How much paint do I need for an average bedroom?

A typical bedroom (around 3.5 m × 4 m with a 2.4 m ceiling) has roughly 34-36 m² of gross wall area. After subtracting a door and one or two windows and applying two coats, most bedrooms need around 6-8 litres of standard matt emulsion — call it two 5 L tins to be safe with some left for touch-ups.

Do I really need two coats of paint?

For a genuine colour change, yes — a single coat almost never fully hides the previous colour or primer, leaving patchy, uneven coverage. If you're simply refreshing a wall with the exact same colour and the existing paint is in good condition, one coat can sometimes be enough, but two coats is the standard, safer assumption this calculator uses by default.

How much extra paint should I buy for a dark-to-light colour change?

Add a third coat to your plan, or budget an extra 30-50% on top of the standard two-coat volume. Dark colours (and reds in particular) are notoriously hard to fully block, and a dedicated stain-blocking primer/undercoat before your colour coats can cut down how many coats you actually need.

Does the paint calculator account for ceilings?

No — this calculator is for wall area only, mirroring how paint is bought and applied in practice (ceiling paint is often a different, flatter formulation). Run the numbers for your ceiling separately: multiply room length by width to get ceiling area, then divide by your ceiling paint's coverage rate.

What waste percentage should I use for a rough or unprepared wall?

For a smooth, previously painted wall in good condition, 5-10% waste is plenty. For a porous, textured, or completely unpainted new-plaster surface, push the waste allowance up to 20-30% — that surface will genuinely absorb far more paint on the first coat, and it's better to buy enough the first time than to hunt for a colour-matched top-up tin later.

How much does it cost to paint a room in the UK?

A typical bedroom with two coats needs about 6-8 litres of paint. With well-known brands (Dulux, Farrow & Ball, Crown) ranging from around £20-30 per litre for standard emulsion (Farrow & Ball runs considerably higher), materials alone come to roughly £120-240, not including labour.

Does Dulux paint coverage match the default figure used in this calculator?

Dulux matt emulsion typically states coverage of around 12-14 m² per litre per coat on a well-prepared wall, slightly ahead of this calculator's default (10 m²/L), which is set conservatively to account for less-prepared surfaces. Always check the exact figure on your chosen tin's label.

Paint Calculator — How Much Paint Do I Need?

How Much Paint Do I Need?

Before you head to the store, it helps to know exactly how much paint to buy — guess too low and you're stuck mid-job with a half-painted wall and no matching tin; guess too high and you've paid for litres you'll never use. The paint calculator above works out the answer from four things: the size of the wall (or room), how many doors and windows cut into that area, how many coats you're planning, and the coverage rate of the paint you've chosen.

The core idea is simple. Paint doesn't go onto floors, ceilings, or the wood/glass of doors and windows, so those areas get subtracted from the gross wall area first. What's left — the net paintable area — then gets multiplied by the number of coats, because a two-coat job needs twice the paint of a one-coat job over the same wall. Divide that by the paint's coverage rate (how many square metres one litre covers) and you get the volume you need to buy.

Most people underestimate two things: the effect of a colour change (going from a dark wall to a light colour, or vice versa, often needs an extra coat that was never in the original plan) and the effect of a porous or previously unpainted surface, which soaks up noticeably more paint than a wall that has already been painted a few times. Both are covered below, along with a waste allowance you can dial up or down depending on how forgiving your surface is.

This calculator also does something most online paint calculators skip: it ships with coverage-rate presets for the main paint types (matt emulsion, silk/gloss, exterior masonry, primer) so you're not stuck guessing a number before you've even bought a tin. If you already have a can in hand, just swap in the figure printed on the label for a more precise result.

How to Use the Paint Calculator

Enter the wall length and wall height (or the total wall length of the room if you're doing more than one wall) and pick your unit — metres or feet.
Enter the number of doors, plus their average height and width. The defaults are standard interior-door sizes, but adjust them if yours are different.
Enter the number of windows, plus their average height and width. Again, sensible defaults are pre-filled.
Set the number of coats you plan to apply. Two coats is standard for a fresh colour on primed plaster; one coat can work for a simple refresh in the same colour.
Pick a coverage rate preset that matches your paint type (matt emulsion, silk/gloss, exterior masonry, primer), or type in the exact figure from your paint tin label if you already have one.
Set a waste allowance to cover spillage, roller loading, and any porous or unprepared patches of wall — 10% is a reasonable starting point, more for rougher or thirstier surfaces.
Read off the net paintable area, the total paint volume needed, and the estimated cost, ready to take to the store.

