How Much Paint Do I Need?

Buying the wrong amount of paint is one of the most common DIY frustrations. Too little and you're back at the shop mid-project with a roller drying out; too much and you've spent money on tins that sit in the garage for years. The good news is that working out paint quantities is straightforward once you know a few numbers. This guide walks you through measuring your walls, choosing the right coverage rate, and buying the correct number of tins. If you'd rather skip the manual maths, our room paint calculator handles all of this in seconds.

How Wall Area Is Calculated

The basic formula for paintable wall area is simple: add up the lengths of all four walls (the perimeter), multiply by the ceiling height, then subtract the areas you won't be painting — doors and windows. That gives you the total area to cover in square metres.

For a rectangular room, the perimeter is twice the length plus twice the width. A room measuring 4 × 3.5 metres has a perimeter of 15 metres. Multiply that by a standard UK ceiling height of 2.4 metres and you get 36 m² of gross wall area before deductions.

Standard deductions to subtract from that total are:

• Internal door — 1.81 m² (0.76 × 2.04 m for a standard UK interior door)
• Standard window — 1.5 m² (1.2 × 1.25 m, a typical double-glazed casement)
• Patio or French door — roughly 3.4 m² (1.5 × 2.1 m, though these vary)

If your room has unusual features — alcoves, chimney breasts, or recessed windows — measure each face separately and add them together. The same wall-measuring technique used for tiling works well here: start at one corner, work your way around the room, and sketch the layout as you go. For L-shaped rooms, split the space into two rectangles and measure each one.

Coverage Rates by Paint Type

Every tin of paint states a coverage rate on the label, but those figures assume ideal conditions — a smooth, sealed surface applied with a medium-pile roller. Real-world coverage varies with the paint type, the surface texture, and how thickly you apply it. The table below gives typical ranges for common interior paints in the UK.

One-coat paints cover less area per litre because the formulation is thicker and more heavily pigmented. They save time on application but cost more per square metre of coverage. For most rooms, two coats of standard emulsion at 12 m²/L works out cheaper and gives a more even finish than a single coat of one-coat paint.

If you're painting over bare plaster or a patched wall, primer coverage drops to around 10–12 m²/L because the porous surface absorbs more paint. Factor this into your quantity calculation — primer and topcoat have different coverage rates, so calculate them separately.

How Many Coats Do You Need?

The number of coats affects your total paint requirement directly. Two coats means you need roughly twice the paint that one coat would require. Here are the standard scenarios and what each demands.

New plaster — three applications in total. Start with a mist coat: standard matt emulsion diluted roughly 70/30 with water. This seals the plaster and gives the topcoat something to grip. After the mist coat dries (at least 24 hours), apply two full coats of your chosen emulsion. The mist coat uses about 30% less paint per square metre than a normal coat because of the dilution, but you still need to account for it.

Previously painted walls (same or similar colour) — two coats is the standard. The first coat may look patchy as it dries, which is normal. The second coat evens out the colour and provides the finished depth. Cutting corners with one coat almost always shows, especially in rooms with natural light from multiple angles.

Colour change (dark to light) — plan for three coats minimum. Going from a deep navy or forest green to a pale grey or white is a multi-coat job. A tinted primer matched to your topcoat colour helps enormously here and can reduce the total from four coats to three. Without primer, you might need four or even five coats to fully hide a dark base colour.

Colour change (light to dark) — two coats usually suffices, though you should ask the paint shop about a tinted undercoat. Deep colours like charcoal or burgundy benefit from an undercoat tinted close to the final colour, which prevents the lighter base from showing through and reduces the risk of a patchy finish.

Ceiling, Woodwork, and Trim

Walls are only part of the job. Most room redecorations also involve the ceiling and some amount of woodwork, and each needs its own paint quantity calculation.

Ceilings: the paintable area of your ceiling equals your floor area. A 4 × 3.5 m room has a 14 m² ceiling. Ceiling paint is typically a matt emulsion (often a specific "ceiling white" formulation that is thicker to reduce dripping), and you should budget for two coats. At 12 m²/L, a 14 m² ceiling needs about 2.4 litres for two coats — so a single 2.5 L tin covers it with a small amount to spare.

Skirting boards: measure the total length of skirting in the room (roughly the perimeter minus doorways) and multiply by the skirting height (typically 0.1–0.15 m). A room with 13 metres of skirting at 0.12 m high has about 1.56 m² of skirting to paint. That's a small area, but skirting usually takes eggshell or gloss paint, which you'll buy separately from your wall emulsion. If you're also refreshing skirting board quantities for a new installation, calculate the paint at the same time.

