How to Calculate Paint Quantities for Any Room in London
A practical guide to calculating exactly how much paint you need for a London room — coverage rates, number of coats, wastage allowance, and how to avoid running short.
Why Getting the Quantity Right Matters
Running out of paint halfway through a job is more than inconvenient — it can be a serious problem. Batch numbers on paint tins differ subtly from production run to production run. If you return to the shop for a top-up tin, there is a real risk the colour will not match precisely, leaving a visible join on your wall. Ordering too much wastes money and leaves you with partially used tins to store or dispose of. Getting the quantity right from the outset is a professional discipline worth understanding.
Coverage Rates: What the Tin Actually Means
Every tin of paint carries a theoretical coverage figure, typically expressed in square metres per litre (m²/L). A standard trade emulsion might claim 12–14 m²/L; a specialist mineral paint might achieve 8 m²/L. These figures are measured under laboratory conditions on a perfectly smooth, sealed surface — not on the textured plasterwork of a typical London period property.
In practice, you should apply a real-world discount to these figures. On smooth, previously painted walls in good condition, assume 10–12 m²/L for emulsions and 8–10 m²/L for eggshell and satinwood. On fresh plaster, newly skimmed surfaces, or rough Victorian brickwork, coverage can drop to 6–8 m²/L because porous surfaces drink in the first coat. Heavily textured Artex or exposed stonework can consume even more.
Calculating Wall Area: The Method
The formula is straightforward. Measure the perimeter of the room (the total length of all four walls), multiply by the ceiling height, and you have your gross wall area in square metres. Then subtract any large openings — typically doors (approximately 2 m²) and windows (typically 1–2.5 m² depending on the window size, but in a London Victorian terrace with a large sash, allow up to 3 m²).
Example calculation for a typical London drawing room:
- Room dimensions: 5 m × 4 m, with a ceiling height of 3 m
- Perimeter: (5 + 4 + 5 + 4) = 18 m
- Gross wall area: 18 m × 3 m = 54 m²
- Subtract two doors (2 × 2 m² = 4 m²) and two sash windows (2 × 2.5 m² = 5 m²)
- Net wall area: 54 − 9 = 45 m²
At a real-world coverage of 12 m²/L for two coats, you need: (45 m² × 2 coats) ÷ 12 = 7.5 litres. Round up to 8 litres and buy a 5-litre and a 2.5-litre tin, giving you a small margin.
Ceiling Area
Ceiling area is simply length × width. For the room above: 5 m × 4 m = 20 m². Ceilings are almost always done in two coats. At 12 m²/L, you need (20 × 2) ÷ 12 = 3.3 litres — a 2.5-litre tin and a 1-litre tin, or a single 5-litre tin if you prefer the margin.
Woodwork: Doors, Skirtings, and Architraves
Woodwork is harder to calculate by area because of the irregular shapes involved. The rule of thumb used by trade decorators is to allow 1 litre of gloss or eggshell for every four standard doors, including both faces and the leading edges. Skirtings and architraves in an average room consume roughly 0.5–1 litre per room for a full two-coat application.
The Wastage Allowance
Even professional decorators allow for wastage. Paint is lost on brush handles, in rollers that are cut open at the end of the job, and in the unavoidable film left in the tin. A 10% wastage allowance on top of your calculated quantity is standard practice. Round up to the nearest available tin size — paint left over from a well-managed job can be stored for touch-ups, which is always useful.
First Coat on Fresh Plaster: A Special Case
New plaster is highly alkaline and extremely absorbent. The first coat on a freshly plastered wall should always be a mist coat — your chosen emulsion diluted 10–20% with water — to seal the surface before full-strength coats are applied. The mist coat will cover poorly and unevenly; this is expected and not a cause for alarm. Allow it to dry fully (typically 24 hours), then apply two full coats of your chosen colour.
Because the mist coat uses diluted paint at higher spread rates, your litre count for the actual colour coats will be closer to standard coverage figures once the surface is sealed.
One Final Rule
Always buy paint from the same batch number. Check the batch code printed on the tin base before purchasing multiple tins. If a shop cannot guarantee the same batch, it is worth ordering all required quantity at once from a single supplier. For bespoke or specialist paints — Farrow & Ball, Little Greene, Papers & Paints — request that the shop checks their stock before you place the order.
For a professional paint specification and quantity schedule tailored to your London property, contact us here or request a free quote.