How this calculator works. Wall area = 2 × (length + width) × ceiling height, minus 21 sq ft per standard door and 15 sq ft per standard window. We divide paintable area by your paint's coverage rate and multiply by coats.
The default coverage of 350 sq ft per gallon is the industry-standard figure published by major manufacturers including Sherwin-Williams and Behr for smooth, previously painted surfaces. Textured, porous, or bare surfaces cover less — drop the rate to 250–300 in advanced settings.
How much paint do I actually need?
A gallon of quality interior paint covers about 350 square feet with one coat on a smooth, sealed surface. A typical 12×10 bedroom with 8-foot ceilings has roughly 320 square feet of paintable wall after subtracting a door and two windows — which means one gallon per coat, and almost every project needs two coats for even color.
The most common mistake is buying for one coat. The second most common is ignoring surface texture: fresh drywall, plaster repairs, and textured finishes drink up 20–40% more paint. When you're covering a dark color with a light one, budget for a tinted primer coat instead of hoping a third coat of finish paint will do it — primer is cheaper per gallon and hides better.
Typical coverage by surface
| Surface | Coverage per gallon |
|---|---|
| Smooth, previously painted drywall | 350–400 sq ft |
| Lightly textured walls | 300–350 sq ft |
| New/bare drywall (after primer) | 300–350 sq ft |
| Heavy texture, stucco, masonry | 200–300 sq ft |
Buying tips
Round up to the next quart, not the next gallon — most brands sell quarts, and a quart covers about 90 square feet. Keep the leftover labeled for touch-ups. If your total lands within a quart of a full gallon, buy the gallon anyway: per-ounce it's cheaper, and running out mid-wall leaves visible lap marks.
Common questions
Do I subtract baseboards and trim?
No — trim height is negligible for wall estimates. Trim itself is painted with different paint (usually semi-gloss) and is estimated by linear feet, not area.
How much paint for a ceiling?
Ceiling area is simply length × width. Check the "Ceiling" box above and it's added to your total. Ceilings usually need only one coat if you're repainting white on white.
Does one coat ever work?
Only when repainting the same color with premium "one-coat" paint on smooth walls. Any color change, sheen change, or budget paint needs two coats for even coverage.
How accurate are the door and window deductions?
We use US standard sizes: 21 sq ft for a 3'0"×7'0" door and 15 sq ft for an average double-hung window. If your windows are unusually large, use the wall-area override in advanced settings.