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)

Work from dry volume, not the finished wet volume. Loose sand and cement bulk up, then lose that air when mixed, so multiply the wet volume by about 1.5 before splitting it by the ratio. Skip this step and you order roughly a third short.

Cement needed (bags) = Wet Volume × 1.5 × (1/4) ÷ 0.035 m³ per bag Sand needed (m³) = Wet Volume × 1.5 × (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. That 49.8 m² is 536 sq ft, which is the figure you type into the area box.

Finish coat (gypsum, 2 mm): 49.8 m² ÷ 6 m² per bag = 8.3 bags. The calculator adds 10% for waste, so you buy 10 bags. Base coat (cement render, 12 mm): a 25 kg bag lays about 0.02 m³, so 49.8 × 0.012 = 0.6 m³ ÷ 0.02 = 29.9 bags. With the same waste allowance, 33 bags.

Run it once per coat type. The calculator works on one plaster type at a time, so a base coat and a finish coat are two separate runs rather than one combined figure.

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