Plaster quantity calculation: area, mortar, cement and deductions

Plaster is measured by area and priced per sqm or sft, but its materials depend on thickness and mix.

Measuring the area

• Measure each face separately: inside and outside of external walls, both faces of internal walls.
• Ceilings and beam soffits are usually separate items.
• Deduct openings larger than the limit your method of measurement sets — commonly openings over 0.5 sqm (about 5 sft) are deducted; jambs, sills and soffits (returns) are then measured and added.

Mortar volume

Wet mortar = area × thickness. Dry mortar = wet × 1.33, plus about 20 % for joint filling, uneven surfaces and droppings on rough brickwork (many engineers use a combined factor of 1.5–1.6 over wet).

For 100 sft of 12 mm (½") plaster:

• wet = 100 × 0.5 / 12 = 4.17 cft
• dry ≈ 4.17 × 1.33 × 1.2 = 6.65 cft

At 1:4: cement = 1.33 cft = 1.06 bags, sand = 5.32 cft. At 1:6: cement = 0.95 cft = 0.76 bags, sand = 5.70 cft.

Per sqm

For 1 sqm of 12 mm plaster at 1:4: wet 0.012 cum, dry ≈ 0.019 cum, cement ≈ 0.0038 cum = 0.11 bag, sand 0.015 cum.

Thickness guide

Worked example

A room 14' × 12' with walls 10' high, one 3' × 7' door and one 5' × 4' window:

• Walls = 2 × (14 + 12) × 10 = 520 sft
• Deduct door 21 sft and window 20 sft → 479 sft
• Add returns (jambs) if your method requires them.

In EstiMatrix

Area items show Nos, Length and Width only. Enter openings as deduction rows; link a second coat or a paint item to the plaster rows with linked measurements so the areas always match.