Plaster Calculator
Last updated: 12 August 2026
Work out how many bags of plaster you need to skim or float a wall or ceiling. Covers multi-finish, board finish, bonding and hardwall at their real coverage rates.
How the calculation works
Bags = area ÷ coverage per bag, plus wastage. Coverage depends entirely on the product and the thickness it is applied at: multi-finish skim goes on at about 2mm in two coats and covers around 10 m² per 25kg bag, while bonding is a float coat at 11mm and covers only about 3 m².
Water is roughly 11 litres per 25kg bag of finish plaster. Always add plaster to water, never the reverse, and mix in batches you can apply before it goes off — multi-finish gives you about 90 minutes.
Plaster coverage per 25kg bag
| Plaster | Thickness | Coverage | Use |
|---|---|---|---|
| Multi-finish | 2mm | ~10 m² | Skim over most backgrounds |
| Board finish | 2mm | ~11 m² | Skim over plasterboard |
| Bonding | 11mm | ~3 m² | Float coat on smooth/low-suction backgrounds |
| Hardwall | 11mm | ~3.5 m² | Float coat on masonry |
| One-coat | 10mm | ~4.5 m² | DIY patch and small repairs |
Do I need to PVA first?
Seal high-suction backgrounds — bare brick, blockwork, old plaster — with diluted PVA (usually 1 part PVA to 4 parts water, then a stronger coat) so the plaster does not dry too fast and crack. Plasterboard normally needs no PVA; skim it straight, scrimming the joints first. One litre of PVA covers roughly 25 m² per coat when diluted.
Frequently asked questions
How many bags of plaster do I need?
For skimming with multi-finish, one 25kg bag covers about 10 square metres at 2mm in two coats. A 4m x 2.4m wall is 9.6 square metres, so one bag plus wastage — order two to be safe.
How much does a bag of multi-finish cover?
About 10 square metres per 25kg bag applied at 2mm in two coats. Board finish covers slightly more at around 11 square metres because plasterboard has lower suction.
How much water do I add to a bag of plaster?
Roughly 11 litres of clean water per 25kg bag of finish plaster. Always add the plaster to the water rather than the other way around, and mix to a smooth, lump-free consistency like thick custard.
What is the difference between bonding and multi-finish?
Bonding is an undercoat or float plaster applied at around 11mm to build out and flatten a surface. Multi-finish is the thin 2mm top coat that gives the smooth finish. Bonding covers only about 3 square metres per bag versus 10 for multi-finish.
How long does plaster take to dry?
Multi-finish is usually touch dry within a few hours and ready to paint after 3 to 7 days depending on ventilation and thickness. Float coats over 11mm can take a week or more. Painting too early traps moisture and causes flaking — wait until the colour is uniformly pale.
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Estimates are for guidance only. Material dimensions, densities and coverage vary by supplier — always confirm quantities before ordering, and consult a qualified professional for structural work.