How we calculate: plastering
Every formula and default figure our plastering calculators use. Each figure can be changed in its calculator to match your product. See how we calculate for the method behind all of them.
Coving calculator
- Perimeter = the sum of the wall lengths
- By waste: lengths = perimeter × (1 + waste) ÷ stock length, rounded up
- By cut list: each wall is cut from full lengths plus one shorter piece (plus the mitre allowance), packed longest first (first-fit decreasing)
- Lengths to buy = the larger of the two
- Mitres: one per corner, each made from two cuts; a square corner is cut at 45°, any other corner at half its angle
- Adhesive tubes = perimeter ÷ metres per tube, rounded up
Coving is cut in a mitre box held upside down, as if the box were the ceiling and wall: follow the coving maker's template.
Join long walls away from the room's focal point, and stagger joins so they're less visible.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PL4 (test CV1 uses 10%). |
| Mitre allowance per piece | 0.1 m | Our assumption Not yet checked against the source | Our choice: change it to suit your job Allowance per piece for squaring ends and mitres (brief v2 FL2). |
Plaster calculator
- 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).
| 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. |
Plasterboard calculator
Open the plasterboard calculator
- Boards = net area × (1 + waste) ÷ board area, rounded up
- Screws = boards × screws per board (maker's fixing guide)
- Tape (m) = boarded area (boards × board area) × tape per m²; rolls = tape ÷ roll length, rounded up
- Joint filler (kg) = boarded area × filler per m²; bags = filler ÷ bag size, rounded up
- Bead (m) = bead run × (1 + waste); lengths = bead ÷ length of one bead, rounded up
Screws, tape, filler and bead come from the maker's rates for the system you use, so enter them from the product or its estimating guide. We haven't sourced typical figures yet.
Fire-resistant and moisture-resistant boards are specified for some rooms: check the maker's guidance and building regulations.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Cuts and offcuts. |
| Screws per board | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product Enter from the board maker's fixing guide. |
| Jointing tape per m² of board | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 §2 (plasterboard: jointing tape from maker rates, null until sourced). Metres of jointing tape per m² of board. Researcher to source from a board or jointing system maker's estimating guide. |
| Length of one roll of tape | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product brief v2 §2 (plasterboard). Length of one roll of jointing tape. Researcher to source from merchant listings; until then the user enters it from the roll. |
| Joint filler per m² of board | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 §2 (plasterboard: joint filler from maker rates, null until sourced). Kg of joint filler per m² of board. Researcher to source from a jointing compound maker's coverage figure. |
| Joint filler bag size | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product brief v2 §2 (plasterboard). Bag size of joint filler. Researcher to source from merchant listings; until then the user enters it from the bag. |
| Length of one bead | Enter from the bag, pack or tin | Common UK size Not yet checked against the source | From your product brief v2 §2 (plasterboard: bead). Length of one angle or stop bead. Researcher to source from merchant listings; until then the user enters it from the product. |
Render calculator
- Per coat: wet volume (m³) = area × coat thickness × (1 + waste); dry volume = wet × dry-volume factor
- Per coat at 1 : r (cement : sand): cement (kg) = dry ÷ (1 + r) × cement density; sand (t) = dry × r ÷ (1 + r) × sand density
- Totals add the coats; cement bags = cement ÷ bag size, sand bulk bags = sand ÷ bulk bag size, both rounded up
- Monocouche or basecoat (kg) = area × thickness (mm) × coverage (kg/m²/mm) × (1 + waste); thin-coat finish (kg) = area × kg/m² × (1 + waste)
- Beads (m) = the perimeter of each opening plus any corner, stop and bellcast bead you add
Coat thicknesses and mixes vary with the background and exposure: follow the render maker's specification or BS EN 13914-1.
Mesh is shown as wall area only: add the overlap the mesh maker asks for.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PL2 (test RN1 uses 10%). Researcher to confirm against render makers' guidance. |
| Dry-volume factor | 1.3 × | Our assumption Not yet checked against the source | Our choice: change it to suit your job Dry materials shrink when mixed with water. Researcher to check against cement makers' guidance. |
| Cement density | 1440 kg/m3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Typical planning figure with no source yet. Researcher to source it and reclassify as typical. |
| Sand density | 1.5 t/m3 Sources range 1.4–1.6 t/m3 | Typical value Not yet checked against the source | aggregatecalculator.co.uk aggregate density table (checked 19 October 2026) |
| Cement bag size | 25 kg | Common UK size Not yet checked against the source | From your product Common UK cement bag. |
| Sand bulk bag size | 0.85 t | Common UK size Not yet checked against the source | builder.tools bulk bag figure (checked 19 October 2026) Varies by merchant; check yours. |
Dot and dab adhesive calculator
Open the dot and dab adhesive calculator
- From a wall area: boards = area after openings × (1 + waste) ÷ board area, rounded up
- Bags = boards ÷ boards per bag (the maker's figure for a flat or an uneven wall), rounded up
Thermal (insulated) boards need mechanical fixings as well as adhesive: follow the board maker's fixing guide for the number and type.
Uneven walls take more adhesive per board: use the maker's figure for your wall, or pack out first.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Board waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Cuts and offcuts. |