How the plaster calculator works
Plaster coverage depends on how thick you apply it. Each product has a typical usage in kg per m² per mm, which the calculator multiplies by your area and thickness:
Plaster coverage per 25kg bag
| Plaster | Typical thickness | Coverage per 25kg |
|---|---|---|
| Multi-Finish | 2mm | ~10.4 m² |
| Board Finish | 2mm | ~10.4 m² |
| Bonding Coat | 8mm | ~3.0 m² |
| Hardwall | 11mm | ~2.8 m² |
| One Coat | 13mm | ~2.3 m² |
These are guide figures based on typical manufacturer data. Real coverage varies with background flatness and suction, so the calculator adds 10% wastage by default.
Choosing the right plaster
- Skimming plasterboard: Board Finish or Multi-Finish at 2mm, with scrim tape over the joints.
- Re-skimming old walls: Multi-Finish, after controlling suction with diluted PVA or a plaster primer.
- Brick or block walls: Hardwall (high-suction backgrounds) or Bonding Coat (low-suction backgrounds like engineering brick or concrete), then a Multi-Finish skim.
- Patching deep holes: Bonding or One Coat plaster, built up in layers.
Mix a full bag only if you can use it within the setting time (around 60–90 minutes for finish plaster). For a large wall, mix half bags so the plaster doesn't go off in the bucket.
Worked examples
Skimming one wall, 4m × 2.4m: 9.6m² × 2mm × 1.2kg = 23kg, plus 10% = 25.3kg. That's just over one bag, so buy 2 × 25kg bags of Multi-Finish. The spare is useful for fixing any patches you're not happy with. You'll need about 23 litres of clean water to mix them.
Re-skimming a whole bedroom (4m × 3.5m, 2.4m high) means about 33m² of walls after the door and window, plus a 14m² ceiling: 47m² in all. At 2mm that's about 124kg, or 5 bags. Do the ceiling first, then the walls.
Hardwall on a 9.6m² block wall at 11mm: about 93kg, so 4 bags, followed by 2 bags of Multi-Finish to skim it.
How a skim coat goes on
- Prepare: scrim-tape plasterboard joints, or PVA high-suction walls and wait until the PVA is tacky.
- First coat: apply about 1mm, working quickly and evenly across the whole wall.
- Second coat: as the first starts to firm up, apply another thin coat to fill the lines and hollows.
- Trowel up: as it sets, flatten and polish it with a clean trowel and a light mist of water, two or three times, until it's smooth.
New plaster usually needs around a week or more to dry, depending on thickness, temperature and ventilation. It's dry when the colour is an even pale pink or light brown with no dark patches. Seal it with a thinned "mist coat" of emulsion before painting. The paint calculator includes a mist-coat option.
Common plastering mistakes
- Too much at once. A full bag that you can't finish before it sets is wasted. Start with half bags.
- Dirty water or buckets. Old set plaster in a bucket makes the new mix go off much faster.
- Skipping suction control. On dry old plaster the skim dries before you can trowel it, leaving cracks and a rough finish.
- Thick skim to hide bad walls. Finish plaster cracks if it's laid much over 3mm. Fill hollows with bonding first.
Plastering new boards? The plasterboard calculator covers sheets and screws, and the stud wall calculator covers the frame. See all floor, wall and decorating calculators.