About the Plaster Calculator
Plastering is one of those jobs where buying the right amount is surprisingly hard. Plaster is sold by the bag, in kilograms, but it goes on by thickness, in millimetres, and different products spread very differently. A bag of finish plaster might cover ten square metres; a bag of undercoat, barely three.
The underlying arithmetic is simple once it is put the right way round. Every plaster has a coverage rate — how many kilograms of dry powder it takes to cover one square metre, one millimetre thick. Multiply that by the area and the thickness and you have the weight of plaster the job needs.
This plaster calculator does exactly that, with typical rates for the common coats built in, and lets you override the rate with the figure from your own bag. It then turns the weight into bags and tells you what each bag will cover at your chosen thickness.
How to Use the Plaster Calculator
Choose metric or imperial units for the area.
Enter the area to plaster — walls and ceilings, less any large openings.
Pick the coat: a finish skim, a lightweight undercoat of the hardwall type, or a bonding undercoat.
Leave thickness and coverage rate blank to use typical values, or enter your own. The coverage rate can be worked out from any bag: divide its weight by the area it says it covers and by the thickness it assumes.
Set the bag size and the waste allowance — 10% is the default.
How the Plaster Is Worked Out
plaster (kg) = area (m²) × thickness (mm) × rate (kg per m² per mm)
bags = plaster × (1 + waste %) ÷ bag weight, rounded upStep-by-Step Example
Skimming 30 m² of wall at 2 mm, 25 kg bags, 10% waste.
Plaster: 30 × 2 × 1.2 = 72 kg
With waste: 72 × 1.10 = 79 kg
Bags: 79 ÷ 25 = 3.2 → 4 bags
Each bag: 25 ÷ (2 × 1.2) = 10.4 m²The same area in a lightweight undercoat at 11 mm would take eleven bags, because each bag covers only about 3.2 square metres at that depth.
Typical Coverage Rates
| Coat | Usual thickness | Rate (kg/m²/mm) | 25 kg bag covers |
|---|---|---|---|
| Finish (skim) | 2 mm | about 1.2 | about 10 m² |
| Lightweight undercoat | 11 mm | about 0.7 | about 3.2 m² |
| Bonding undercoat | 8 mm | about 1.1 | about 2.8 m² |
These are typical figures for UK gypsum plasters. Undercoats are made with lightweight aggregates such as perlite and vermiculite, so they weigh less per millimetre; finish plasters are dense and smooth. Brands and products vary, and the coverage printed on the bag should always take priority.
Why Thickness Matters Most
Plaster use is directly proportional to thickness. An undercoat meant to be 11 mm that averages 13 mm across an uneven wall uses 18% more plaster than planned. Old walls, walls with patches of missing plaster, and walls that are out of plumb all push the real average thickness up.
That is why it pays to check the depth in several places before ordering, and why the waste allowance is set at 10% by default. For a wall that is badly out of true, 15 to 20% is safer.
Which Coat Do You Need?
New plasterboard usually needs only a finish skim, about 2 mm, sometimes over a bonding primer.
Brick and block walls need an undercoat first to build up a flat surface, then a skim. Lightweight undercoats of the hardwall type suit most masonry; bonding coats suit low-suction surfaces such as smooth concrete.
Existing plaster in good condition can often simply be skimmed after treating the surface with a suitable primer.
Mixing and Working Time
Gypsum plaster does not dry out like mortar; it sets chemically, usually within an hour or so. Once it starts to stiffen it cannot be brought back with more water — that weakens it and spoils the finish. So mix in batches you can apply comfortably within the working time on the bag, and use clean water and a clean bucket each time, since old set plaster in the bucket speeds up the next mix.
Preparing the Surface
Good preparation saves plaster as well as giving a better finish.
Clean off dust and loose material so the plaster bonds to a sound surface.
Control suction. Very absorbent backgrounds such as brick suck water out of the plaster too quickly, causing cracks and weak adhesion; a PVA or proprietary primer evens this out. Very smooth surfaces need a bonding agent to give a key.
Fill deep holes first with an undercoat or filler rather than trying to fill them with the finish coat, which wastes plaster and cracks as it shrinks.
Fix scrim tape over plasterboard joints before skimming, to stop cracks along the joints later.
Understanding Your Result
Bags needed is the number of bags including waste.
Weight gives the total dry plaster in kilograms.
Coverage per bag shows how far each bag goes at your thickness.
Area echoes the area in both units.
Worth knowing covers mixing, thickness and storage.
When Should You Use This Calculator?
Before a plastering job, to buy the right number of bags.
When comparing products, to see how coverage changes the count.
For a skim over new plasterboard, where the figure is predictable.
Checking a plasterer's materials estimate.
Common Mistakes
Assuming all plaster covers the same area. Undercoats cover about a third as much.
Underestimating thickness on old walls. Check the depth in several places.
Mixing too much at once. Gypsum plaster sets and cannot be revived.
Using old bags. Plaster past its date sets unpredictably.
Skipping the primer. Suction differences cause cracking and poor adhesion.