Every tin claims about 400 ft² per gallon. That figure is measured on smooth, sealed, primed drywall — the easiest surface there is.
| Surface | First coat | Second coat |
|---|---|---|
| Primed drywall | 375 | 431 |
| Previously painted, sound | 400 | 460 |
| New unprimed drywall | 320 | 400 |
| Smooth plaster | 400 | 460 |
| Light texture (orange peel) | 325 | 374 |
| Heavy texture (knockdown) | 275 | 316 |
| Popcorn ceiling | 200 | 240 |
| Bare wood | 300 | 360 |
| Previously painted wood trim | 400 | 460 |
| Smooth fibre cement | 350 | 400 |
| Lap siding | 275 | 316 |
| T1-11 plywood | 200 | 230 |
| Stucco | 160 | 200 |
| Bare brick | 125 | 160 |
| Concrete block | 150 | 190 |
| Bare metal | 400 | 460 |
The second coat goes about 15% further
Once the surface is sealed, paint spreads further because it is no longer soaking in. Two coats is roughly 1.87× one coat, not 2×.
Doubling the first-coat figure is the most common over-buy in painting, and the surplus separates in the garage.
The effect is largest on the most absorbent surfaces — on bare brick the second coat can spread 30% further than the first, because the first coat did the sealing.
Three things that change the number
Application method. A roller is the baseline. Brushing uses roughly 10–15% more on the same area because the film is heavier. Spraying without back-rolling uses 20–40% more, because overspray and a heavier wet film both cost paint — which is why spraying is chosen for speed and finish, not economy.
Colour change. Coverage per coat does not change; the number of coats does. Light over dark is three coats, or two over a tinted primer. Deep reds and yellows are notorious for needing three coats regardless, because the pigments are transparent.
Temperature and thinning. Paint applied hot goes on thinner and covers more area with a weaker film. Paint thinned to spray covers more area per gallon of paint but not per gallon of mixed material, and the film is thinner — so it is not a saving.
Working out the gallons
area ÷ first-coat rate + area ÷ second-coat rate = gallons
A 365 ft² wall on primed drywall: 365 ÷ 375 + 365 ÷ 431 = 0.97 + 0.85 = 1.8 gallons.
Gallons by room size, ceiling height, coats and openings does this for every combination, and the exterior version by siding type does the same outdoors.
Where the label is not wrong
On a previously painted, sound, smooth interior wall going a similar colour, 400 ft² per gallon is achievable. That is the case the label describes, and it is a common one — most repaints are exactly that.
The label is misleading only when it is applied to a surface it does not describe, which is what happens when people compare two products on their coverage claims.
Buy in round gallons and box them
Paint keeps for years sealed and running out mid-wall costs a trip and shows as a line.
Pour every tin of the same colour into one bucket and stir before starting. Batches differ slightly, and the change shows exactly where you stopped — the single most visible difference between a professional job and a careful amateur one.
Ceilings are their own calculation
A ceiling is length × width — no perimeter arithmetic, no openings to deduct beyond light fixtures, and usually one colour throughout.
The surface, though, is often not drywall. A popcorn or acoustic ceiling takes roughly 200 ft² per gallon on the first coat, half what a flat ceiling takes, because the texture is deep.
And it must be sprayed or rolled with a very long nap, because rolling a popcorn ceiling with a standard nap pulls the texture off in sheets. If the ceiling has never been painted, test a corner before committing to the method.
Wastage that never reaches the wall
The published spread rates assume all the paint ends up on the surface. In practice some does not:
- In the tray and roller at the end: a few percent, more on a small job
- In the brush for cutting-in: absorbed and washed out
- Overspray, if spraying: 20–40%
- Left in the tin: the last quarter-inch that will not pour
Add roughly 5% to any calculated figure for a rolled interior job, and buy in whole gallons on top of that. Paint keeps; a second trip does not come back.