How Much Paint Do I Need for My Walls?

How the calculation works

How to calculate paint needed for a room

Working out how much paint you need for a room is simpler than most people think, but three variables are often underestimated: the actual wall area, the coverage stated by the manufacturer, and the number of coats. This calculator handles all three and adds a 5% margin for touch-ups, a practice recommended by professional decorators but often overlooked by generic online calculators.

Calculating wall area

The area of a rectangular wall is length multiplied by height. A typical UK room (2.5 m ceiling height, four walls of 4 m) has a total wall area of 4 × 4 × 2.5 = 40 m². This calculator accepts up to 20 walls, allowing you to handle rooms of irregular shape wall by wall.

Useful to know: you do not need to deduct the area of doors and windows in small rooms. The difference is usually offset by natural paint wastage (excess in the tray, paint left on the roller, touch-ups). In larger spaces or with unusually large doors, you can mentally subtract these areas before entering your measurement.

Paint coverage: where most people go wrong

Coverage is the relationship between paint used and area covered, expressed in square metres per litre (m²/litre). It is always stated on the tin, usually in small print on the side label. Typical coverage figures for interior paints in the UK are:

• Standard interior emulsion (matt), most common brands: 10 to 12 m²/litre
• Satin or silk interior emulsion: 11 to 13 m²/litre
• Premium or specialist ranges: 12 to 14 m²/litre
• Primers and sealers: 8 to 10 m²/litre

Critical point: the coverage on the tin assumes one coat on a properly prepared and primed surface. If your wall is already painted in a dark colour, you will need more coats, which this calculator allows for in the 'number of coats' field.

How many coats do you really need?

The standard practice in interior decoration is:

• One coat: only for touching up the same colour
• Two coats: the standard. Covers nearly all situations of light colour over light
• Three coats: when changing from dark to light colours, or using very dilute paints (some eco-friendly ranges fall into this category)
• Primer plus two coats: new plasterboard, walls with repaired cracks, or a radical colour change

There is no saving in skipping a coat to use less paint; the result will have patchy opacity, especially noticeable in side light. Professional decorators recommend a minimum of two coats even in straightforward situations.

Why include a 5% margin?

This calculator automatically adds 5%. The reason is practical: paint does not all end up on the wall. Some stays in the tray, some is lost in splashes, and some will be needed for later touch-ups (corners, accidental marks, difficult-to-cover patches). Experienced decorators in the UK work with margins between 5% and 10%. The more conservative margin protects you from an unexpected return to the shop halfway through, an irritating situation when you discover the paint batch is no longer in stock and a new mix has a slight colour variation.

Choosing paint: quality tiers and how to select

Paint choice depends on the end use and budget. In the UK market, interior paints typically divide into three tiers: economy (budget ranges, own-brand in large retailers), mid-range (the bulk of the market, reliable brands with good coverage and colour selection), and premium (specialist formulations, higher coverage, extended durability). All three categories are available from large DIY retailers, specialist decorating suppliers and online merchants.

Checking the coverage figure on the specific tin you plan to use is always wise. Even within a manufacturer's range, coverage can vary; a premium silk may deliver better coverage than a standard matt from the same maker. For small projects, prefabricated sizes of 1–2.5 litres are often better value than opening a 5-litre tin you may not finish. For larger jobs, 5-litre or 10-litre tins are more economical per unit volume.

Common mistakes to avoid

• Buying paint in oversized tins for small rooms. Once opened, paint degrades. For one bedroom, prefer a 5-litre tin to a 15-litre drum.
• Mixing different batches of the same colour. Even 'pure white' varies between production runs. If you need to buy more mid-job, take a sample of what you have.
• Trusting the coverage figure without checking the label. The 12 m²/litre figure is a conservative estimate. Economy paints often fall well short of their claimed coverage in real use.
• Skipping primer on new walls. Without primer, paint absorbs unevenly and patches appear.
• Overestimating how much coverage a thin first coat will deliver. Most people underbuy on the first attempt and regret it.

If you are unsure about a specific situation, ask the staff where you buy the paint. Most large retailers have staff trained by manufacturers and can offer practical advice. In Scotland, Building Standards require certain fire-safety paints in communal areas; always check whether your project is affected.

The Building Research Establishment (BRE) is the UK's national body for building research and standards, publishing guidance on materials, including paint and surface preparation best practice.