One litre of paint covers about 10–12 m² per coat, so paint needed = wall area ÷ coverage × number of coats.
One litre of emulsion covers roughly 10 to 12 square metres per coat, so the calculation is wall area divided by coverage, multiplied by the number of coats. A standard 4 m by 3 m bedroom needs about 5.5 litres for two coats on the walls. The step people skip is that second coat, which doubles the requirement and is almost always necessary, and the step people overdo is subtracting every door and window when only the large openings meaningfully change the answer. This guide walks through measuring a room properly, converting to litres or gallons, and adjusting for surfaces that drink paint. The CalcRange Paint Calculator does the sums from your measurements.
The Calculation
Litres needed = (wall area x number of coats) / coverage per litre
Coverage for standard interior emulsion runs about 10 to 12 square metres per litre per coat. Use 11 as a working figure for smooth painted walls, and drop toward 8 or 9 for anything porous.
Note where the coats belong in that formula. They multiply the area, not divide the coverage. A room needing 30 square metres of wall at two coats is a 60 square metre painting job.
Measuring the Room
Wall area is the perimeter of the room multiplied by its height.
Wall area = 2 x (length + width) x height
Measure floor to ceiling for the height, and measure the actual room rather than trusting a floor plan. Then decide what to subtract.
A standard door is about 1.6 square metres and a typical window around 1.8. Subtracting both from a small room removes over 3 square metres, which is a third of a litre once you account for two coats. That is worth doing. Subtracting a small high-level window from a large open-plan space is not, and the leftover is useful for touch-ups anyway.
Rule of thumb: subtract openings if they come to more than about 10% of the wall area. Otherwise ignore them and treat the surplus as your margin.
Worked Example
Bedroom 4 m by 3 m, ceiling height 2.4 m, one door and one window.
- Perimeter = 2 × (4 + 3) = 14 m
- Wall area = 14 × 2.4 = 33.6 m²
- Door 2.0 × 0.8 = 1.6 m²; window 1.5 × 1.2 = 1.8 m². Together 3.4 m²
- Net wall area = 33.6 − 3.4 = 30.2 m²
- Two coats = 60.4 m² of coverage required
- At 11 m² per litre: 60.4 ÷ 11 = 5.5 litres
Paint is sold in fixed tin sizes, so buy 6 litres, most simply as a 5 litre tin plus a 1 litre. The leftover is genuinely useful, since matching a colour years later is far harder than storing a part-tin.
The ceiling
Ceiling area is just length by width: 4 × 3 = 12 m². Two coats makes 24 m², which at 11 m² per litre is 2.2 litres. Ceilings often need the full two coats even over existing paint, because the change in sheen shows badly under overhead light.
Work Out Your Quantity
Enter your room dimensions into the CalcRange Paint Calculator for litres required. If your walls are an awkward shape, the area calculator handles the geometry, and the unit converter switches between metric and imperial.
How Many Coats You Actually Need
Two is the realistic default and the figure the calculation above assumes.
One coat only works when you are repainting the same colour on a sound, previously painted wall, and even then the finish is often patchy under raking light. Manufacturers who advertise one-coat coverage are describing ideal conditions on a well-prepared surface.
Three coats become necessary for a few specific situations: covering a dark colour with a light one, painting a strong colour that is naturally thin in pigment, working over bare plaster or filler without a primer, or dealing with stains that bleed through. Deep reds and yellows are notorious for needing an extra pass.
Priming changes this calculation more than people expect. A tinted primer over dark walls can save an entire finish coat, and primer is generally cheaper per litre than good emulsion, so the sequence primer plus two coats often costs less than four coats of emulsion.
What Changes the Coverage Rate
- Bare plaster and new plasterboard absorb heavily, and coverage can fall to 8 m² per litre or lower on a first coat. A thinned mist coat is standard practice here.
- Textured or rough surfaces have more actual surface area than their measured dimensions suggest, sometimes 20% more.
- Bare wood and masonry drink the first coat, so prime rather than trying to compensate with volume.
- Matt finishes typically cover slightly further than silk or gloss.
- Application method matters. Rollers are efficient, brushes less so, and sprayers waste a significant proportion in overspray, sometimes requiring a third more paint.
- Cheap paint often carries less pigment, which means more coats and frequently a higher total cost than buying better paint once.
Working in Gallons
A US gallon of interior paint typically covers around 350 to 400 square feet per coat.
Gallons = (wall area in sq ft x coats) / 350
The example bedroom is 30.2 m², which is about 325 square feet. Two coats gives 650 square feet, divided by 350 comes to 1.9 gallons, so two gallons.
To convert between systems: one square metre is 10.76 square feet, and one US gallon is 3.79 litres.
Common Mistakes
- Calculating for one coat. This is the most expensive error, because a second trip for more paint risks a batch difference in colour.
- Using floor area instead of wall area. Floor area belongs in the ceiling calculation only.
- Subtracting every opening in a large room, which produces a shortfall with no margin for touch-ups.
- Ignoring the surface. Bare plaster can consume a third more than a previously painted wall.
- Buying exactly the calculated amount. Tin sizes are fixed, and rounding down leaves you short.
- Mixing batches mid-wall. Where a large area needs more than one tin, decant them together into a bucket first so any slight colour variation is invisible.
Frequently Asked Questions
How much paint do I need for one room?
Calculate the perimeter times the height, subtract large openings, multiply by two coats, and divide by 11 square metres per litre. A typical 4 m by 3 m bedroom with a 2.4 m ceiling needs about 5.5 litres for the walls, plus around 2.2 litres for the ceiling.
How much does one litre of paint cover?
Around 10 to 12 square metres per coat for standard interior emulsion on a smooth, previously painted surface. That drops to 8 or 9 on porous or textured walls, and manufacturers quote coverage on ideal surfaces, so treat their figure as an upper limit.
Do I really need two coats?
Almost always. One coat only holds up when repainting the same colour on sound existing paint, and even then the finish can look uneven in strong light. Colour changes, bare surfaces, and strong pigments all need two as a minimum.
Should I subtract doors and windows?
Subtract them if they add up to more than roughly 10% of the wall area, which is usually the case in small rooms. In larger spaces the difference is small enough that leaving them in gives you a helpful margin and some paint for touch-ups.
How much paint for a ceiling?
Ceiling area is the room’s length times width, so a 4 m by 3 m room has a 12 m² ceiling. Two coats at 11 m² per litre comes to about 2.2 litres. Ceilings usually need both coats because sheen differences are very visible from below.
Does primer count as a coat?
It does the job of one in terms of labour and material, but it is not a finish coat. On bare plaster, wood, or a dark existing colour, priming first usually reduces the number of finish coats needed and often lowers total cost, since primer is cheaper than quality emulsion.
How much extra paint should I buy?
Round up to the next tin size rather than buying a fixed percentage extra. That naturally gives 5% to 15% spare, which covers touch-ups and future repairs. Keeping a part-tin is far easier than trying to match a colour years later.
The Bottom Line
Perimeter times height gives wall area, subtract the large openings, multiply by two coats, and divide by 11 square metres per litre. Then round up to the next tin size, because paint sold in fixed volumes never matches your calculation exactly and the surplus is genuinely useful later. Adjust coverage downward for bare plaster or textured surfaces, and consider a tinted primer whenever you are covering a dark colour, since it usually saves a whole finish coat. Run your room through the paint calculator and buy it all in one go so the batch matches.
Coverage figures are typical values for standard interior emulsion and vary by product, surface, and application method. Always check the manufacturer’s stated coverage on the tin. Last reviewed: August 2026. Recommended editorial review: every 12 months.