How far a gallon of paint really goes

The tin says 350 square feet per gallon. That figure is measured on smooth, primed, non-porous drywall, rolled by someone who does it for a living, and almost nobody is painting that wall.

Everything that makes a real wall interesting takes a bite out of it.

What eats the coverage

Texture is the big one. A lightly textured wall has more actual surface than its measured area suggests, because the paint has to coat every peak and valley. Orange peel costs you roughly 10 to 15%, and a heavy knockdown or popcorn ceiling can take 30% or more. The wall still measures 120 square feet, but the paint behaves as though it were 150.

Porosity is the second. Fresh drywall, bare plaster, new joint compound and raw wood all drink the first coat. That is what primer exists to stop, and skipping it on a porous surface does not save a gallon, it just moves the gallon into the first coat of finish paint.

Dark to light is a coat, not a nuisance

Covering a strong colour with a pale one is the one that catches people out, because it is invisible until the second coat goes on and the old colour is still faintly there.

Red, deep blue and anything approaching black often need three coats of a light colour, sometimes four, and the honest fix is a tinted primer rather than another finish coat. A grey-tinted primer under a white is the standard trade answer and it turns a four-coat job into a two-coat one.

Sheen changes the number slightly

Flat and matt paints cover marginally better per gallon than gloss, because they are more heavily pigmented and less self-levelling. The difference is small, usually within the rounding when you are buying in whole gallons, so it is not worth planning around. It is worth knowing when a job comes out half a gallon short.

Working from your own walls

Measure the walls rather than the floor. Perimeter times ceiling height gives wall area, and that is the number that matters, not the square footage of the room, which is what people reach for first and which bears no fixed relationship to it.

A 12 by 14 foot room with 8 foot ceilings has 416 square feet of wall, not 168. At 350 square feet per gallon that is 1.2 gallons for one coat, or around 2.4 for two, which means buying three gallons or two gallons and a quart. The paint calculator does this with doors and windows deducted and the ceiling added if you want it, and it will take a coverage figure other than 350 if your tin says something different.

Then add 10% on top. Not for the wall, for everything else. What stays in the tray, what soaks into the roller sleeve, the bit left in the tin that skins over before the touch-ups, and the second attempt at the cutting-in you were not happy with.

Buy the whole job at once

Paint is mixed in batches, and two tins of the same colour from different batches can differ enough to see in daylight on a long wall. If the job needs three gallons, buy three gallons now rather than two and a top-up next weekend.

If you do end up mixing batches, box them. Pour all the tins into one bucket, stir, and work out of that. It averages any difference across the whole job instead of leaving a visible line where one tin ran out.

The number that actually decides the day

Coverage sets how much you buy. Surface preparation sets how long the job takes, and the two are not related.

A wall that needs filling, sanding and spot-priming is a two day job whatever the paint covers, and a clean previously painted wall in a similar colour is an afternoon. People plan around the gallons because the gallons are the number printed on the tin, then lose the weekend to the part nobody calculated.