How many square feet does a gallon of paint cover?

Plan on 350 to 400 square feet for one coat on a prepared, previously painted surface. Heavy texture, bare drywall and porous masonry all pull that number down.

How much paint do I need for a 12x12 room?

With 8 ft ceilings and a couple of openings, about 340 sq ft of net wall. Two coats is roughly 1.8 gallons — so two gallons, or three if you want a touch-up can.

Do I subtract windows and doors?

Yes, once you have more than one or two. Each door is around 21 sq ft and each window around 15. On a small room, leaving them in is a useful safety margin; on a whole house it is a wasted gallon.

Is primer really necessary?

On new drywall, bare substrate, stains, or a dark-to-light change, yes. Over a sound painted wall in a similar colour, no — a second topcoat does more.

How many coats of paint do I need?

Two, in almost every case. One coat only works when you are repainting the identical colour with the identical sheen. Deep reds and yellows sometimes want three regardless of primer.

Should I buy 5-gallon pails or single gallons?

Pails, when the job needs more than three gallons. It is cheaper per gallon and it eliminates batch-to-batch colour variation across a wall.

Paint Calculator

Enter the room size, the ceiling height and how many doors and windows there are, and this calculator returns gallons for the walls, the ceiling and the trim separately, plus primer where the surface calls for it. It uses an honest spread rate, not the number on the can.

How are you measuring?

Room length(ft)

Room width(ft)

Known wall area(sq ft)

Used only when the option above is set to a known wall area.

Ceiling height(ft)

Doors

Windows

Coats on the walls

Two is standard. One coat only works over the same colour.

Surface condition

Paint the ceiling?

Paint the trim and doors?

Coverage per gallon(sq ft)

350–400 is realistic. The can usually claims 400+.

Wall paint (2 coats)

2.25gal

396 sq ft of wall after openings

Ceiling paint (2 coats)168 sq ft of ceiling: 1.00gal
Trim and door enamel118 linear ft of trim and casing: 0.75gal
PrimerSound painted surface, same colour family: Not needed
Buy as: 3 × 1 gal cans
Estimated material costPaint and primer only: $218

What this figure includes

Paint volume only. It excludes tape, plastic, brushes, rollers, patching compound and labour. Doors are counted at 21 sq ft and windows at 15 sq ft — adjust the counts if yours are much bigger.

How this is calculated

The formula

Everything here comes from one line:

Gallons = (surface area × coats) ÷ coverage per gallon

The surface area for walls is the room perimeter times the ceiling height, minus the openings:

Perimeter = 2 × (length + width)
Gross wall area = perimeter × ceiling height
Net wall area = gross − (doors × 21) − (windows × 15)

Ceilings are simply length × width. Trim is a linear-foot measure converted to area: baseboard around the perimeter, plus about 17 ft of casing per door and 16 ft per window.

Worked example

A 14 ft × 12 ft bedroom with 9 ft ceilings, two doors and two windows, two coats over sound painted walls.

1. Perimeter = 2 × (14 + 12) = 52 ft 2. Gross wall = 52 × 9 = 468 sq ft 3. Openings = (2 × 21) + (2 × 15) = 72 sq ft 4. Net wall = 468 − 72 = 396 sq ft 5. Two coats = 396 × 2 = 792 sq ft of coverage needed 6. At 375 sq ft per gallon = 792 ÷ 375 = 2.1 gallons

Buy a 5-gallon pail if you are also doing the hall, or three gallons if you are not. The ceiling adds 14 × 12 = 168 sq ft, two coats, about 0.9 gallons.

Coverage: what the can says and what happens

Labels commonly claim 400 square feet per gallon. That figure comes from a controlled spread on a smooth, sealed, non-absorbent panel. On a real wall — knockdown texture, a patch or two, a roller cover that holds paint, and a person who does not want holidays — 350 to 400 is the honest range, and heavy texture drops it below 300.

Plan on 375 and you will not be driving back to the store. The half-gallon left over is worth having anyway; it is the touch-up can, and it is the correct colour from the correct batch.

When you need primer, not a third coat

Three situations where primer is the right answer and more topcoat is not:

New drywall. Fresh paper and joint compound absorb at different rates. Paint straight onto it and the joints flash — they look duller than the field, at a slightly different sheen, and no number of topcoats fixes it because the substrate is still uneven. Drywall primer (a PVA sealer) equalises the surface. It is cheap and it is the whole difference between a good job and a job that looks fine until the light rakes across it.

Dark to light. Going from a deep red or navy to an off-white takes four coats of topcoat, or one coat of grey-tinted primer and two coats of finish. The second option is cheaper, faster, and finishes flatter.

Bare wood, plaster, masonry, or any stain. Water stains, smoke and tannin bleed through latex indefinitely. They need a stain-blocking primer, usually shellac or oil-based. Latex primer will not hold them back.

Buy the big container

Paint is tinted in batches, and two gallons of the same colour code mixed on different days can differ enough to see on a long wall in raking light. Buying a single 5-gallon pail instead of five gallons in separate cans removes that risk entirely. If you do end up with separate cans, box them — pour them all into a clean 5-gallon bucket and stir — before you start.

One more practical note: cut in and roll the same wall in the same session. Cut-in edges that dry before the roller reaches them leave a visible band, because brushed and rolled paint dry to slightly different textures.

Cost figures last reviewed 9 Sept 2026 · US retail prices for mid-grade interior latex, primer and trim enamel, with the manufacturer coverage claim discounted to a real-world spread rate.

Frequently asked questions

How many square feet does a gallon of paint cover?: Plan on 350 to 400 square feet for one coat on a prepared, previously painted surface. Heavy texture, bare drywall and porous masonry all pull that number down.
How much paint do I need for a 12x12 room?: With 8 ft ceilings and a couple of openings, about 340 sq ft of net wall. Two coats is roughly 1.8 gallons — so two gallons, or three if you want a touch-up can.
Do I subtract windows and doors?: Yes, once you have more than one or two. Each door is around 21 sq ft and each window around 15. On a small room, leaving them in is a useful safety margin; on a whole house it is a wasted gallon.
Is primer really necessary?: On new drywall, bare substrate, stains, or a dark-to-light change, yes. Over a sound painted wall in a similar colour, no — a second topcoat does more.
How many coats of paint do I need?: Two, in almost every case. One coat only works when you are repainting the identical colour with the identical sheen. Deep reds and yellows sometimes want three regardless of primer.
Should I buy 5-gallon pails or single gallons?: Pails, when the job needs more than three gallons. It is cheaper per gallon and it eliminates batch-to-batch colour variation across a wall.

How to measure square footageMeasure length times width, split odd shapes into rectangles, and add. The full method, wh…