A standard 25kg bag of multi-finish plaster covers roughly 10m² at 2mm thick, which works out at about 2.5 to 3 plasterboards per bag. One 2400mm x 1200mm board is 2.88m², so budget about a third of a bag per board. Undercoat plasters like bonding go on much thicker and only cover around 3m² per bag. Here is how to work out exactly what you need.
Key facts
- A 25kg bag of multi-finish covers roughly 10m² at 2mm, about 2.5 to 3 plasterboards
- One standard 2400mm x 1200mm plasterboard is 2.88m²
- Bonding coat covers just 3 to 3.5m² per 25kg bag at 8mm
- The standard trade bag is 25kg; DIY stores also sell 12.5kg bags and small patching packs
- Bagged plaster keeps for 3 to 4 months, check the date printed on the bag
How many bags of plaster do you need per board?
A standard sheet of plasterboard measures 2400mm x 1200mm, which is 2.88m². Divide the 10m² coverage of a 25kg bag of multi-finish by 2.88 and you get 3.5 boards per bag on paper.
In practice you will get 2.5 to 3 boards per bag. A proper skim plastering job uses two coats totalling nearer 3mm than 2mm, and you always lose some in the bucket and on the spot board. Smaller 1800mm x 900mm boards are 1.62m² each, so a bag covers around 5 of those.
| Number of boards (2400 x 1200mm) | Area to skim | Bags of 25kg multi-finish |
|---|---|---|
| 1 to 3 boards | Up to 8.6m² | 1 bag |
| 4 to 6 boards | Up to 17.3m² | 2 bags |
| 7 to 9 boards | Up to 25.9m² | 3 bags |
| 10 to 12 boards | Up to 34.6m² | 4 bags |
Figures assume a two coat skim at around 2mm finished thickness plus 10% wastage, rounded up to full bags.
How much does a bag of plaster cover?
Coverage depends almost entirely on how thick the plaster goes on. Finishing plasters are applied at about 2mm, while undercoat plasters go on at 8 to 13mm, which is why a bag of bonding covers a third as much as a bag of multi-finish. If you are not sure which product you need, see our guide to the types of plaster explained.
| Plaster type | Typical thickness | Coverage per 25kg bag | Typical use |
|---|---|---|---|
| Multi-finish | 2mm skim | Roughly 10m² | Skimming boards and backing coats |
| Board finish | 2mm skim | Around 11m² | Skimming new plasterboard only |
| Bonding coat | 8mm | 3 to 3.5m² | Undercoat on most backgrounds |
| Hardwall | 11mm | 2.5 to 3m² | Undercoat on brick and block |
| One coat | 13mm | 2.25 to 2.5m² | Undercoat and finish in one |
These are planning figures for reasonable surfaces. A wall in poor condition, or a heavy hand, will eat into them. Hardwall plaster in particular varies with the suction of the background.
What size bags does plaster come in?
The standard trade size is the 25kg bag. Multi-finish, board finish, bonding, hardwall and one coat plasters all come in 25kg bags, and it is the cheapest way to buy by some distance.
DIY stores also stock 12.5kg bags of finishing plaster for smaller jobs, plus 2kg to 5kg patching packs and ready-mixed tubs that need no water at all. Tubs suit filling cracks and small holes, but the cost per kilo is several times higher than bagged plaster.
A 25kg bag of multi-finish mixes with roughly 11.5 litres of clean water and gives you about 40 minutes of working time, so plan your mixes around what you can use in that window.
Essential measurements you need
Before calculating your plaster requirements, measure the space accurately:
- Wall height (floor to ceiling)
- Wall width (measure each wall separately)
- Total area of doors and windows (to subtract from the wall area)
- Ceiling dimensions (if plastering the ceiling)
Multiply the height by the width of each wall, then subtract the openings. Do not forget the internal reveals of windows and doors if they need plastering. Small measuring errors turn into whole bags at the merchant.
How many bags of multi-finish per room?
