The "400 square feet per gallon" on the tin is measured on smooth, primed drywall. That is the best case, and it is rarely the case.
Spread rate belongs to the surface, not the paint.
| Surface | ft² per gallon |
|---|---|
| Primed drywall | 375 |
| New unprimed drywall | 320 |
| Previously painted, sound | 400 |
| Light texture (orange peel) | 325 |
| Heavy texture (knockdown) | 275 |
| Bare wood | 300 |
| Smooth plaster | 400 |
| Rough plaster | 275 |
Choosing paint by the label's coverage claim compares products on a number that describes a wall you do not have.
The arithmetic
wall area = perimeter × ceiling height minus openings ÷ spread rate = gallons per coat
A 12 × 14 room with 8 ft ceilings: perimeter 52 × 8 = 416 ft². One door (21 ft²) and two windows (15 ft² each) = 51 ft². So 365 ft² of wall.
- First coat on primed drywall: 365 ÷ 375 = 0.97 gallons
- Second coat: spreads about 15% further, so 365 ÷ 431 = 0.85 gallons
- Total: 1.8 gallons → buy 2
Gallons by room size, ceiling height, coats and openings does this for every combination, including the ceiling.
The second coat is cheaper than the first
Once the surface is sealed, paint spreads about 15% further. Two coats is 1.87× one coat, not twice.
Doubling the first-coat figure is the most common over-buy in painting, and the surplus sits in the garage until it separates.
Deduct openings properly
Deduct doors and windows at their full area — unlike drywall, you are not cutting paint around them and wasting it.
But do not deduct small openings. Anything under about 15 ft² is offset by the extra cutting-in around its frame, which uses paint slowly and carefully rather than quickly.
Primer is a coat, and sometimes it is two coats saved
New drywall needs primer for a reason that is not adhesion: paper and joint compound absorb at different rates. Paint straight onto unprimed board dries at different sheens over the joints and the field, and every taped seam shows through in the right light.
A gallon of drywall primer costs less than the extra finish coat it replaces.
Over a dark colour, a primer tinted toward the new colour is almost always cheaper and faster than three finish coats. The full case is here.
Trim does not scale with the room
Doors, trim and ceilings are separate calculations, and trim is linear-foot work.
A room that takes three gallons of wall paint usually takes under a quart of trim enamel — and about three times as long to cut in. That imbalance is most of why painting is priced the way it is.
Box the paint before you open a roller
Pour every tin of the same colour into one bucket and stir. Tins from different batches differ slightly, and a wall painted from two of them shows the change exactly where you stopped for lunch.
This costs nothing and is the single most visible difference between a professional job and a careful amateur one.
Buy the round-up
Gallons are sold in whole units and paint keeps for years sealed. Running out mid-wall means a second batch, a visible line, and a trip.
Round up, and keep the remainder labelled with the room name and the date.
The ceiling is a separate line
Length × width, no perimeter arithmetic, and almost nothing to deduct. A 12 × 14 room is 168 ft² of ceiling — about half a gallon per coat on flat drywall.
Ceiling paint is a different product: flatter, thicker, formulated to spatter less and to hold a wet edge longer so lap marks do not show. It is worth using rather than the wall paint, and it is why a room takes three tins rather than two.
A popcorn or acoustic ceiling takes roughly 200 ft² per gallon, half a smooth one, and it wants spraying or a very long nap roller — a standard roller pulls the texture off. Test a corner before committing.