Paint Calculator
Buying the right amount of paint the first time saves a wasted trip back to the shop and avoids the risk of a slightly mismatched top-up batch partway through a wall. Enter your room dimensions, how many doors and windows to deduct, and how many coats you're planning below to get a litres estimate.
Work out how much paint you need
Enter your room dimensions, doors/windows, and how many coats you plan to apply.
Wall area to paint (after deducting doors and windows): 33.2m²
approximately 5.5 litres of paint
A 5L can (or combination of cans) should cover this, based on average emulsion coverage of 12m² per litre per coat.
Recommended products
A few well-reviewed decorating essentials for your next paint job.
- Interior walls
£24.27
Pros
- Wipeable finish shrugs off scuffs in hallways and kids rooms
- Good one-coat opacity over most existing colours
- Covers around 32.5m² per coat
Cons
- Matt finish shows roller marks if overworked
- Exterior wood
Currently unavailable
Pros
- Touch dry in around an hour so fences get two coats in a day
- Water repellent and rainproof in as little as 30 minutes
- Water-based formula cleans up with soap and water
Cons
- Needs reapplying yearly on south-facing fences
- Interior walls
£61.00
Pros
- Chalky, richly pigmented finish sought after by interior designers
- Low-VOC formula suited to bedrooms and nurseries
- Exceptional depth of colour in muted heritage shades
Cons
- Premium price per litre compared with high-street brands
- Mixed to order, so cannot be returned once opened
Frequently asked questions
How many coats of paint do I actually need?
Two coats is standard for most emulsions and gives an even, opaque finish, especially over a different existing colour. A single coat can work for touch-ups or when using a dedicated one-coat paint, while very dark colours or big colour changes sometimes need three coats.
Does paint coverage vary between brands?
Yes — this calculator uses an average coverage of 12m² per litre per coat, which is typical for mid-range emulsion, but budget paints can cover less (nearer 10m²/L) and premium paints can cover more (up to 14-16m²/L). Check the coverage figure on your chosen tin and adjust your estimate accordingly.
Should I paint the ceiling too?
This calculator estimates wall area only. If you're painting the ceiling as well, add its area (room length × width) separately, usually with a ceiling-specific paint that resists drips and gives a flat, non-reflective finish.
Is it better to buy slightly more paint than the estimate?
Yes — buying a little extra avoids running out mid-wall and having to colour-match a top-up later, which can leave a visible join if the batch differs slightly. Keeping leftover paint is also useful for future touch-ups.
How this calculator works
The calculator works out the total wall area by multiplying the room's perimeter (2 × length + 2 × width) by the wall height, giving the gross wall area in square metres. It then subtracts standard allowances for doors (1.6m² each) and windows (1.2m² each) to get the net wall area. This net area is multiplied by the number of coats you choose and divided by the average coverage of 12m² per litre per coat to give the total litres needed.
The result is rounded up to the nearest sensible can size. Standard UK paint cans are 2.5L, 5L and 10L. The calculator suggests the largest can size that doesn't exceed your total litres need — for example, if you need 7.5L, it suggests a 5L can (or combination of cans) rather than a 10L can that would leave too much leftover.
Paint coverage and can sizes
| Can size | Coverage per can (2 coats) | Suitable for | Typical cost |
|---|---|---|---|
| 2.5L | ~30m² | Small room, feature wall | £8-£15 |
| 5L | ~60m² | Standard bedroom, bathroom | £15-£25 |
| 10L | ~120m² | Large room, multiple rooms | £25-£45 |
Worked example: A 4m x 3.5m x 2.4m bedroom with 1 door and 1 window
A bedroom 4m long, 3.5m wide and 2.4m high has a perimeter of 2 × (4 + 3.5) = 15m. The gross wall area is 15 × 2.4 = 36m². Deducting 1 door (1.6m²) and 1 window (1.2m²) gives a net wall area of 36 - 1.6 - 1.2 = 33.2m². For 2 coats, the paintable area is 33.2 × 2 = 66.4m². At 12m² per litre, you need 66.4 ÷ 12 = 5.53L, rounded up to 5.5L.
The calculator would suggest buying a 5L can plus a 2.5L can (7.5L total) to give a little extra for touch-ups. At a typical mid-range price of £18 for 5L and £10 for 2.5L, that's £28 for materials. If you're painting a ceiling too, add the ceiling area (4 × 3.5 = 14m²) to the calculation — ceilings usually need the same number of coats and similar coverage.
