Plastering Calculator Guide. Plaster calculations require wall area and coats. Formula: Square Yards equals Area divided by 9. Plaster sold by square yard coverage. Standard three-coat plaster: scratch coat 3/8 inch, brown coat 3/8 inch, finish coat 1/8 inch, total 7/8 inch thick. Example 500 sq ft walls: 500 divided by 9 equals 55.6 square yards. Three coats times 56 yards equals 168 yard-coats. Material needs: One bag plaster (80 pounds) covers approximately 60-80 sq ft per coat depending on thickness. 500 sq ft three-coat: 500 divided by 70 times 3 equals 22 bags total. Sand ratio 3 parts sand to 1 part plaster by volume for base coats, finish coat uses lime putty or gypsum. Costs: Traditional plaster 8-15 per sq ft installed (three-coat system). Veneer plaster (two-coat) 5-10 per sq ft. Skim coat over existing 2-4 per sq ft. Materials only 1-3 per sq ft. Labor intensive work. Lath types: Wood lath historical, strips of wood, labor-intensive. Metal lath modern standard, diamond or flat sheets, best for curved surfaces. Rock lath (gypsum board) smooth economical base. Blue board special gypsum for veneer plaster. Each affects cost and application. Three-coat process: Scratch coat first layer 3/8 inch, scratched with rake for bond, dries 2-7 days. Brown coat second layer 3/8 inch, floated smooth and level, dries 5-14 days. Finish coat final layer 1/8 inch, troweled smooth or textured, dries 24-48 hours. Total drying 2-3 weeks. Rush drying causes cracking. Veneer plaster: Faster two-coat system. Base coat 1/16-1/8 inch over blue board. Finish coat 1/16-1/8 inch. Total thickness 1/8-1/4 inch. Dries 24-48 hours ready to paint. More economical than traditional three-coat, very strong hard surface. Plastering vs drywall: Plaster advantages: harder more durable, better sound insulation, seamless appearance, superior fire rating, can create curves. Disadvantages: expensive, slow (weeks to cure), requires skilled tradesman, difficult repairs. Drywall cheaper and faster, most homes use drywall now. Ceiling plaster: More difficult than walls, requires scaffolding. Three-coat process same as walls. Furring channels needed for suspended ceilings. Expect 50-100 percent higher labor cost than walls due to difficulty. Common mistakes: Not allowing proper drying time between coats causes failure. Applying coats too thick cracks as dries. Mixing batches wrong consistency. Poor lath prep, plaster won't bond. Rushing finish coat shows imperfections. Plastering over painted surfaces without preparation, won't adhere. Not protecting floors and fixtures, messy process. Pro tips: Keep room temperature 50-70 degrees F and humidity moderate for proper curing. Mix plaster to consistency of thick cream, test on board. Apply in continuous strokes, don't stop mid-wall. Keep leading edge wet feathers together better. Use hawk and trowel for application takes practice. Clean tools immediately, plaster sets quickly. Mask and protect everything, dust and splatter inevitable. Repair vs new: Small holes and cracks repair with patching plaster or joint compound. Large damaged areas skim coat 1/4 inch over lath. Extensively damaged walls remove to lath and replaster. Sometimes covering with 1/4 inch drywall more economical than full replaster. Decorative finishes: Smooth trowel traditional formal. Skip trowel textured rustic. Knockdown modern subtle texture. Venetian plaster multiple troweled layers polished, high-end look. Stucco coarse sand finish. Each affects cost and appearance. Historical plaster: Lime plaster traditional, lime putty and sand, breathable, flexible. Gypsum plaster modern, sets faster, harder. Horsehair reinforced old plaster prevents cracking. Clay plaster natural, earthen homes. Each has specific properties and applications. FAQ: How much plaster for 100 sq ft? Three-coat approximately 5 bags or 75-100 pounds. Cost to plaster room? 10×12 room with 8 ft ceiling approximately 3,500-6,000 installed traditional three-coat. Can I DIY plaster? Very difficult, requires significant skill and practice. Consider hiring professional for quality results. How long until I can paint? Traditional three-coat 2-3 weeks. Veneer plaster 48 hours. Paint too soon causes failure. Plaster or drywall? Plaster more expensive but superior durability, sound control, seamless appearance. Drywall cheaper and faster, most modern homes.