Paint Calculator

How Much Paint You Need for a Room

British paint tins do not all cover the same area, and the difference between brands is exactly why so many DIY projects end with a half-empty wall and a closed B&Q at 7pm. Coverage in the UK market ranges from roughly 10 m² per litre on a budget own-brand emulsion to a comfortable 14 m² per litre on Dulux Trade Diamond Matt, and getting the figure right at the planning stage is the single biggest factor in not overspending or underbuying. This calculator uses real UK coverage rates, the standard 2.5L, 5L and 10L tin sizes that B&Q, Wickes and Homebase actually stock, and the metric measurements that match British plasterboard and ceiling heights.

The headline numbers worth remembering are these. Dulux Trade Diamond Matt covers around 14 m² per litre on a properly prepared interior wall. Crown Trade Vinyl Matt sits in the 12–14 m² per litre band, with the lower end being more honest on a slightly textured surface. Farrow & Ball Estate Emulsion is rated at roughly 14 m² per litre per coat, although the deep pigments in the 132-strong Farrow & Ball palette often need an extra coat to reach full opacity, which means the effective coverage is lower than the tin suggests. A standard mid-range trade emulsion such as Leyland Trade or Johnstone's Covaplus will give you a reliable 12–14 m² per litre. Drop down to the cheapest own-brand emulsion at B&Q, Wickes or Homebase and you should plan for around 10 m² per litre and frequently three coats rather than two — the apparent saving on the tin price is usually wiped out by the extra litres you have to buy.

This calculator defaults to a sensible 12 m² per litre because that is what a real-world DIY job actually achieves once you account for roller absorption, edges that need cutting in twice, and walls that are not as smooth as the showroom sample. Adjust the coverage figure up to 14 if you are buying premium trade emulsion onto sound, sealed plaster, and down to 10 if you are using bargain emulsion or painting over a textured or absorbent surface. Coverage figures quoted by manufacturers are also brand-specific data — if you are uncertain, the back of the tin is always the source of truth, and the British Coatings Federation (BCF), the UK trade body, publishes annual industry coverage averages if you want a benchmark.

How to Calculate Paint Coverage for Walls and Ceilings

Enter the room dimensions in metres (length, width, height), the number of internal doors and windows, the number of coats you plan to apply, and the coverage rate printed on your tin. Click Calculate to see total litres needed and the most economical mix of 2.5L, 5L and 10L tins from B&Q, Wickes or Homebase.

1. Measure the length and width of the room in metres along the skirting board (use a laser measure for accuracy on rooms over 4m).
2. Measure the floor-to-ceiling height — most UK new builds are 2.4m, Victorian and Edwardian rooms are commonly 2.7m to 3.0m.
3. Count internal doors at roughly 1.5 m² each (standard UK door is 1.98m × 0.76m) and standard windows at 1.2 m² each (adjust upwards for bay or French windows).
4. Choose your number of coats: 1 for a same-colour refresh, 2 for most colour changes, 3 when going from dark to light or covering bare plaster.
5. Enter the coverage rate from your tin label — default 12 m²/L, raise to 14 for Dulux Trade Diamond Matt or Farrow & Ball Estate, drop to 10 for budget own-brand emulsion.
6. Read off the litres needed and the calculator's recommended combination of 2.5L, 5L and 10L tins.
7. For the ceiling, run a separate calculation using floor area (length × width) as the paintable surface — ceilings almost always need 2 coats, even in the same colour, because of how light catches them.

Paint Formula: Wall Area ÷ Coverage Rate per Litre

The formula used: Paint Needed (litres) = (Total Wall Area − Unpainted Areas) × Number of Coats / Coverage Rate (m² per litre)

Paint Estimates for Small, Medium, and Large Rooms

Room 5m × 4m × 2.4m, 1 door (1.9m²) and 2 windows (1.5m² each). Wall area = 43.2m². Minus openings: 43.2 − 4.9 = 38.3m². Two coats at 12 m²/L: 38.3 × 2 ÷ 12 = 6.38L. Buy 1 × 5L Dulux Trade Vinyl Matt + 1 × 2.5L tin (£40-£55 total at trade prices).

When to Calculate Paint Quantity Before Buying

Run this calculator before you set foot in B&Q, Wickes, Homebase or your local Dulux Decorator Centre — not while standing in the aisle trying to do mental arithmetic with a screaming toddler in the trolley. Knowing your litres figure in advance lets you compare brand prices on a fair like-for-like basis, because a 5L tin of premium Farrow & Ball at £58 and a 5L tin of Wickes own-brand at £22 are not the same thing once you factor in the extra coat the cheap tin will need.

