How Much Paint Do I Need? A Room-by-Room Calculation Guide

Guessing Costs You Twice

Buy too little paint and you are back at the shop mid-job, hunting for a tin from a different batch that may not quite match. Buy too much and you have half a tin of a colour you will never use again slowly skinning over in the garage. Both mistakes come from the same place: nobody measures the walls.

The good news is that working out how much paint you need takes four steps and about two minutes, and the maths is the same whether you work in metres or feet.

Step 1: Measure the Wall Area

You do not need to measure each wall separately. Add up the length of every wall to get the room's perimeter, then multiply by the ceiling height:

Wall Area = Perimeter × Height

For a simple rectangular room, the perimeter is easier still:

Perimeter = 2 × (Length + Width)

A 4 m × 3 m room has a perimeter of 2 × (4 + 3) = 14 m. At a 2.4 m ceiling height, that is 14 × 2.4 = 33.6 m² of wall.

Step 2: Subtract the Doors and Windows

You are not painting the glass, so take those areas back off. Rather than measuring every opening, these standard allowances are close enough for buying purposes:

Standard door: roughly 1.9 m² (about 20 sq ft)
Average window: roughly 1.4 m² (about 15 sq ft)

Our room has one door and two windows: 1.9 + (2 × 1.4) = 4.7 m² to deduct. That leaves 33.6 − 4.7 = 28.9 m² of paintable wall.

If your windows are unusually large, measure them properly — a set of patio doors is nothing like 1.4 m² and will throw the estimate out badly.

Step 3: Multiply by the Number of Coats

This is the step people forget, and it is the one that doubles the answer. Paint coverage figures are quoted per coat, and almost every job needs two.

28.9 m² × 2 coats = 57.8 m² of surface to cover.

Step 4: Divide by the Coverage on the Tin

Litres Needed = Total Area to Cover ÷ Coverage per Litre

At a typical 12 m² per litre: 57.8 ÷ 12 = 4.8 litres. Round up to the next tin size and you are buying a 5 litre tub.

Always round up. Paint is sold in fixed sizes, and a job that needs 5.2 litres needs a 5 litre tin plus a 2.5 litre one.

How Far Does a Litre Actually Go?

Coverage varies more than most people expect, and the figure printed on your tin always beats a rule of thumb. As a guide, per litre per coat:

Matt emulsion: 12–14 m²
Vinyl silk emulsion: 12–14 m²
Gloss or satinwood on trim: 12–16 m²
Primer or undercoat: 10–12 m²
Masonry paint: 6–8 m²

Rough, textured, and porous surfaces drink paint. Bare masonry, artex, and heavily filled walls can pull coverage down by a third or more, so take the lower end of the range whenever the surface is not smooth.

When Two Coats Is Not Enough

Budget for a third coat when:

You are going from a dark colour to a light one — deep reds and navy blues are the worst offenders.
You are using a strong, saturated colour, especially bright yellows and reds, which have poor natural opacity.
You are covering stains, marker, or water damage, which need a stain-blocking primer first.

A tinted primer close to your final colour will often save you an entire finish coat, which is cheaper than a third tin of premium emulsion.

Bare Plaster Needs a Mist Coat

New plaster is porous and will suck a normal coat of emulsion straight in, leaving a patchy finish that peels later. The standard fix is a mist coat: matt emulsion thinned with water, usually around 70% paint to 30% water, applied as the first coat so it soaks in and seals the surface.

Count the mist coat as one of your coats when you calculate quantities. Fresh plaster typically needs the mist coat plus two full coats, so multiply your paintable area by three, not two.

Ceilings and Floors

Ceilings and floors are simpler, because there is nothing to deduct:

Area = Length × Width

Our 4 m × 3 m room has a 12 m² ceiling. Two coats is 24 m², which at 12 m² per litre needs just 2 litres.

The same calculation works for floor coverings, laminate, and tiles — though for those you should add 10% for cuts and waste, which paint does not need.

Working in Feet and Gallons

The method is identical in imperial units. A 20 ft × 15 ft room with 8 ft ceilings has a perimeter of 70 ft and 560 sq ft of wall. Deduct one door and two windows (20 + 30 = 50 sq ft) and you have 510 sq ft, or 1,020 sq ft across two coats. At 350 sq ft per gallon, that is 3 gallons.

Note that 350 sq ft per gallon works out at about 8.6 m² per litre — noticeably more conservative than the 12 m² per litre quoted on most UK emulsion. That is deliberate; it builds in a margin. If you would rather trust the tin, enter your own coverage figure instead of the default.

Before You Head to the Checkout

Round up to whole tins. Two 2.5 litre tins usually cost more than one 5 litre tub of the same paint.
Buy it all at once, and check the batch numbers match. Mixed batches of the same colour can dry very slightly different, which shows badly on a long wall.
Keep the leftovers. A sealed half-litre is your touch-up supply for the next few years.
Do not forget the trim. Skirting, architrave, and doors are a separate paint, a separate tin, and a separate calculation.

Conclusion

Perimeter times height, minus the doors and windows, times the number of coats, divided by the coverage on the tin. That one chain of arithmetic answers the question for any room in the house, in either measurement system. Run your dimensions through a Paint Coverage Calculator to have the deductions, coats, and rounding handled in one go — and measure the awkward rooms rather than eyeballing them.

Try our Paint Coverage Calculator

Related Calculators: Tiling/Flooring Calculator