Last updated: October 2026
What the label is actually promising
Manufacturers typically quote between 250 and 400 square feet per gallon, and the test conditions behind that figure are consistent: a smooth, sealed, primed surface, applied by brush or roller at the recommended wet-film thickness, in a single coat, at the recommended temperature and humidity. It is a laboratory coverage rate, and it is honest as far as it goes.
What it does not describe is a bedroom with rolled drywall texture, a colour change from magnolia to charcoal, and the two coats any decorator would actually apply. Each of those moves the number further from the label, and they compound.
Factor one: the second coat
Almost every finish needs two coats. The first flattens the surface and seals it; the second provides the colour and the even sheen. On a colour change from dark to light, three is common. So the first adjustment is simply to double the single-coat figure — and to resist the temptation to stretch one coat by rolling it thinner, which trades a saving of about 40% of a tin for a patchy wall.
Factor two: the surface drinks
New drywall, fresh plaster and bare timber absorb paint and hold it at the same time, so the first coat disappears faster than the label allows for. Texture works the same way: a stippled wall has more surface area than its footprint suggests, and brick, block or stucco more again.
| Surface | Extra paint above the label figure | Why |
|---|---|---|
| Smooth, primed drywall, repaint in the same colour | 0–10% | close to the tested condition |
| Light roller texture (orange peel) | 10–15% | more surface area, plus absorption |
| New, unprimed drywall or fresh plaster | 20–40% on the first coat | absorbent substrate |
| Brick, block, stucco, bare timber | 25–50% | deep texture and high absorption |
| Sprayed application | 15–30% | overspray and bounce-back |
Factor three: openings and trim
Subtracting doors and windows is worth doing, but be realistic about it. A standard interior door is about 21 square feet and a medium window about 15, and each usually has a casing that has to be painted anyway. If you are painting the ceiling and the trim, those are separate areas with their own coverage rates: ceilings take paint like a large flat wall, while trim is usually measured in linear feet and priced in quarts.
A worked example
A 12 × 14 ft bedroom with an 8 ft ceiling, two coats on walls and ceiling:
- Wall perimeter: 2 × (12 + 14) = 52 ft, so wall area = 52 × 8 = 416 sq ft.
- Minus one door (21 sq ft) and one window (15 sq ft) = 380 sq ft of wall.
- Ceiling: 12 × 14 = 168 sq ft.
- One coat over both = 548 sq ft; two coats = 1,096 sq ft.
- At 350 sq ft per gallon: 3.13 gallons. With a 10% texture allowance: 3.44 gallons.
- Buy 4 gallons — the fourth covers the texture allowance, the touch-ups after furniture goes back in, and the inevitable patch where a roller slipped.
Trim for the same room — baseboard, two door casings and one window — comes to roughly 100 linear feet, which at an average 6 inches of painted width is about 50 sq ft, or 100 sq ft across two coats. That is a quart, not a gallon.
When the label really is accurate
There are jobs where the tin's number holds up: repainting a smooth, previously painted wall in the same colour with one refresh coat, or rolling a sealed and primed new-build where the primer has already done the absorbing. In those cases the estimate can land within a few percent. Everything else needs the allowances above.
Practical rules that save a second trip
- Buy one tin more than the arithmetic says if the colour is mixed to order — a different batch can be a shade off, and running out mid-wall is worse than storing a spare.
- Keep the leftovers. Label the lid with the room and the date; a sealed tin of water-based paint keeps for years and pays for itself the first time something needs touching up.
- Primer is not wasted money. One coat of primer over patches and bare drywall usually replaces one coat of paint and does it at a lower cost per litre.
- Do not thin to stretch. Adding water to reach the last few square metres reduces film thickness, and thin film is exactly what shows every imperfection underneath.
Size your own job
Our paint calculator takes room dimensions, the number of coats, door and window counts, ceiling and trim, and applies the allowances above so you get tins rather than guesses. For the parallel problem of flooring and wall coverings, the tile calculator uses the same waste-factor logic. The guides index collects the rest of these write-ups.
Frequently asked questions
How much paint do I need for a 12 by 14 foot room?
With an 8 ft ceiling, the walls come to about 416 sq ft; subtract a door and a window and you have roughly 380 sq ft, plus 168 sq ft of ceiling. Two coats over both surfaces is about 1,096 sq ft, which at 350 sq ft per gallon is 3.1 gallons — so buy 4, or a 5-gallon pail if you are doing more than one room.
Does the second coat use as much paint as the first?
Yes, essentially. The first coat may soak in slightly more on new or porous surfaces, but the second coat still needs a full film thickness, so plan for roughly double the single-coat figure. Skipping the second coat on a colour change is the most common cause of a patchy finish.
How much extra paint do textured walls need?
Add roughly 10% on light texture such as orange peel or a roller stipple, and 20–30% on heavy texture including brick, stucco, concrete block and bare timber. New drywall and fresh plaster behave like heavy texture on the first coat because they absorb paint as well as holding it.
Is primer cheaper than an extra coat of paint?
Usually yes. Primer is formulated to seal and to bond rather than to build colour, so it costs less per litre than a quality topcoat and hides the substrate in one pass. Two coats of paint over bare, patched drywall frequently turns into three; one coat of primer plus two coats of paint gives the finish the manufacturer actually tested.