Cost data reviewed August 2026. Sources listed at the foot of this page.
The arithmetic, in full
Paint coverage is one of the few home improvement numbers that is genuinely simple, and it is worth knowing the steps so you can sanity-check any quote:
- Perimeter = 2 × (length + width)
- Gross wall area = perimeter × ceiling height
- Subtract openings — about 21 sq ft per door, 15 sq ft per average window
- Multiply by coats — two is standard
- Divide by 350 sq ft per gallon, then round up
A 14 × 12 ft room with 9 ft ceilings: perimeter 52 ft, gross area 468 sq ft, less one door and two windows (51 sq ft) leaves 417 sq ft. Two coats is 834 sq ft of coverage, which is 2.4 gallons — so you buy three.
Why 350 square feet per gallon
Published coverage runs 300 to 400 square feet per gallon and manufacturers quote the top of that band. We use 350 because the higher figure assumes smooth, primed, previously painted drywall and an experienced hand with a roller. Bare drywall drinks paint. Textured walls can consume 20 to 50 percent more. Deep colors, and any color going over a much darker one, may need a third coat regardless of what the can says.
Buy a little more than you calculate, and buy it in one batch. Paint is mixed in lots and two gallons of the same color from different lots can differ enough to see on a long wall. The leftover is also your touch-up supply for the next five years, which is worth more than the $30 it cost.
DIY versus hiring out
Painting has the widest DIY-to-professional gap of any job on this site, because labor is 70 to 85 percent of a professional painting bill. Materials for that 14 × 12 room are perhaps $150. A painter will charge $800 to $1,300.
What you are buying for the difference is preparation and edges. Filling, sanding, caulking, masking and cutting in a clean line at the ceiling is most of the work and all of the skill. If a quote seems high, ask specifically what preparation is included — surface prep alone runs $0.50 to $0.75 per square foot, and a cheap quote is often a quote with no prep in it.
Paint grade is worth more than it looks
| Grade | Per gallon | Where it makes sense |
|---|---|---|
| Builder grade | $15 – $22 | Rentals, ceilings, closets, a room you will redo soon |
| Mid-range | $25 – $40 | Most rooms — the practical default |
| Premium | $45 – $60 | Kitchens, bathrooms, hallways, trim, deep colors |
The premium is small in absolute terms — three gallons at $50 rather than $32 is a $54 difference on a job where labor is the real cost. Better paint covers in fewer coats, which can pay for itself outright, and it scrubs without burnishing, which matters in any room where people put their hands on the walls.
Common questions
How many gallons of paint for a 12x12 room?
A 12 × 12 room with 8 ft ceilings has 384 sq ft of gross wall. Less one door and one window, about 348 sq ft. Two coats needs 696 sq ft of coverage, so 2 gallons at 350 sq ft per gallon — buy 3 if the walls are bare, textured, or you are going over a darker color.
Does one gallon really cover 400 square feet?
On smooth, primed, previously painted drywall with a good roller, close to it. On bare drywall, textured walls or a rough surface, expect closer to 250 to 300. We calculate at 350 as a realistic middle.
Do I need primer?
Yes if you are painting bare drywall, covering a dark color with a light one, painting over glossy or oil-based paint, or sealing stains. No if you are refreshing a similar color on sound, previously painted walls — a good paint-and-primer product is adequate there.
How much does it cost to paint a whole house interior?
The national average for interior painting is around $2,000, with a typical range of $350 to $5,800 depending on square footage and whether trim and ceilings are included. Walls-only runs about $2.75 per square foot; adding trim and ceilings takes it to roughly $4.70.