Paint + Primer Coverage Calculator UK — Litres & Leftover
Painting a room? This free paint and primer coverage calculator helps UK homeowners and contractors work out exactly how many litres of paint and primer a room needs — and unlike other calculators, we show you exactly how much paint you'll have left over for future touch-ups. Enter your room dimensions or wall area directly, choose your surface type, number of coats, and paint tier, and get an instant breakdown in metres and pounds, sized to the 5L/2.5L/1L cans stocked by suppliers like Wickes and B&Q. Results update live and include a full cost breakdown.
How to use this calculator
1. Measure your room or enter wall area directly
Get the length and width of the room in metres, plus the ceiling height — 2.4m is the default and covers most UK new-builds, but older homes often run higher, so adjust it. Prefer to skip the geometry? Switch to the "By wall area" tab and enter your total area in square metres directly, handy if you already have it from an EPC report or a builder's measurement. Toggle "Paint ceiling too?" on if the ceiling is going the same colour, since that adds real area to your total.
2. Choose your surface type and number of coats
Pick the surface that matches your walls: bare drywall, previously painted, textured, or masonry. This matters more than people expect — bare plasterboard soaks up noticeably more paint than a smooth, previously painted wall, because the plaster and joint filler are porous. Then set your number of coats. Two coats is the default and the right call for most repaints over emulsion. Drop to one coat only if you're matching a very close colour on a wall already in good condition; bump to three over a dark colour or new plaster.
3. Pick a paint tier and primer option
Choose economy, mid, or premium paint — this changes both the price per litre and the coverage rate, since better emulsion carries more pigment and spreads further. Leave "Include primer?" on Auto and the calculator recommends primer for bare drywall automatically, and skips it for previously painted walls in good condition. New plaster is a special case: it needs a mist coat (diluted emulsion) rather than a dedicated primer, so force primer off and thin your first coat instead if that's your situation.
4. Review your paint, primer, and leftover estimate
Results update live as you type. You'll see your paintable area after doors and windows are subtracted, the litres needed for your chosen coats, and how many litres to buy — rounded up to the nearest combination of 5L, 2.5L, and 1L cans, since UK suppliers like Wickes and B&Q don't sell odd litre amounts. Below that is our leftover box: how much paint you'll have left after the job, in litres and in square metres of future touch-up coverage. That's the figure most calculators skip entirely.
The formula we use
Here's the exact math, start to finish. First we work out your wall area: perimeter × height, using 2 × (length + width) × ceiling height, in metres. Painting the ceiling too adds length × width on top. Next we subtract openings — 1.86 sqm per door and 1.39 sqm per window, standard sizes for a residential interior door and window — to get your paintable area.
From there: paintable area ÷ coverage rate = litres per coat, multiplied by your number of coats. Coverage rate is where most calculators cut corners. We derive our metric rates from the same US sqft/gallon matrix — previously painted drywall at mid-tier works out to 9.8 sqm per litre, bare drywall at the same tier to 7.35 sqm per litre. These figures run noticeably below what's printed on most tins: Dulux and Crown often quote 16-17 sqm per litre on smooth, primed surfaces under lab conditions. Real UK walls — especially older plaster with texture and uneven absorbency — rarely hit that, so our conservative numbers are there to stop you running short mid-wall.
Finally, we round up to the smallest combination of 5L, 2.5L, and 1L cans that covers your need — even a small shortfall bumps you to the next can size, and that rounding gap is exactly where the leftover figure comes from.
A worked example
Let's walk through a real bedroom: 3.7m × 3.7m, 2.4m ceilings, previously painted walls, one door, two windows, mid-tier paint, two coats.
Wall perimeter: 2 × (3.7 + 3.7) = 14.8m. Wall area: 14.8 × 2.4 = 35.52 sqm. Subtract openings: 1 door (1.86 sqm) + 2 windows (2.78 sqm) = 4.64 sqm. That leaves 30.9 sqm of paintable area.
Previously painted drywall at mid-tier covers 9.8 sqm per litre, so one coat needs 30.9 ÷ 9.8 = 3.15 litres. Two coats: 6.3 litres. Rounded up to the smallest covering combination of cans, that's 6.5 litres — a 2.5L can plus four 1L cans of Dulux Trade Vinyl Matt at £51 per 5L works out to £66.30, which we round to £66.
Those 6.5 litres cover a touch more than you strictly need for two coats, leaving roughly 0.2 litres — enough for a few scuffs and nail holes down the line, not a full extra wall. Previously painted walls in good condition don't need primer, so £66 is your full paint cost — no primer line to add.
Regional cost context
UK paint prices span a wide range by tier, and — like in the US — primer isn't the budget option people assume.
Cost estimates are based on prices from named UK retailers (Wickes, B&Q, others) as of August 2026 and may vary by store and season. As an Amazon Associate we earn from qualifying purchases. Always verify current prices and confirm any planning requirements with your local authority before starting work.
| Product | Tier | Price | Container |
|---|---|---|---|
| Economy | Economy | £16 | 5L |
| Mid | Mid | £51 | 5L |
| Premium | Premium | £70 | 5L |
| Primer | Zinsser Bulls Eye 1-2-3 | £49 | 2.5L |
Prices from Wickes, current as of our last check — see our Methodology page for sources and verification dates.