The Formula Behind the Numbers

The calculation runs in three stages. First, the gross wall area: length × height. Second, subtract the openings — each door's height × width, each window's height × width, summed and taken off the gross area — to get the net area for a single coat. Third, multiply that net area by the number of coats to get the total paintable area, since each additional coat covers the same surface again from scratch.

To turn area into a volume of paint, divide the total paintable area by the coverage rate (how many square metres one litre covers), then add your waste percentage on top: volume = (net area × coats × (1 + waste%)) ÷ coverage rate.

Worked example: a wall 4 m long and 2.4 m high, with one door (2.03 m × 0.86 m) and one window (1.2 m × 1.0 m), painted with two coats of matt emulsion (10 m²/L coverage), with a 10% waste allowance.

Gross wall area: 4 × 2.4 = 9.6 m². Door area: 2.03 × 0.86 = 1.75 m². Window area: 1.2 × 1.0 = 1.2 m². Net area for one coat: 9.6 − 1.75 − 1.2 = 6.65 m². For two coats: 6.65 × 2 = 13.3 m². With 10% waste: 13.3 × 1.10 = 14.63 m². Divide by the coverage rate: 14.63 ÷ 10 = 1.463 litres of paint needed.

In practice you'd round that up to the nearest tin size available — most retailers sell 1 L, 2.5 L, 5 L, and 10 L tins, so this small wall would need a single 2.5 L tin with plenty left over for touch-ups.

Typical Paint Coverage Rates

Coverage rate is the single biggest variable in this calculation, and it varies a lot by paint type, brand, and the condition of the surface underneath. These are typical trade figures for a single coat on a reasonably prepared wall — always check the tin, since real coverage can vary 20-30% either side of these averages.

Matt/emulsion wall paint: roughly 10-12 m² per litre (around 350-400 sq ft per US gallon).
Silk/satin/gloss paint: roughly 12-14 m² per litre (around 400-450 sq ft per US gallon) — smoother finishes tend to spread a little further.
Exterior masonry paint: roughly 6-8 m² per litre (around 200-250 sq ft per US gallon) on textured render or brick — rough exterior surfaces soak up far more than smooth interior walls.
Primer/undercoat: roughly 8-10 m² per litre (around 300-350 sq ft per US gallon), though bare plaster or plasterboard can pull this rate down significantly on the first coat.
Ceiling paint (flat, high-opacity): roughly 10-11 m² per litre, similar to standard matt emulsion.

Pro Tips and Common Mistakes

The single most common underestimate is not accounting for a big colour change. Painting a dark wall (navy, charcoal, deep red) over with white or a pale colour routinely needs a third coat to fully block the old colour, even with a good primer — budget for it rather than being surprised mid-job.

At around £24 per litre, our worked example above (a small wall, two coats, 1.463 L needed) rounds up to a single 2.5 L tin for roughly £60, leaving plenty for touch-ups. For an average UK bedroom needing two coats, budget roughly £145-210 in paint materials, before considering primer, brushes, or a decorator's labour.

Always prime bare plaster, plasterboard, or new render before the first colour coat — unprimed porous surfaces can drink up 20-30% more paint than the same wall once it's sealed.
A mini roller and brush for cutting in around doors, windows, and skirting boards uses noticeably more paint per square metre than a full roller on open wall — factor this in for small, detail-heavy rooms like bathrooms and hallways.
Don't skip the waste allowance even for a simple job — spillage, a roller that never quite empties, and offcuts of tape and dust sheets all eat into your usable paint.
Let each coat dry fully (check the tin for recoat time, often 2-4 hours for emulsion, longer for gloss) before applying the next — painting over a tacky coat causes lifting and an uneven finish that can force a redo.
Buy slightly more than the calculator says and keep the receipt — it's far easier to return an unopened tin than to find an exact colour match again in six months for a small touch-up.
If you're painting a ceiling as well as walls, calculate it separately — a ceiling's area is length × width, not length × height, and it usually needs its own coverage-rate preset since ceiling paint is formulated differently.