The calculator is also the fastest way to sanity-check a decorator's quote. Most professional UK decorators charge £150-£250 per day plus materials, and a common rough quote in 2025 is £200-£350 for a single bedroom (around 11 m² floor area), £350-£600 for a typical living room (around 16 m² floor area), and £1,800-£3,000 for a full 3-bed terrace painted throughout. If a decorator's materials line says they need 25 litres for one medium bedroom, your calculator will tell you that something is wrong. Equally, if they have only quoted for 1 × 5L tin to repaint a whole living room with two coats of dark blue, you can flag that they have undersized the order before you sign.

Finally, run the numbers when you are choosing between buying retail and trade. The Dulux Decorator Centre and Brewers will sell to the public, and a 10L trade tin often works out 25-40% cheaper per litre than the equivalent retail product on a B&Q shelf — but only if your calculation justifies a 10L tin in the first place. If you only need 4 litres, retail is usually fine.

Coverage Rate, Coats, and Paint Types

Emulsion vs Eggshell vs Satinwood vs Gloss: Coverage and Cost Compared

Tips for Buying the Right Amount of Paint

• <strong>New plaster:</strong> 1 mist coat (emulsion thinned 70:30 with water) followed by 2 full topcoats. Skipping the mist coat is the most expensive mistake on any new build or extension — your topcoat will peel off in sheets within 18 months.
• <strong>Old painted wall, similar colour:</strong> 1-2 coats. Edges and corners almost always need a second pass even when the wall faces look fine.
• <strong>Old painted wall, dark to light:</strong> 2-3 coats plus a stain-blocking primer if the original was a deep red, navy, or any kitchen wall that has absorbed cooking oils.
• <strong>Bare wood (skirting, new doors, MDF):</strong> wood primer + undercoat + 2 topcoats of eggshell or satinwood. Knotting solution first if there are visible knots in pine.
• <strong>Radiators and metal:</strong> radiator-specific metal primer (Hammerite or equivalent for cast iron) + 2 coats of radiator enamel or water-based satinwood. Standard emulsion will yellow and crack within a heating season.
• <strong>Bathrooms and en-suites:</strong> always use a moisture-resistant emulsion (Dulux Easycare Bathroom, Crown Bathroom Mid-Sheen, Johnstone's Aqua Guard). Standard matt will mildew at the ceiling line within months.
• <strong>Kitchens:</strong> use grease-resistant kitchen emulsion (Dulux Easycare Kitchen). It scrubs clean without burnishing, which standard matt will not.
• <strong>Buy slightly more than the calculation:</strong> add 10% for spillage, roller absorption, and a sealed touch-up tin. A properly stored tin (lid hammered shut, stored upside down to seal) lasts 5-10 years.
• <strong>Listed buildings and conservation areas:</strong> exterior colour changes may need planning permission or listed building consent — check with the local planning authority before buying.

Paint Quantity Errors That Cause Mid-Job Shortages

• <strong>Underestimating coats and ending up short:</strong> the single most common DIY mistake. A 5L tin claiming 70 m² coverage actually does ~35 m² across two coats, not 70 m² of finished wall.
• <strong>Forgetting to subtract doors and windows:</strong> overbuys by 10-15% on a typical bedroom and 20%+ on a glazed living room.
• <strong>Buying budget own-brand emulsion to 'save money':</strong> it covers ~10 m²/L versus 14 m²/L for trade, so you buy 40% more litres and apply 50% more coats. The trade tin almost always wins on total cost.
• <strong>Using standard matt in a bathroom or kitchen:</strong> mildew or grease damage within one season, then a full repaint. Specify the right sub-product first time.
• <strong>Skipping the mist coat on new plaster:</strong> topcoat peels off the powdery plaster face within 12-18 months. There is no shortcut — thinned emulsion or a dedicated plaster sealer, every time.
• <strong>Choosing the wrong paint type for trim:</strong> oil-based gloss yellows badly in north-facing rooms. Water-based satinwood (Dulux Quick Dry, Crown Fastflow) is now the UK trade default for skirting and doors.
• <strong>Over-paying a decorator on materials:</strong> for context, typical 2025 UK decorator costs (Checkatrade, MyBuilder, HouseholdQuotes data) are £200-£350 for a single bedroom (~11 m² floor), £350-£600 for an average 16 m² living room, and £1,800-£3,000 for a full 3-bed terrace. Materials are usually ~25-30% of those totals — anything significantly higher is worth questioning.