Door frames and architraves: a standard door frame (both sides of the architrave plus the frame edge) adds roughly 1.5–2 m² per doorway. Doors themselves are about 1.6 m² per face. Use the same eggshell or gloss as your skirting for a consistent trim finish.

A quick method for all woodwork in a typical bedroom: budget 1 litre of trim paint. That covers two coats on skirting, one door frame, and any picture rails or window sills. For a living room with two doorways and a bay window, budget 1.5–2 litres.

Buying the Right Amount

Paint in the UK comes in standard tin sizes: 1 L, 2.5 L, and 5 L. Some brands also sell 0.75 L tester pots and 10 L trade buckets. The price per litre drops significantly as you go up in size — a 5 L tin typically costs 30–40% less per litre than two 2.5 L tins of the same paint.

When your calculation lands between tin sizes, round up to the next available size. Running out mid-wall is far worse than having half a litre left over. Here are some practical rules for rounding.

• Need 4.8 litres? Buy a 5 L tin — not two 2.5 L tins.
• Need 6.2 litres? Buy a 5 L plus a 2.5 L. You'll have 1.3 litres spare for touch-ups.
• Need 2.1 litres? A single 2.5 L tin does the job.

Keep leftover paint for touch-ups. Store tins upside down (this creates an airtight seal with the paint itself) in a frost-free location. Well-sealed paint lasts 2–5 years in storage. Label the tin with the room and date so you can find it later.

If you're considering wallpaper as an alternative for a feature wall, remember that reduces your paint area. Subtract the feature wall dimensions from your total before calculating paint quantities.

A Worked Example

Let's walk through a complete paint calculation for a standard bedroom. The room is 4 × 3.5 m with a 2.4 m ceiling height. It has 1 internal door and 2 standard windows. The walls are previously painted magnolia and will be repainted in a light grey matt emulsion. The ceiling gets two coats of white matt. Skirting boards need a fresh coat of eggshell.

Step 1 — Wall perimeter: (4 + 3.5) × 2 = 15 m

Step 2 — Gross wall area: 15 × 2.4 = 36 m²

Step 3 — Deductions: 1 door at 1.81 m² + 2 windows at 1.5 m² each = 1.81 + 3.0 = 4.81 m²

Step 4 — Net wall area: 36 − 4.81 = 31.19 m²

Step 5 — Wall paint (2 coats): 31.19 × 2 = 62.38 m² total coverage needed. At 12 m²/L, that's 62.38 ÷ 12 = 5.2 litres. Buy a 5 L tin plus a 1 L tin, or two 2.5 L tins plus a 1 L tin. A 5 L + 1 L combination gives 6 litres total with 0.8 litres spare — enough for later touch-ups.

Step 6 — Ceiling paint (2 coats): 4 × 3.5 = 14 m². Two coats = 28 m² coverage. At 12 m²/L, that's 28 ÷ 12 = 2.33 litres. A single 2.5 L tin covers the ceiling.

Step 7 — Skirting (eggshell, 2 coats): Perimeter is 15 m, minus 0.76 m for the doorway = 14.24 m. At 0.12 m skirting height, the area is 14.24 × 0.12 = 1.71 m². Two coats = 3.42 m². At 13 m²/L (eggshell rate), that's 0.26 litres. A 1 L tin of eggshell is more than enough and leaves paint for the door frame too.

Shopping list summary:

• Light grey matt emulsion — 1 × 5 L tin + 1 × 1 L tin (6 L total)
• White matt ceiling paint — 1 × 2.5 L tin
• White eggshell for trim — 1 × 1 L tin

Total spend for this bedroom is typically in the range of £55–£90 depending on the paint brand. Premium brands like Farrow & Ball or Little Greene will push that higher; trade-quality own-brand emulsions from Dulux Trade or Crown Trade sit at the lower end.

While you're measuring up for painting, it's worth thinking about related projects. Bathroom walls that aren't being tiled need moisture-resistant paint — a kitchen and bathroom emulsion rather than standard matt. And if the bedroom overlooks a garden, you might want to factor in painting the fence or shed before laying new turf or artificial grass, since paint splashes on fresh landscaping are best avoided.

Getting your paint quantities right the first time saves money, avoids colour-batch mismatches from buying extra tins weeks later, and means you can finish the job in one go. Measure carefully, round up to the next tin size, and keep the leftovers for inevitable touch-ups down the line. For a faster route to the same answer, plug your room dimensions into the paint quantity calculator and get a materials list in seconds.