Plaster Calculator

Plaster (render) is a cement, lime, or gypsum-based coating applied to walls and ceilings to create a smooth, durable surface. Correctly calculating material quantities prevents costly mid-job shortages and wasted materials.

Coverage formula: Area to Plaster = Length × Height (per wall section) Volume of Plaster = Area × Coat Thickness Bags Required = Volume ÷ Coverage per Bag

Mix ratio formula (site-mixed plaster): For a 1:3 cement:sand mix: 1 part cement + 3 parts sand + water (0.5 water:cement ratio) Cement needed (bags) = Volume × (1/4) ÷ 0.035 m³ per bag Sand needed (m³) = Volume × (3/4)

What each variable means:

• Coat Thickness: plaster is applied in layers:
  • Scratch coat (base coat): 10–15 mm (⅜–⅝ inch)
  • Brown coat (leveling): 8–10 mm (⅓–⅜ inch)
  • Finish coat: 2–3 mm (⅛ inch)
  • Total typical thickness: 20–25 mm for three-coat system
• Coverage per Bag: a 25 kg bag of premixed plaster covers approximately:
  • Base coat: 1.5–2.0 m² at 10mm depth
  • Finish coat: 5–8 m² at 2mm depth

Plaster types:

• Gypsum plaster: interior use only. Fast-setting, smooth finish. Not waterproof.
• Cement render: exterior or wet areas. Harder, waterproof, slower drying.
• Lime plaster: historic buildings, breathable walls. Slow-setting, flexible.

Worked example: Plastering a 4m × 3m room (4 walls + ceiling). Ceiling: 4 × 3 = 12 m². Walls: perimeter × height = 14m × 3m − 2 doorways (2 × 2.1m²) = 42 − 4.2 = 37.8 m². Total area = 49.8 m². Finish coat (2mm depth): bags needed = 49.8 ÷ 6 (m²/bag avg) = 8.3 → 9 bags Base coat (12mm): bags needed = 49.8 × 0.012 ÷ 0.02 (m³/bag) = 30 bags

Always add 10% overage for mixing waste and coverage variation across substrates.