Plaster Calculator
Calculate how much plaster or render a wall needs, based on the net wall area, the layer thickness and the consumption printed on the bag.
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Formula and method
Net wall area (gross area minus openings) is multiplied by the layer thickness to give a volume, and by the consumption rate to give the required mass. That mass is divided by the bag weight and rounded up.
Gypsum, lime and cement use different quantities
Gypsum plaster covers about 0.9 kg/m² at 1 mm, cement-lime about 1.3 and cement-sand about 1.7. The difference is density, so the same wall thickness costs a different number of bags depending on the mix, and the mix is chosen by the room, not by the price per bag.
Wet rooms and external walls need cement-based or lime-based plaster; gypsum is for dry internal walls. Choosing gypsum everywhere and hoping is how a bathroom wall becomes a soft, chalky surface within a year.
Thickness comes from the wall, not from you
The average thickness is the wall's deviation, measured over a 2 m straightedge. A wall that is 20 mm out over 2 m needs about 10 mm average, which at 0.9 kg/m² per mm is 9 kg/m² — nearly double the figure a 5 mm assumption would give.
The minimum over the high spots is what limits the job: gypsum plaster needs at least 5 mm over the proudest point, and if the wall is worse than 30 to 50 mm the right answer is to straighten it with a backing coat first rather than to float it out with finish plaster.
Water, beads and the working day
Water goes in at the manufacturer's ratio, usually 0.5 to 0.7 litres per kilogram. Overwatering makes it easier to trowel and weaker when set, and the cracks that appear at the corners are the invoice for that convenience.
Beads, scrim and PVA are counted by the running metre and the square metre respectively, and they are small line items that stop the plaster cracking at every corner. Plan the day around the set time: a mix that goes off before you reach the end of the wall costs the whole batch.
How to measure the inputs
- Measure each wall's length and height and multiply by the number of identical walls.
- Add up the area of doors and windows you want to deduct.
- Read the consumption rate and bag weight from the bag you will actually use.
Worked example
Wall length 16.4 ft; Wall height 9.84 ft; Wall count 1; Openings area (optional) 5.38 ft²; Layer thickness 0.39 in; Consumption rate 1 kg/m² per mm; Package weight 55.12 lb; Waste 10 % → Plaster bags: 7
Reference data
| Mix | kg/m² per 1 mm | 25 kg bags per 10 m² at 10 mm |
|---|---|---|
| Gypsum | 0.9 | 4 |
| Cement-lime | 1.3 | 6 |
| Cement-sand | 1.7 | 7 |
Thicker layers on uneven walls are counted by the average thickness, not the deepest point.
Practical tips
- Order whole bags — an opened bag absorbs moisture and goes off quickly.
- Apply in two passes on thick layers rather than one heavy coat.
- Check the substrate is not too dry or too absorbent before starting.
Sources
- СП 71.13330.2017 — Изоляционные и отделочные покрытия
- ГОСТ 31377-2008 — Смеси штукатурные на гипсовом вяжущем
Checked and updated October 22, 2026