Paint is sold by the gallon, but rooms are measured in square feet — and one gallon covers roughly 350–400 square feet. The gap between those units is where most painting mistakes happen. Too little and you're back at the hardware store praying the dye lot matches. Too much and you're storing half-used cans in the garage for a decade.
The Basic Calculation
Step 1: Calculate your wall area. Add up the perimeter of the room (total of all wall lengths) and multiply by ceiling height. A 12×10 room with 8-foot ceilings: perimeter = 44 feet, wall area = 44 × 8 = 352 sq ft.
Step 2: Subtract doors and windows. Standard door: 20 sq ft. Standard window: 15 sq ft. Two of each: subtract 70 sq ft → 282 sq ft of paintable surface.
Step 3: Divide by 350 (conservative coverage per gallon) = 0.81 gallons. Round up: 1 gallon for one coat.
💡 Buy one quart extra. Always purchase a quart can beyond your calculated need. Quarts are cheaper than gallons per ounce, and you'll want touch-up paint for scuffs and nail holes for years after painting. Keep it stored upside-down to prevent skin from forming on top.
Coverage Varies by Paint Type
- Flat/Matte: 350–400 sq ft/gallon — hides imperfections, harder to clean
- Eggshell: 350–400 sq ft/gallon — good for living areas
- Satin: 300–350 sq ft/gallon — slight sheen, washable
- Semi-gloss: 250–300 sq ft/gallon — kitchens, bathrooms, trim
- Gloss: 200–250 sq ft/gallon — trim and doors only
Coats of Paint
Most projects need two coats. Going dark to light (or light to dark) can require three. If you're painting over an existing color that's very similar to the new color and using a paint-and-primer-in-one, one coat may suffice. When in doubt, assume two coats and multiply your total accordingly.
Ceiling and Trim Are Separate
Ceiling paint is different from wall paint — it's formulated to prevent spattering and has a flat finish to hide imperfections. Calculate ceiling square footage separately (length × width) and buy dedicated ceiling paint. Trim typically takes a quart or less per room at semi-gloss or gloss finish.