How Much Paint Do I Actually Need?
The arithmetic takes thirty seconds, tin coverage figures are optimistic, and the second coat is not negotiable. A guide to buying paint once.
There are two ways to get paint wrong. Buy too little and you make a second trip mid-job, possibly to find a different batch. Buy too much and you’ve spent money on tins that will sit in a cupboard for a decade.
The calculation that avoids both takes about thirty seconds.
The arithmetic
- Perimeter = 2 × (length + width)
- Gross wall area = perimeter × wall height
- Subtract openings: about 1.8 m² per door, 1.5 m² per window
- Multiply by the number of coats
- Divide by coverage: usually 10 m² per litre
For a 4 × 4 m room with 2.4 m ceilings, one door and two windows:
- Perimeter: 16 m
- Gross area: 16 × 2.4 = 38.4 m²
- Openings: 1.8 + (2 × 1.5) = 4.8 m²
- To paint: 33.6 m²
- Two coats: 67.2 m²
- At 10 m²/L: 6.7 litres
So a 5 L tin plus a 2.5 L tin. Buy the 5 L, it’s almost always better value per litre than two 2.5s.
Tin coverage figures are optimistic
The number on the tin assumes a smooth, sealed, previously painted surface, applied by someone who does this professionally. What you actually get:
| Surface | Realistic coverage |
|---|---|
| Previously painted, smooth | 11–13 m²/L |
| Typical painted wall | 10 m²/L |
| Lightly textured | 8–9 m²/L |
| Heavily textured or woodchip | 6–7 m²/L |
| Bare plaster, first coat | 5–7 m²/L |
| Bare wood or MDF | 6–8 m²/L |
Bare plaster is the one that catches people out. It’s porous and drinks paint. The first coat can use nearly double what you budgeted.
New plaster also needs a mist coat first: emulsion thinned with 10–20% water, which soaks in and seals the surface. Skip it and your topcoat sits on top rather than bonding, and peels off in sheets a year later. The mist coat doesn’t count as one of your two coats.
Two coats. Always two coats.
One coat looks acceptable while wet and patchy the next morning in daylight. The second coat is what makes it look like a finished room rather than a job in progress.
Go to three if:
- You’re painting a light colour over a dark one
- You’re using a strong red, yellow or orange. Those pigments have genuinely poor opacity, and three coats is normal
- You’re covering stains, smoke or marker (use a stain-blocking primer instead, or the stain will bleed through however many coats you apply)
If you’re facing three coats of an expensive colour, a tinted primer in a similar shade is usually cheaper and gives a better result than a third topcoat.
Subtracting doors and windows
Do it, but don’t be precise about it. Approximations are fine:
- Standard door: about 1.8 m²
- Average window: about 1.5 m²
- Large window or patio door: 3–4 m²
The point is to avoid buying a whole extra tin for area that doesn’t exist. Deliberately leave the small overage. You want a litre or so spare for touch-ups, and running out three-quarters of the way up a wall is genuinely miserable.
What else goes in the trolley
The paint is the obvious cost. The rest adds up:
- Primer or mist coat for any bare surface
- Woodwork paint: skirting, architrave, doors. A 750 ml tin does the trim in a typical room, twice
- Ceiling paint if you’re doing it, flat finish, hides imperfections, drips less. Same arithmetic on the floor area
- Filler and sandpaper: paint hides nothing and highlights everything
- Dust sheets, masking tape, roller sleeves, a decent brush for cutting in
A cheap roller sleeve sheds fibres into the finish. This is a false economy you will be looking at for years.
The two things that matter more than the paint
Preparation. Filling holes, sanding rough patches, washing off grease and dust. Expensive paint over bad prep looks worse than cheap paint over good prep, and no amount of coats fixes a wall you didn’t fill.
Cutting in. Brush a 5 cm band around edges, corners and the ceiling line, then roller the middle while that band is still wet so the two blend. Let the band dry first and you’ll see a visible ridge where the textures meet, in every light, forever.
Keep the leftovers
Write the room name and date on the lid and keep the part-tin somewhere that doesn’t freeze.
Touch-ups always happen, a scuff from moving furniture, a picture hook in the wrong place. Matching a colour from memory a year later is hopeless, and even the same product from a different batch can differ slightly. Half a litre in the shed saves repainting a whole wall.
Sources
Every factual claim above traces to one of these. Follow them rather than taking this article's word for it.
- Approved Documents (building regulations) UK Government
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