Wickes' own Vinyl Silk Emulsion at £16 for 5L is a solid economy choice for ceilings, rentals, or a first coat you'll paint over again. Dulux Trade Vinyl Matt at £51 for 5L is the mid-tier workhorse most tradespeople reach for on living rooms and bedrooms — better hide and a flatter, more forgiving finish than budget emulsion. Dulux Trade Diamond Matt at £70 for 5L is built for high-traffic areas like hallways and kids' rooms, with a scuff-resistant finish that survives a lot more wiping down.
Zinsser Bulls Eye 1-2-3 primer, at £49 for a 2.5L can, is noticeably pricier per litre than any of the topcoats above — it's a stain-blocking, multi-surface primer, not a budget undercoat. All figures reflect our last direct check of Wickes' site (see Methodology); prices at B&Q or in different regions may vary by a few pounds.
Shop on Amazon: Zinsser Bulls Eye 1-2-3 primer
A quick safety note: lead paint
Many UK homes built before the 1960s may still have old lead-based paint under newer layers, particularly on woodwork, skirting boards, and window frames. If you're sanding or stripping old paintwork in an older property, test first rather than assume, and take extra care with dust — wet-sand where you can and avoid dry power sanders on suspect layers. Modern paint sold in the UK has its VOC content listed on the tin — check gov.uk for current limits if you're working with a specialist product, and ventilate the room well regardless of what you're using.
Common mistakes to avoid
- Skipping the mist coat on new plaster. Fresh plaster needs a diluted first coat, not full-strength emulsion or a US-style primer — skip it and your topcoat can flake or patch unevenly.
- Forgetting to subtract doors and windows. A room with two windows and one door has nearly 4.6 sqm less wall than its raw perimeter × height suggests — miss that and your leftover figure will be off too.
- Mixing tins from different batches. Even the same colour code can shift slightly between production runs. Buy everything you need in one trip, or box-mix your tins together before starting if you're topping up mid-project.
- Rounding down to save a few pounds. If the maths says 6.3 litres, buy the covering combination the calculator shows — running short mid-wall with a half-dry roller is a worse Saturday than buying one extra 1L tin.
- Treating masonry like drywall. Brick and render soak up far more paint than plasterboard. Pick "Masonry/brick" as your surface type rather than guessing, or you'll run short.
- Planning a bigger project? If you're also building a deck, see how many boards you'll need with our composite decking calculator. Adding a retaining wall to the garden too? Check our retaining wall block calculator for blocks and gravel.
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Frequently asked questions
How much paint do I need for a room?
It depends on room size, but a typical 3.7m × 3.7m bedroom with 2.4m ceilings needs about 6.5 litres for two coats on previously painted walls. That's a 2.5L can plus a couple of 1L top-ups, since UK suppliers sell in 5L, 2.5L, and 1L sizes rather than odd amounts. Bare plaster or a dark-to-light colour change will need more. Use the calculator above with your exact room dimensions for a precise litre and cost estimate.
How many m² does a litre of emulsion cover?
Previously painted walls cover roughly 8.6-11 m² per litre depending on tier (economy to premium); bare plasterboard covers less, around 6.1-8.6 m² per litre, since raw plaster is more absorbent. These figures sit below what's printed on most tins — Dulux and Crown often quote 16-17 m² per litre, measured on smooth, primed surfaces in ideal conditions. Real UK walls, especially older plaster with texture, rarely hit that, so we keep our numbers conservative on purpose.
Do I need to mist coat new plaster?
Yes — new plaster needs a mist coat, not a standard primer or full-strength emulsion straight onto the wall. A mist coat is your first coat of emulsion thinned roughly 50/50 with water, which lets fresh plaster absorb it evenly instead of flashing patchy and drying unevenly. Skip it and your proper topcoat can flake or bond poorly later. In our calculator, set primer to "Never include" for new plaster and thin your first coat yourself instead.
How much paint for two coats of emulsion?
For a 3.7m × 3.7m room with previously painted walls, two coats of mid-tier emulsion need 6.3 litres — rounded up to 6.5 litres once you factor in real UK can sizes (a 2.5L can plus 1L top-ups). One coat alone needs about 3.15 litres for the same room. Two coats is the standard recommendation for most repaints; new plaster or a big colour change may need a third coat on top of the mist coat.
Do I need primer over a dark colour?
Usually yes, if you're going from dark to light — primer or a tinted undercoat stops the old colour bleeding through and can save you a third topcoat. Like-for-like colour changes or dark-over-dark generally don't need it. Our calculator's Auto primer setting is based on surface type rather than colour, so for a dramatic colour change, manually switch primer to "Always include" regardless of what surface type you select.
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Cost estimates are based on prices from named UK retailers (Wickes, B&Q, others) as of August 2026 and may vary by store and season. As an Amazon Associate we earn from qualifying purchases. Always verify current prices and confirm any planning requirements with your local authority before starting work.
Last updated: August 2026. Estimates are for planning purposes only. Verify with local suppliers before purchase. Coverage rates are generic averages; actual coverage varies by product, application method, and surface condition — check your specific paint can's label before buying.