How we calculate: decorating
Every formula and default figure our decorating 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.
Paint calculator
- Net area (m²) = walls (perimeter × height) and ceilings, less doors and windows
- Litres = net area × coats ÷ coverage (m²/L per coat) × (1 + waste)
- Mist coat: paint before thinning = thinned mix ÷ (1 + water parts); water = thinned mix − paint
- Tins: the mix of tin sizes with the least left over (ties go to fewer tins), and the fewest-tins alternative
A maker's coverage figure is a best case on a smooth, sealed surface. Fresh plaster, rough walls and strong colour changes use more.
Masonry paint coverage varies a lot with the surface: smooth render takes far less than pebbledash or brick. Use the figure on the tin for your surface.
Mist coat: a thinned first coat that seals new plaster. Thin it as the maker says, then follow with full coats (switch back to Walls and ceilings). Our guide to painting new plaster explains why.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Coverage per coat: Walls and ceilings | 12 m2/L Sources range 12–17 m2/L | Typical value Not yet checked against the source | BuildBatch's typical emulsion coverage per coat (checked 19 October 2026) Makers state up to 14 (Dulux Matt, via masseys-diy.co.uk) and up to 16–17 (Dulux Trade, via buildbatch.co.uk). A maker's figure is a best case on a sealed surface. |
| Coats: Walls and ceilings | 2 coats | Our assumption Not yet checked against the source | Our choice: change it to suit your job Two coats is our starting point; new plaster or a colour change may need more. |
| Coverage per coat: Masonry paint (outside walls) | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 §2 (paint: masonry paint mode). Coverage per coat for masonry paint; depends heavily on the surface (smooth render vs pebbledash). Researcher to source from makers' data sheets; until then the user enters the figure from the tin. |
| Coverage per coat: Mist coat (new plaster) | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product brief v2 §2 (paint: mist coat mode). Coverage of a thinned mist coat on new plaster. Researcher to source from makers' mist-coat instructions; until then the user enters the figure from the tin. |
| Waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Allowance for rollers, trays and touch-ups. |
| Tin sizes you can buy | 1, 2.5, 5, 10 L | Common UK size Not yet checked against the source | From your product Common UK tin sizes; not every paint comes in every size. |
Wallpaper calculator
- Drop = wall height + trim, rounded up to a whole number of pattern repeats
- Drops per roll = roll length ÷ drop, rounded down
- Drops = (wall width − full-height openings) ÷ roll width, rounded up
- Rolls = drops ÷ drops per roll, rounded up, plus any spare rolls
Buy rolls from one batch: colours can vary between batches.
Half-drop patterns: follow the maker's guidance.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Roll length | 10.05 m | Common UK size Not yet checked against the source | Roll size quoted by trade calculators (checked 19 October 2026) Standard UK roll. |
| Roll width | 0.53 m | Common UK size Not yet checked against the source | Roll size quoted by trade calculators (checked 19 October 2026) Sources quote 0.52 or 0.53 m wide; check the label. |
| Trim allowance per drop | 0.1 m per drop | Our assumption Not yet checked against the source | Our choice: change it to suit your job Trimming allowance at top and bottom. |
| Spare rolls | 0 rolls | Our assumption Not yet checked against the source | Our choice: change it to suit your job Add a spare if the pattern is hard to match later. |
Fence and decking stain calculator
Open the fence and decking stain calculator
- Panels: area = panels × width × height × sides; deck: area = deck m²
- Litres = area × coats ÷ coverage per coat (m²/L, from the tin) × (1 + waste)
- Tins: the combination of tin sizes with the least left over, and the one with the fewest tins
Rough-sawn timber soaks up far more than planed timber, so use the tin's figure for your surface. The first coat on bare wood often covers less than later coats.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Panel width | 1.83 m | Common UK size Not yet checked against the source | From your product 6 ft panel. |
| Coverage per coat: Rough sawn | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product From the tin (brief v2 D1: null; the user enters it). |
| Coverage per coat: Smooth (planed) | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product From the tin (brief v2 D1: null; the user enters it). |
| Coats | 2 coats | Our assumption Not yet checked against the source | Our choice: change it to suit your job Two coats is our starting point; new plaster or a colour change may need more. |
| Waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Allowance for rollers, trays and touch-ups. |
| Tin sizes you can buy | 1, 2.5, 5, 10 L | Common UK size Not yet checked against the source | From your product Common UK tin sizes; not every paint comes in every size. |
Sealant and caulk calculator
Open the sealant and caulk calculator
- Sealant (ml) = length (m) × bead width (mm) × bead depth (mm) × (1 + waste), since 1 m × 1 mm² = 1 ml
- Cartridges = ml ÷ cartridge size, rounded up
A triangular bead in a corner uses about half the volume of a square one, so this errs on the generous side.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Cartridge size | 310 ml | Common UK size Not yet checked against the source | From your product brief v2 D2 (310 ml default). |