50 = 14.4 gallons of water, plus the 2 bags of veneer finish on top.
Quick reference
| Wall area | Veneer (50 lb bags) | Basecoat (80 lb bags) | Two-coat over lath (80 lb bags) | Water for veneer |
|---|---|---|---|---|
| 100 sq ft | 2 | 3 | 10 | 6 gal |
| 200 sq ft | 3 | 5 | 19 | 9 gal |
| 300 sq ft | 4 | 7 | 28 | 12 gal |
| 400 sq ft | 5 | 9 | 37 | 15 gal |
| 500 sq ft | 6 | 11 | 46 | 18 gal |
| 800 sq ft | 9 | 18 | 74 | 27 gal |
Veneer plaster vs traditional three-coat
Veneer plaster is the modern system. Blueboard goes up like drywall with 1-1/4 in screws at 12 in on center and the joints get mesh tape. One or two thin coats are troweled over the whole surface, and the room is ready for paint in two or three days, harder and smoother than taped drywall. It is standard practice in New England; elsewhere blueboard is a special order and plasterers are scarce, so book the plasterer before you order board.
Traditional three-coat plaster over 2.5 lb diamond mesh lath is a 1/2 in scratch and brown coat of gypsum basecoat under a finish coat. It uses four to eight times the material per square foot and cures a week or more. Pros use it to match existing walls in old houses and for curved work; blueboard gives the same finish for a fraction of the material.
Mixing and applying plaster
Add the plaster to clean water, never water to plaster, and mix one bag at a time with a drill paddle. Working time is about an hour, and a batch that has started to set cannot be retempered. Wash the bucket and paddle between mixes; set plaster left behind kicks the next batch early. The blue face paper does the bonding, so board that sat in sunlight and faded to tan needs bonding agent. ASTM C843 calls for 55 degrees F or warmer and no fans on the wet coat; a dryout is soft and chalky.
Over painted walls or standard drywall, scrape loose paint, dull any gloss and brush on Larsen Plaster-Weld or USG Plaster Bonder; both are pink so you can see coverage. Apply the first coat tight to the board, double back to fill and smooth, and trowel to the finish as it firms up. Prime with latex primer once dry, usually a day or two. Keep leftover dry plaster sealed; an open bag sets faster.
What this calculator leaves out
The result is plaster, board and water. Add mesh tape for every joint, expanded-flange plaster corner bead on outside corners, and 1-1/4 in screws for 1/2 in board (1-5/8 in for 5/8). Bonding agent for a skim coat covers roughly 300 to 400 sq ft per gallon. Lath work adds nails, plaster stop and casing bead. Primer, paint, board delivery and labor are separate.
Mistakes that cost money
- Veneer plaster over regular drywall or paint peels off in sheets; use blueboard, or brush on Plaster-Weld or USG Plaster Bonder first.
- Blueboard left in sunlight for weeks fades from blue to tan and loses its bond; faded board gets bonding agent before plaster.
- Set plaster left in the bucket kicks the next batch in minutes; wash the bucket and paddle clean between every batch.
- Fans or open windows in hot weather dry the coat before it sets; a dryout is soft and chalky and gets scraped off.
- Drywall corner bead under 1/16 in of veneer telegraphs through; use expanded-flange plaster bead made for veneer thickness.
- Mixing a full bag for half a wall wastes it; working time is about an hour, and a set batch cannot be retempered.
Key facts
- Veneer plaster is applied only 1/16 to 3/32 in thick over blueboard, a gypsum board with an absorbent face paper made for plaster.
- A 50 lb bag of veneer finish plaster covers about 100 sq ft; an 80 lb bag of gypsum basecoat covers about 50 sq ft at 1/8 in.
- Traditional plaster over metal lath is about 5/8 in thick: a 1/2 in scratch and brown basecoat plus a 1/16 to 1/8 in finish.
- Veneer plaster sets in about an hour or two and can usually be painted after a day or two; joint compound needs a full day to dry per coat.
- Blueboard comes in the same 4 x 8 ft sheets and 1/2 and 5/8 in thicknesses as drywall and is hung with the same screws.
Frequently asked questions
How much does a bag of veneer plaster cover?
About 100 sq ft per 50 lb bag at 1/16 to 3/32 in. A 12 x 12 ft room with 8 ft ceilings has about 334 sq ft of wall after openings, which needs 4 bags with 10% waste.
Can I put veneer plaster over regular drywall?
Not directly. Standard drywall paper is not absorbent enough and the plaster can peel. Use blueboard for new work, or brush a plaster bonding agent over existing drywall or painted walls first.
How much water do I mix with plaster?
About 3 gallons (12 quarts) per 50 lb bag of veneer plaster, so 2 bags take about 6 gallons. Follow the ratio on the bag and add the plaster to the water, not the other way around.
Can a plasterer skim coat my painted or textured walls?
Yes. Scrape loose paint, dull any gloss, brush on Plaster-Weld or USG Plaster Bonder, then skim 1/16 to 3/32 in of veneer finish. Heavy texture gets knocked down first or a basecoat under the finish. Figure one 50 lb bag per 100 sq ft and pick the existing wall option here.
Sources and references
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.