How the plaster calculator works
Plaster, render and screed are all sold by the bag with a coverage figure at a given thickness. The calculator turns that figure into a consumption rate and applies it to your job.
- Net area. Wall width × height or floor length × width, minus doors and windows.
- Consumption. Bag weight ÷ (coverage on the bag × thickness on the bag) gives kilograms per square metre per millimetre.
- At your thickness. Consumption × your coat thickness gives kilograms per square metre.
- Material. Kilograms per square metre × net area, plus a waste allowance of 10 % for mixing losses and what stays in the bucket.
- Bags. Total kilograms ÷ bag weight, rounded up.
The material presets fill in typical figures: gypsum finishing plaster at 1.25 kg per square metre per millimetre, undercoat at 0.8, cement render at 1.8 and sand-cement screed at 2.0. Change any field to match the bag you are buying and the preset switches to custom.
Measuring
Measure each wall to be plastered and subtract the door and window openings; reveals can be ignored. For ceilings use length × width. For screed use the floor area and the depth you have decided on, allowing for the finished floor level and any underfloor heating pipes.
Thickness matters more than area
Doubling the thickness doubles the material. A skim coat at 2 mm over a whole room takes fewer bags than an undercoat at 11 mm on one wall. If the wall is uneven, measure the deepest point and enter the average you expect to apply.
Imperial units
Switch to imperial to enter feet for the wall and inches for the thickness, with bag weights in pounds and coverage in square feet. The calculation runs in metric behind the scenes.