How Much Paint Do You Need for a Room? The Complete Calculation
One gallon covers 350-400 square feet. But walls are not just length times height — here is the full math.
A gallon of paint covers 350-400 square feet with one coat. Most rooms need two coats. A 12×12 room with 8-foot ceilings has about 384 square feet of wall space — one gallon per coat, two gallons total. But you need to subtract windows and doors and add ceiling if painting that too.
The Calculation
Wall area: perimeter × height = total wall square footage. Subtract: each standard door (21 sq ft) and each standard window (15 sq ft). Multiply by 2 for two coats. Divide by 350 to get gallons needed. Always round up — buying an extra quart is cheaper than a second trip to the store mid-project.
Primer: When You Need It
New drywall: always. Dark to light color change: always. Over stains or patches: always. Same color refresh: skip primer (one coat may suffice). Using primer when needed saves paint — primer is cheaper and creates a uniform base so your finish coat covers evenly in fewer coats.