What the calculator works out
Plaster is a thin layer of cement mortar over brickwork. The quantity depends on three things: how much wall there is, how thick the coat is, and how rich the mix is. Everything else — bags, cft, cost — follows from those.
Formula
Wet volume = area × thicknessDry volume = wet volume × 1.30— sand bulks up and mortar fills the voids in the brickworkCement = dry volume ÷ (1 + ratio), and the rest is sandBags = cement volume × 1,440 kg/m³ ÷ 50 kg
Worked example
1,000 sq ft of inside wall at 12 mm in a 1:5 mix: the wet volume is 1.11 m³, the dry volume 1.45 m³. That is 0.24 m³ of cement — 6.96 bags, so seven — and 42.6 cft of sand.
How to use the plaster calculator
- Measure the wall in square feet. Deduct large openings; small ones roughly cancel out against the reveals.
- Pick the thickness: 12 mm inside, 20 mm outside, more on rough brickwork.
- Pick the mix your mason is using, then read the bags and cft. Add prices for a material cost.
On site
- Buy a little more than the calculation — about 5% — for waste, spillage and uneven brickwork.
- The 30% dry-volume allowance is the usual site figure. Very wet sand bulks more, so check the sand before ordering.
- Labour, scaffolding, curing water and any waterproofing compound are not included here.
Frequently asked questions
How much cement is needed for 1000 sq ft of plaster?
At 12 mm thickness in a 1:5 mix, about 7 bags of cement and 43 cft of sand. A 20 mm outside coat in 1:4 needs roughly double the cement.
What is the right plaster thickness?
12 mm for internal walls, 15 mm for ceilings and rough surfaces, and 20 mm for external walls that take the weather. Thicker coats are usually applied in two layers.
Why multiply by 1.30 for dry volume?
Mortar is measured wet but mixed from dry materials. Damp sand bulks up, and the mix sinks into the joints and pores of the brickwork, so you need about 30% more dry material than the finished volume.