Running out of paint mid-wall — or ending up with four leftover tins — is easily avoided with a straightforward calculation. The key figures are your room’s surface area and the coverage rate printed on the paint tin. Once you have both, the maths takes about two minutes.
This guide walks through the method room by room, with worked examples for walls, ceilings, and woodwork, plus adjustments for porous new plaster and dark colour changes.
Coverage Rates by Paint Type
Coverage rates vary by product quality and surface porosity. Here are realistic figures for UK residential use:
| Paint Type | Coverage Rate (per litre) | Notes |
|---|---|---|
| Contract/flat matt emulsion | 10–12 m² | Low cost, high pigment load |
| Vinyl matt emulsion (standard) | 11–14 m² | Most common wall paint |
| Silk / soft sheen emulsion | 12–15 m² | Slightly better coverage |
| Eggshell (water-based) | 12–16 m² | Woodwork and walls |
| Satin wood (water-based) | 12–15 m² | Doors, skirting, architraves |
| Gloss (water-based) | 13–16 m² | Woodwork, exterior |
| Masonry paint | 5–8 m² | Textured render absorbs more |
| Primer / mist coat | 8–12 m² | New plaster needs more coats |
Premium paints at the higher end of a price range tend to offer better coverage per litre due to higher pigment concentration. Always check the coverage rate on your specific tin — manufacturers publish this on the label and on their website.
Step-by-Step: Calculating Paint for Walls
1. Measure wall height and perimeter
Measure floor-to-ceiling height and the perimeter (total distance around the room). Multiply them:
Wall area = perimeter × height
For a room 4 m × 3.5 m with 2.4 m ceilings:
Perimeter = (4 + 3.5 + 4 + 3.5) = 15 m
Wall area = 15 × 2.4 = 36 m²
2. Subtract doors and windows
Each standard interior door (roughly 2.0 m × 0.8 m) = 1.6 m²
A typical window (1.2 m × 1.0 m) = 1.2 m²
If your room has one door and one window:
36 − 1.6 − 1.2 = 33.2 m² paintable wall area
3. Divide by coverage rate and multiply by coats
For a vinyl matt emulsion at 12 m²/litre, two coats:
(33.2 ÷ 12) × 2 = 5.5 litres
Round up to the nearest tin size. A 5-litre tin is the sensible purchase — you will have a small amount left over for touching up later.
Ceiling Calculation
Ceiling area is simply length × width:
For a 4 m × 3.5 m room: 4 × 3.5 = 14 m²
Using a ceiling white matt at 12 m²/litre, two coats:
(14 ÷ 12) × 2 = 2.3 litres
A 2.5-litre tin covers this comfortably.
Woodwork: Doors, Skirting & Architraves
Woodwork areas are smaller but require primer plus two topcoats in most cases, so the total paint usage adds up.
A standard interior door (both sides, including edges): approximately 4–4.5 m² of surface.
At 14 m²/litre for eggshell, two coats: 0.6 litres per door.
Skirting board: roughly 0.2 m² per linear metre (100 mm skirting painted front and top).
A 15 m perimeter room = 3 m² of skirting.
At 14 m²/litre, two coats: 0.4 litres — a 500 ml tin or sample pot typically suffices.
Architraves: approximately 0.1–0.15 m² per linear metre.
One doorset (two sides, roughly 6 m total) = 0.7–0.9 m².
For most rooms, a 1-litre tin of eggshell or satin covers all woodwork with some left over.
Worked Example: A Typical UK Living Room
| Element | Dimensions | Area | Coats | Litres Needed |
|---|---|---|---|---|
| Walls (less door + window) | 15 m perimeter × 2.4 m | 33.2 m² | 2 | 5.5 L |
| Ceiling | 4 × 3.5 m | 14 m² | 2 | 2.3 L |
| 1 door (both sides) | — | ~4.5 m² | 2 | 0.6 L |
| Skirting | 15 m run | 3 m² | 2 | 0.4 L |
Total paint to buy: 5.5 L walls (purchase a 5 L tin) + 2.5 L ceiling (purchase a 2.5 L tin) + 1 L woodwork tin.
When to Add Extra Paint
New plaster is porous and soaks up paint. The first coat (mist coat — emulsion diluted 10% with water) is mostly absorbed, so you will use significantly more. Add 25–35% extra for new plaster, or plan for three coats rather than two.
Covering a dark colour (dark grey, strong red, deep navy) with a pale colour is notoriously inefficient. Two coats of a standard white matt often fail to hide a deep colour. Options:
- Use a specialist stain-blocking primer first — one coat can save you a coat of topcoat
- Choose a paint with a higher opacity rating (check the spreading rate on the tin; a lower m²/litre often means denser coverage)
- Plan for three coats and budget accordingly
Rough or textured surfaces (pebbledash, artex, rough render) absorb significantly more paint than smooth plaster. For heavily textured masonry, halve the standard coverage rate to get a realistic estimate.
Paint Tin Sizes and Buying Strategy
Standard UK paint tin sizes are: 500 ml, 750 ml, 1 L, 2.5 L, 5 L, and occasionally 10 L for trade products. Buying the right size avoids waste but also keeps you from running short.
Rule of thumb: always round up to the next tin size, not down. Running out mid-wall and buying a second small tin from a different batch risks a colour variation (a slightly different production batch, called a dye lot), which can be visible on the wall.
If you are ordering a mixed colour, buy all the paint you need in one order. Mixed colours are difficult to match exactly if you need more later.
Leftover paint can be stored with a tight-fitting lid in a frost-free location for two to three years. Note the room name, colour reference, and purchase date on the lid with a permanent marker.
Common Calculation Mistakes
- Forgetting to account for two coats — the single most common underestimate
- Ignoring ceilings — they add up quickly in large rooms
- Not subtracting doors and windows — makes little difference in a window-heavy room but can lead to over-buying
- Using the tin’s maximum coverage figure — manufacturers quote best-case figures on smooth surfaces; real-world coverage is typically 10–15% lower
- Buying a single 10-litre tin to save money — this only makes sense if you are painting a very large area in one go; leftover paint in a large tin deteriorates faster once opened
With a tape measure, a calculator, and the coverage rate from your chosen tin, the whole calculation takes five minutes and can save you two extra trips to the merchant.