Common mistakes when buying paint
- Forgetting to deduct doors and windows. A typical bedroom has 3-5m² of openings, which can make 10-15% difference to the total paint needed. Always measure or count them, especially in rooms with large windows or bi-fold doors.
- Buying exactly the calculated litres with no spare. A 5-10% extra allowance covers touch-ups, mistakes, and the fact that paint coverage varies with surface texture. Rough-textured walls absorb more paint than smooth plaster.
- Not checking the paint's actual coverage figure. This calculator uses 12m²/L as an average, but premium paints can cover 14-16m²/L, while budget paints might only cover 10m²/L. Check the label on your chosen paint and adjust the estimate if needed.
- Mixing paint batches for large jobs. If you need more than one can of the same colour, try to buy all the paint at once from the same batch. Different batches can have slightly different tints, which shows as a visible join when two walls meet.
What to look for when buying paint
Paint type matters as much as colour. Matt emulsion hides imperfections well and is good for ceilings and imperfect walls, but it's less wipeable. Silk or satin emulsion is more durable and easier to clean, making it better for kitchens, bathrooms and hallways. For woodwork, use gloss or satinwood — water-based versions are easier to clean up and have less odour, but oil-based gives a harder, more traditional finish.
For rooms with high humidity (bathrooms, kitchens), look for paint with a mildew-resistant additive or a specific bathroom and kitchen emulsion. These paints are formulated to resist mould and condensation staining, which standard emulsion can suffer from in damp conditions. A good bathroom paint also handles scrubbing better when you need to clean splashes from tiles or walls.
Finally, think about primer and undercoat. If you're painting over a dark colour with a light one, or over bare plaster, you'll need a primer or mist coat first. A white emulsion mist coat (heavily diluted with water) is a cheap way to seal new plaster before the topcoat. For woodwork, a separate primer and undercoat are essential for adhesion and to prevent tannin staining from knots.
More questions answered
How many coats of paint do I actually need?
Two coats is standard for most emulsions and gives an even, opaque finish, especially over a different existing colour. A single coat can work for touch-ups or when using a dedicated one-coat paint, while very dark colours or big colour changes sometimes need three coats.
Does paint coverage vary between brands?
Yes — this calculator uses an average coverage of 12m² per litre per coat, which is typical for mid-range emulsion, but budget paints can cover less (nearer 10m²/L) and premium paints can cover more (up to 14-16m²/L). Check the coverage figure on your chosen tin and adjust your estimate accordingly.
Should I paint the ceiling too?
This calculator estimates wall area only. If you're painting the ceiling as well, add its area (room length × width) separately, usually with a ceiling-specific paint that resists drips and gives a flat, non-reflective finish.
Is it better to buy slightly more paint than the estimate?
Yes — buying a little extra avoids running out mid-wall and having to colour-match a top-up later, which can leave a visible join if the batch differs slightly. Keeping leftover paint is also useful for future touch-ups.
What's the difference between matt, silk and satin emulsion?
Matt emulsion has a flat, non-reflective finish that hides wall imperfections well but is less wipeable. Silk emulsion has a slight sheen, is more durable and easier to clean, and works well in kitchens and bathrooms. Satin is somewhere between matt and silk — it has a subtle sheen, is more hardwearing than matt, and is good for hallways and woodwork. For most living rooms and bedrooms, matt is the most popular choice.
Do I need a primer before painting?
If you're painting over new, bare plaster, use a primer or mist coat first to seal the surface and prevent the topcoat from being absorbed unevenly. If you're painting over a dark colour with a light one, a primer or undercoat helps cover the old colour with fewer topcoats. For previously painted walls in good condition, you can usually paint directly with emulsion, but spot-priming any repairs is essential.
How long does paint take to dry between coats?
Most water-based emulsions are touch-dry in 1-2 hours and can be recoated after 2-4 hours in normal conditions. Oil-based gloss takes longer — typically 4-6 hours to touch-dry and 12-16 hours before the next coat. Cold, damp weather slows drying significantly, so allow extra time in winter and keep the room well ventilated.
Can I use emulsion on woodwork and doors?
You can use emulsion on woodwork, but it's not ideal — it's softer than gloss or satinwood and will scratch and mark more easily. For skirting boards, doors and window frames, use a dedicated water-based or oil-based gloss or satinwood for a hardwearing, wipeable finish. If you prefer the look of emulsion, use a water-based eggshell or satin finish designed for woodwork.