HomeChiselPaint Calculator — How Much Paint Do I Need?
Calculation breakdown
How much paint do I need for a room?
The standard formula used by professional painters: measure the room perimeter (2 × length + 2 × width), multiply by wall height to get gross wall area, then subtract 21 sq ft per door and 15 sq ft per window. Divide the remaining paintable area by 350 sq ft (the realistic coverage per gallon confirmed by Sherwin-Williams and Benjamin Moore), multiply by the number of coats, and add 10% for touch-ups. For a typical 12×12 ft bedroom with 8-ft ceilings, 1 door, and 2 windows: the net paintable area is 333 sq ft, requiring approximately 2 gallons for 2 coats.
Most paint manufacturers claim 400 sq ft per gallon on the can label. This is measured under ideal lab conditions on perfectly smooth, primed surfaces. Real-world coverage on typical residential drywall is closer to 350 sq ft per gallon. Using 350 prevents the most common painting mistake: running out of a custom-mixed color mid-project. If this happens, the replacement batch may not match exactly due to color variation between mixing runs.
How many coats of paint do I need?
For a same-color or similar-shade refresh, 2 coats delivers a professional finish with complete coverage. For moderate color changes (light gray over beige), 2 coats is usually sufficient. For drastic changes — painting dark walls white, or any dramatic light-over-dark transition — apply 1 coat of tinted primer first, then 2–3 coats of paint. The primer creates a uniform base that reduces the total number of paint coats needed, saving both paint and money on large projects.
How much does it cost to paint a room?
DIY paint costs range from $50–$170 for a standard 12×12 ft bedroom depending on paint quality. Budget paint (Behr flat, store brands) runs $25–30 per gallon. Mid-range paint (Behr Premium Plus, Valspar Signature) costs $35–45 per gallon. Premium paint (Benjamin Moore Regal Select, Sherwin-Williams Duration) runs $50–60 per gallon. Professional painting costs $2–6 per sq ft including labor and materials, or $300–800 for a typical bedroom.