Plaster calculator: bags per m² and per board
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Walls and ceilings | Room: room walls 4 m × 3 m × 2.4 m, less 0.8 × 2 m |
| Plaster | Finish (multi-finish, skim) |
| Coverage from the bag: kg per m² per mm of thickness | 1.15 kg/m²/mm |
| Thickness per coat | 2 mm |
| Coats | 1 |
| Waste allowance | 10% |
| Bag size | 25 kg |
| Board size for the per-board answer | 2.88 m² |
Order: 4 × 25 kg bags of plaster (81.0 kg)
| Result | Value |
|---|---|
| Bags of plaster | 4 |
| Plaster (with waste) | 81 kg |
| Plaster per board | 7.3 kg |
| Bags per board | 0.29 bags |
| Boards per bag | 3.4 boards |
| Area (after openings) | 32 m² |
| Beads (opening edges) | 5.6 m |
Show the working
- Area: 33.6 m² − 1.6 m² of openings = 32 m²
- Beads: opening edges add up to 5.6 m
- Plaster: 32 m² × 2 mm × 1 coat × 1.15 kg/m²/mm × 1.1 (10% waste) = 81 kg
- Per board: 2.88 m² × 2.53 kg/m² = 7.29 kg
- Bags: 81 kg ÷ 25 kg, rounded up = 4
- Per board: 7.29 kg ÷ 25 kg = 0.29 bags (one bag covers 3.4 boards)
How it works
- Plaster (kg) = area (m²) × thickness per coat (mm) × coats × coverage rate (kg/m²/mm) × (1 + waste)
- Bags = kg ÷ bag size, rounded up
- Per board = the same sum for one board's area (2.88 m² for a 2400 × 1200 mm board)
- PVA or primer (L) = area ÷ coverage from the tin
- Beads (m) = the perimeter of each opening (2 × (width + height))
Coverage rates differ between makers and products: use the figure printed on your bag or the maker's data sheet.
Bead length counts every edge of each opening; leave out edges that don't need a bead (such as the bottom of a door).
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Coverage from the bag: kg per m² per mm of thickness: Finish (multi-finish, skim) | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 PL1: coverage from the maker's data sheet (British Gypsum / Knauf). Enter from the bag; Researcher fills. |
| Coverage from the bag: kg per m² per mm of thickness: Bonding | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 PL1: coverage from the maker's data sheet. Enter from the bag; Researcher fills. |
| Coverage from the bag: kg per m² per mm of thickness: Hardwall | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 PL1: coverage from the maker's data sheet. Enter from the bag; Researcher fills. |
| Coverage from the bag: kg per m² per mm of thickness: Browning | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 PL1: coverage from the maker's data sheet. Enter from the bag; Researcher fills. |
| Coverage from the bag: kg per m² per mm of thickness: One-coat | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 PL1: coverage from the maker's data sheet. Enter from the bag; Researcher fills. |
| Waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PL1 (test PL-T1 uses 10%). Researcher to confirm against plaster makers' guidance. |
| Bag size | 25 kg | Common UK size Not yet checked against the source | From your product brief v2 PL1 (test PL-T1 uses 25 kg bags). Check the bag. |
| Board size for the per-board answer | 2.88 m2 | Common UK size Not yet checked against the source | From your product brief v2 PL1: 'bags per board equivalent (2.88 m²)', a 2400 × 1200 mm board. |
| PVA or primer coverage | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 PL1: PVA or primer coverage from the tin. Researcher fills. |
FAQs
Guides
How many plasterboard sheets do I need?
Plasterboard sizes, walls vs ceilings, offcuts, fire and moisture-resistant boards, and when to hire a plasterer.