Room size to litres and gallons, with doors and windows deducted and the number of tins you actually have to buy.
Wall area is the room's perimeter multiplied by the height — that is 2 × (length + width) × height. It is not length × width, which is the floor. A 14 × 12 ft room with 9 ft walls has a perimeter of 52 ft and therefore 468 sq ft of wall.
From that, deduct the openings. A standard interior door is about 20 sq ft and an average window about 15. One door and two windows takes 50 sq ft off, leaving 418 sq ft. At two coats that is 836 sq ft to cover, which at 350 sq ft per gallon comes to 2.39 gallons, or about 9 litres.
The standard figure is 350 to 400 sq ft per US gallon, or roughly 10 to 12 m² per litre, for one coat on a smooth, previously painted surface. The tin will state its own figure and that is the one to use — coverage varies considerably by product.
It also varies by surface, always downward. New drywall, bare plaster and anything textured absorbs noticeably more — 25% more is a reasonable allowance, and heavily textured surfaces can be worse. Priming first is not just about adhesion; it seals a porous surface so the topcoat spreads at its rated coverage instead of soaking in.
One coat almost never looks finished. Even repainting the same colour, a single coat tends to leave patchy sheen and roller lap marks visible in raking light. Two is the working standard.
Three is realistic when you are going light over dark, covering a strong or saturated colour, painting bare plaster or new drywall, or using a deep base tint — reds and yellows in particular have poor opacity and routinely need three. A tinted primer in a shade close to the topcoat often saves a coat and is cheaper than the extra paint.
Tinted paint is mixed to order, and batch-to-batch variation is real. Two tins mixed at different times can differ enough to show as a seam on a long wall. If you need more than one container, buy them together and — for a large job — "box" them: pour all the tins into one large bucket and remix, so any variation is spread evenly rather than appearing as a line where you switched tins.
Keep a labelled leftover for touch-ups, and write the colour code and finish on the tin. Trying to match a wall colour years later without the code is a losing exercise.
| Job | Typical area | Two coats |
|---|---|---|
| Small bedroom, 10 × 10 ft | ~320 sq ft walls | ~2 gal / 7.5 L |
| Average bedroom, 12 × 14 ft | ~420 sq ft walls | ~2.5 gal / 9.5 L |
| Living room, 15 × 20 ft | ~600 sq ft walls | ~3.5 gal / 13 L |
| Ceiling only, 12 × 14 ft | 168 sq ft | ~1 gal / 3.8 L |
| Interior door, both sides | ~40 sq ft | Under 0.5 L |
Matt hides surface imperfections best and is the usual choice for ceilings and low-traffic walls, but it marks easily and does not wash well. Eggshell and satin are the practical compromise for most rooms. Semi-gloss and gloss are hard-wearing and wipeable, which is why they go on trim, doors, kitchens and bathrooms — but they reflect light, so every dent and filler mark shows.
Sheen also affects apparent colour: the same tint looks lighter in gloss than in matt, because more light bounces back. Test on the actual wall rather than judging from a chip, and look at it in both daylight and artificial light before committing to a full room.
Multiply the perimeter by the height: 2 × (length + width) × height. A 14 by 12 ft room with 9 ft walls has a 52 ft perimeter and 468 sq ft of wall. Then subtract the openings — roughly 20 sq ft per interior door and 15 per average window. Do not use length × width, which gives the floor area and will substantially underestimate what you need.
Around 350 to 400 square feet for one coat on a smooth, previously painted surface — about 10 to 12 square metres per litre. Always check the tin, since it varies by product. Expect noticeably less on new drywall, bare plaster or textured surfaces, where 25% more is a sensible allowance. Priming first seals a porous surface so the topcoat achieves its rated coverage.
Almost always. A single coat usually leaves patchy sheen and visible roller lap marks even when repainting the same colour. Three coats is realistic when going light over dark, covering a saturated colour, painting bare plaster, or using reds and yellows, which have poor opacity. A tinted primer close to your topcoat colour often saves a coat and costs less than the extra paint would.
Yes. Tinted paint is mixed to order and batches vary slightly, enough that two tins mixed at different times can show as a seam across a long wall. Buy the full quantity together, and on a large job pour all the tins into one bucket and remix — "boxing" — so any variation is spread out invisibly rather than appearing at the point where you opened the second tin.
Matt for ceilings and low-traffic walls, since it hides imperfections best but marks easily. Eggshell or satin for most rooms, as a practical balance of durability and appearance. Semi-gloss or gloss for trim, doors, kitchens and bathrooms, where washability matters — though the higher the sheen, the more every dent and filler mark shows. Sheen also changes apparent colour, so the same tint reads lighter in gloss than in matt.
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