Painting & Walls
Paint Calculator
Enter your room dimensions in metres to get the litres of emulsion and number of tins to buy, with standard door and window deductions and an optional VAT-aware cost estimate.
Formula tested · Local units · No sign-up
Project inputs
Enter measurements
Use your preferred units. Results update automatically.
Show the calculation methodFormula, conversions, rounding, and assumptions+
Wall area = 2 × (room length + room width) × wall height. A 5 m × 4 m room with 2.4 m ceilings has 2 × 9 × 2.4 = 43.2 m² of wall. You can also enter a measured wall area directly.
Standard openings are deducted: 1.9 m² per door and 1.4 m² per window (editable typical values). The remaining paintable area is multiplied by the number of coats.
Litres = total area ÷ coverage per litre. Typical emulsion covers 10–12 m² per litre per coat — the calculator defaults to 11, but check your tin. The result is rounded up to whole tins, because paint is sold that way.
Real-world example
Worked example: 5 m × 4 m living room, 2.4 m ceilings, 2 coats
- Wall area: 2 × (5 + 4) × 2.4 = 43.2 m².
- Deduct openings: 1 door (1.9 m²) + 1 window (1.4 m²) = 3.3 m² → 39.9 m² paintable.
- Two coats: 39.9 × 2 = 79.8 m² to cover.
- Paint needed: 79.8 ÷ 11 m²/L = 7.25 litres.
- Round up to whole 2.5 L tins: 7.25 ÷ 2.5 = 2.9 → 3 tins.
Buy 3 × 2.5 L tins. At an example price of £30 per tin plus 20% VAT, that's £90.00 + £18.00 = £108.00.
Before you start
How to measure
- Measure the room's length and width in metres at skirting level, and the wall height from floor to ceiling (standard UK ceilings are around 2.4 m).
- Count doors and windows — the calculator deducts standard areas (1.9 m² per door, 1.4 m² per window). Measure bay windows and patio doors separately.
- Skirting boards, architraves and other woodwork take gloss or satinwood, not emulsion — calculate them separately.
Local guidance
Notes for United Kingdom
- UK wall and ceiling paint is sold as emulsion in litres; common tin sizes are 1 L, 2.5 L, 5 L and 10 L. The 2.5 L tin is the standard retail size.
- Coverage on UK tins is quoted in m² per litre — typically 10–12 for standard emulsion on previously painted plaster; bare or fresh plaster needs a mist coat and absorbs more.
- VAT at the standard 20% rate applies to retail paint and is normally included in shelf prices; trade-counter prices are often quoted ex-VAT.
Quick reference
Typical coverage by surface (planning values — check your tin)
| Surface | Typical coverage per coat |
|---|---|
| Previously painted plaster | 10–12 m²/L |
| Fresh plaster (after mist coat) | 9–11 m²/L |
| Lining paper / woodchip | 8–10 m²/L |
| Bare masonry | 6–8 m²/L |
Editable planning values only — the coverage figure on your tin governs.
Good to know
Common mistakes to avoid
- Painting fresh plaster without a watered-down mist coat first — full-strength emulsion won't bond and coverage figures go out the window.
- Using emulsion coverage figures for skirting and doors, which take gloss or satinwood with different coverage.
- Assuming one coat will cover a strong colour change — budget for two, or more over deep reds and blues.
- Comparing a trade ex-VAT price against a retail VAT-inclusive one.
Need help?
Frequently asked questions
How much emulsion do I need for a 3 m × 3 m box room?
Wall area is 2 × 6 × 2.4 = 28.8 m². Deduct one door (1.9 m²) → 26.9 m²; two coats is 53.8 m²; at 11 m²/L that's 4.9 litres, so two 2.5 L tins covers it exactly (5 L).
How many square metres does a 5 L tin of emulsion cover?
At a typical 10–12 m² per litre, a 5 L tin covers roughly 50–60 m² in one coat — or 25–30 m² of wall at two coats. Check your tin: coverage varies by brand and finish.
Do I paint skirting boards with the same paint?
No — skirting, architrave and doors take a wood paint (gloss, satinwood or eggshell), which covers differently and is sold in smaller tins. This calculator is for the walls and ceiling emulsion.
Keep planning
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About this calculator
- Written by:
- BuildMeasure Editorial Team
- Technically reviewed by:
- Pending independent technical reviewer (formula unit-tested; see methodology)
- Last reviewed:
- 2026-07-16
- Formula version:
- 1.0.0
- Region reviewed for:
- United Kingdom
- Spotted an error?
- Report a correction
Methodology
- Wall area comes either from room dimensions (perimeter × wall height, i.e. 2 × (length + width) × height) or from a directly entered area. All arithmetic runs internally in SI units (m² and litres) to avoid unit drift; regional units are converted on the way in and out.
- Standard opening deductions (door and window areas, clearly labelled next to the inputs and editable via the counts) are subtracted from the wall area, floored at zero. The paintable area is multiplied by the number of coats to get the total area to cover.
- Paint volume = total area ÷ the coverage rate you enter. The number of containers is the paint volume divided by your container size, rounded UP to the next whole container, because paint is sold in whole cans.
- The cost estimate multiplies the container count by the price you enter, then applies the tax rate you enter. No prices are built in.
- The formula is covered by automated unit tests, including hand-calculated worked examples, and is versioned (see formula version on this page).
Sources & standards
- Unit definitions: Metric units used throughout; 1 L covers coverage-rate m² per coat as stated on the tin.
- Coverage defaults: 10–12 m² per litre per coat is the typical emulsion label range; the figure on your tin governs.
This tool provides a material estimate for planning purposes only. It is not a quotation, and it does not assess surface condition, primer requirements or colour-change coverage. Confirm quantities and the coverage rate on your specific product with your paint supplier before buying.