Room size in, gallons out — with doors and windows subtracted and the second coat counted, so the color matches wall to wall.
Room details
ft
ft
ft
Paint to buy
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Wall area (openings subtracted)—
Ceiling paint (if selected)—
Primer for bare/patched walls—
Based on 350 sq ft per gallon per coat — the standard for most interior paints on smooth walls. Textured walls and drastic color changes drink more; check your can’s label.
How paint coverage works
Interior paint covers about 350 square feet per gallon per coat on smooth, previously painted walls. The takeoff is perimeter times height, minus roughly 20 sq ft per door and 15 per window — then multiplied by your coat count, which for a real color change is almost always two.
gallons = (2(L+W) × H − openings) × coats ÷ 350
Worked example
A 14 × 12 room with 8-ft walls, one door and two windows: 416 − 50 = 366 sq ft of wall. Two coats is 732 sq ft ÷ 350 = 2.1 gallons — buy two gallons and a quart, or round to three if the color change is dramatic. Adding the ceiling (168 sq ft × 2 coats) is another gallon.
Buy it all at once
Paint is tinted per batch, and batch-to-batch variation is real. Buying all your gallons together (or having the store box-mix them) is the difference between an invisible wall and a faint stripe where can two started. Leftover paint is touch-up insurance — running out mid-wall is the expensive outcome.
Frequently asked questions
How much does one gallon of paint cover?
About 350 sq ft per coat on smooth, sealed walls — the number most manufacturers print. Textured surfaces, bare drywall, and deep color changes can drop that to 250–300.
Do I really need two coats?
For any real color change, yes — one coat almost always shows roller marks and uneven sheen. One-coat claims hold up only when repainting a similar color on well-prepped walls.
Should I use primer?
On bare drywall, patched spots, stains, or when going from dark to light: yes. Primer is cheaper than paint and covers about 300 sq ft per gallon, so priming first often reduces total cost.