Calculating paint for a room is one of those jobs people assume is simple until they're standing in a decorating shop unable to remember whether 2.5 litres covers one coat or two. I've ended up short before — one wall left to finish and the pot empty — and had to drive back to match a colour that was, of course, slightly different from the second batch because the formula had shifted. Here's how to calculate what you need before you drive anywhere.
What You're Calculating
You need the net paintable area: the total wall and ceiling surface area minus the windows and doors. Paint is sold by coverage — typically 10–12 m² per litre for standard emulsion on a prepared surface. On bare plaster or heavily absorbent surfaces, expect 8–10 m². Multiply your net area by the number of coats and divide by the coverage rate to get your litre requirement.
Step-by-Step: Walls
Measure each wall individually: width × ceiling height. Add them all together for the gross wall area. Then subtract the openings. A standard UK internal door is roughly 0.76 m × 1.98 m = 1.5 m². A standard single window might be 0.9 m × 1.0 m = 0.9 m². A standard double window around 1.5 m × 1.0 m = 1.5 m². Subtract the total opening area from the gross wall area to get your net wall area.
Our area calculator handles rectangle, circle, and triangle calculations — useful if your room has arched windows, bay windows, or any non-rectangular feature you need to account for.
Example: A Standard Bedroom
Room: 3.8 m × 3.2 m, 2.4 m ceilings, one door (1.5 m²), one window (0.9 m²).
- Two 3.8 m walls: 2 × (3.8 × 2.4) = 18.24 m²
- Two 3.2 m walls: 2 × (3.2 × 2.4) = 15.36 m²
- Gross wall area: 33.6 m²
- Minus door and window: 33.6 − 1.5 − 0.9 = 31.2 m²
- Ceiling: 3.8 × 3.2 = 12.2 m²
For two coats of wall paint at 11 m² per litre: 31.2 × 2 ÷ 11 = 5.67 litres. A 5-litre tin plus a 2.5-litre tin gives 7.5 litres — enough with margin. For one coat of ceiling paint at 11 m²/litre: 12.2 ÷ 11 = 1.1 litres. A 2.5-litre tin covers this with plenty to spare.
Coverage Rates Vary — Know What to Expect
The coverage figure on the tin is based on a smooth, already-painted surface with a roller. In practice, expect variation. Freshly plastered walls absorb more paint — they need a mist coat first (paint diluted 10–20% with water) before the full-strength emulsion. Heavily textured surfaces like pebbledash or thick artex cover less efficiently than the tin suggests. Dark colours going over light may achieve adequate coverage in two coats; light colours over dark often need three.
Matt emulsion hides imperfections better than silk or eggshell but marks more easily. Silk provides a washable surface good for kitchens and bathrooms. Kitchen and bathroom paints include fungicide and moisture resistance — worth the small extra cost in those environments.
Woodwork and Trim
Skirting boards, door frames, and window surrounds are typically painted in a different finish (usually satinwood or gloss). Calculate the linear metres of trim and multiply by the trim width (typically 0.1–0.15 m for skirting). A room with 16 linear metres of skirting board at 0.13 m wide has about 2 m² of skirting surface. This is usually covered by 750 ml or 1 litre of satinwood for two coats.
How Much Margin to Add
Always round up to the next available tin size. If you need 5.7 litres, buy a 5-litre and a 2.5-litre rather than two 5-litres — that's less waste while still giving you adequate margin. Buy one size up from your calculated requirement if the colour is custom-mixed: a shop can often mix more later if they have the formula on file, but batch variation is real and a slightly-off patch on a feature wall is difficult to fix without repainting the entire surface.
Tracking the Calculation
Write all your measurements down before going to the shop. A note with "walls: 31.2 m², 2 coats, ceiling: 12.2 m², 1 coat" is all you need to make a correct purchase. Do the net area calculation at home when you can measure carefully — not from memory in the shop aisle. Measuring correctly once saves at least one return trip.
Going Dark to Light — or Light to Dark
Coverage estimates change significantly when you're making a dramatic colour shift. A good-quality mid-sheen emulsion in a mid-tone colour typically covers in two coats over an existing painted surface. Going from a dark colour to a white or very pale shade is a different proposition — the dark pigment bleeds through. In this situation, a tinted primer or a mist coat matched to your top colour is almost always the better first move. It costs less per litre than the top coat, covers the dark base more effectively than an extra full coat, and means the final two coats of your chosen colour land on a neutral ground rather than fighting against the original.
Going from light to dark is usually straightforward in two coats, provided the previous surface is clean and not glossy. Where people go wrong is applying the second coat too quickly — most emulsions need at least two hours between coats, and in poorly ventilated rooms or damp conditions, longer. Applying a second coat before the first has fully dried seals moisture in and creates a surface that peels far sooner than it should.
Why Batch Numbers Matter More Than People Realise
Every tin of custom-mixed paint carries a batch number. Two tins mixed on different days, even to an identical formula, can show a perceptible colour difference under certain lighting conditions — especially with whites, off-whites, and muted mid-tones where the eye is most sensitive to subtle variation. The rule is simple: order everything you need for a room in a single visit. If you run short and return for a second batch, buy a new tin and test it against the first in natural light before applying it. If there's a visible difference, use the new batch only on a complete wall where the join will be a corner rather than mid-surface — a corner break in colour is almost invisible; a mid-wall join is not.