How Much Paint Do I Need?

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:

Those are working figures for a real room, and they sit deliberately below what the tin claims. The manufacturer’s number is a best case on a sound, non-porous, uniformly coloured surface. For comparison, Dulux Trade quotes up to 18 m² per litre for Supermatt and up to 17 m² per litre for Vinyl Matt, and both figures carry the qualifier “on sealed surfaces”.¹² On woodwork the published figures are lower and closer to what you actually get: up to 16 m² per litre for Diamond Eggshell, up to 12 m² per litre for Diamond Satinwood, and up to 12 m² per litre for water-based Quick Dry Gloss.¹ Treat any figure above the manufacturer’s own as impossible rather than optimistic.

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: 5.5 litres means a 5-litre tin plus a 1-litre tin, which leaves a little 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 3.2–3.5 m² of surface. The common UK internal leaf is 1,981 × 762 mm, which is 1.5 m² a face.³ That is the old imperial 6 ft 6 in × 2 ft 6 in size; BS 4787-1 covers the metric door sizes instead.⁴
At 14 m²/litre for eggshell, two coats: 0.5 litres per door.

Skirting board: roughly 0.12 m² per linear metre (100 mm skirting painted front and top).
A 15 m perimeter room = about 1.8 m² of skirting.
At 14 m²/litre, two coats: about 0.3 litres, a 500 ml tin or sample pot typically suffices.

Architraves: approximately 0.08–0.1 m² per linear metre (face and edge).
One doorset (both sides, roughly 10 m total) = 0.8–1.0 m².

For a room like this, a 1-litre tin of eggshell or satin just about covers two topcoats on all the woodwork (roughly 0.5 + 0.3 + 0.1 litres, about 0.9 litres); buy a 2.5-litre tin if there is more than one door.

Worked Example: A Typical UK Living Room

Total paint to buy: 5.5 L walls (purchase a 5 L and a 1 L tin) + 2.3 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, the mist coat, is mostly absorbed, so you will use significantly more. Dilute it more heavily than most people expect: Dulux Trade tells you to seal new or bare surfaces with a first coat of Supermatt thinned with up to 1 part clean water to 3 parts paint.¹ 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 risks a colour variation between production batches, which can be visible on the wall. Dulux Trade’s own advice for Supermatt is to buy enough tinted paint for each job at one time and from the same source.⁵

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 often 20–40% lower, as the working figures in the table above show
• 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.