Use this method for any room:
- Total wall area (m²) = wall height x wall width, added up for every wall
- Subtract door and window areas
- Divide the total by the coverage per bag for your plaster type
- Add 10% for wastage and round up to full bags
Worked example for a typical 4m x 3m room with 2.4m ceilings:
Wall area = 2 x (4m + 3m) x 2.4m = 33.6m²
Minus a door (1.98m x 0.76m = 1.5m²) and a window (1.2m x 1.2m = 1.44m²) = 30.66m²
Using multi-finish at 10m² per bag: 30.66 ÷ 10 = 3.07 bags
Add 10% wastage = 3.37 bags
Final requirement: 4 bags for the walls, or 5 bags including the 12m² ceiling
How long does a bag of plaster last?
Bagged gypsum plaster has a shelf life of three to four months from manufacture. The use-by date is printed on the bag, so check it at the merchant before you buy. Out-of-date plaster sets too fast, and sometimes not at all, which ruins a skim coat.
Store bags off the floor in a dry room and keep opened bags tightly sealed. Damp is the killer, a bag left on a concrete garage floor can be lumpy within weeks.
Once mixed, the clock really starts. Get the water measured and the batch size right using our guide on how to mix plaster, and only mix what you can apply in about 40 minutes.
Factors that affect plaster usage
Several factors can push your usage above the planning figures:
- Wall condition (poor surfaces need more material)
- Application thickness (deeper backing coats need more bags)
- Temperature and humidity (these change drying times and workability)
- Skill level (inexperienced plasterers use more material)
- Surface suction (different substrates pull differently, and application technique matters)
Professional tips for accurate ordering
- Always round up to the nearest full bag
- Add one extra bag for repairs and surprises
- Store plaster dry and off the floor
- Check the date stamp on every bag when purchasing
- Buy from the same batch where you can for consistency
If you are weighing up doing it yourself against getting a quote, our plastering costs guide shows what supply-and-fix work costs across the county.
Common calculation mistakes to avoid
- Forgetting to subtract windows and doors
- Not adding a wastage allowance
- Assuming all plaster types cover the same
- Overlooking internal reveals and returns
- Ignoring the condition of the existing surface
It is better to have one spare bag than to run short mid-job. Running out part way through a ceiling means a joint line you will see forever. And if you would rather not do the sums or the skimming, our plasterers in Kent will price the whole job for you, materials included.
Frequently asked questions
How many bags of plaster do you need per board?
Budget about a third of a 25kg bag of multi-finish per plasterboard, so one bag skims 2.5 to 3 boards. A standard 2400mm x 1200mm board is 2.88m² and a bag covers roughly 10m² at 2mm. Two coats and normal wastage bring the practical figure down from the theoretical 3.5 boards per bag.
How much does a bag of plaster cover?
A 25kg bag of multi-finish covers roughly 10m² at 2mm thick. Board finish is similar at around 11m². Undercoat plasters cover far less because they go on thicker: bonding coat manages 3 to 3.5m² at 8mm, hardwall 2.5 to 3m² at 11mm and one coat plaster about 2.5m² at 13mm.
What size bags does plaster come in?
The standard trade size is 25kg, used for multi-finish, board finish, bonding, hardwall and one coat plasters. DIY stores also sell 12.5kg bags of finishing plaster, small 2kg to 5kg patching packs and ready-mixed tubs for minor repairs. Buy 25kg bags for anything bigger than a patch, they cost far less per kilo.
How many bags of multi finish do I need per room?
Four bags of multi-finish will skim the walls of a typical 4m x 3m room, and five will do walls and ceiling. The walls come to about 30.7m² after subtracting a door and window, and each 25kg bag covers roughly 10m² at 2mm. Add 10 per cent for wastage and round up.
How long does a bag of plaster last?
Bagged gypsum plaster keeps for three to four months from the date of manufacture, which is printed on the bag. After that it sets too quickly and may not harden properly. Store bags somewhere dry, off the floor and sealed. Once mixed with water, multi-finish gives you about 40 minutes of working time.
How many bags of plaster do I need for one wall?
One 25kg bag of multi-finish usually covers a single wall. A 4m x 2.4m wall is 9.6m² and a bag covers roughly 10m² at 2mm, so one bag does it with little to spare. For walls larger than 10m², or if you are new to plastering, buy two bags.