How paint math works
Paintable area is wall length × height, minus about 20 sq ft per door and 15 sq ft per window. Divide by coverage (350 sq ft per gallon is the standard interior figure) and multiply by coats, then round up. Two coats is the honest default for a colour change — one coat rarely covers evenly.
Tips that save a gallon
- Prime first on bare drywall, patched areas, or a dark-to-light change — primer is cheaper than a third coat of paint.
- Measure the ceiling separately if you are painting it: length × width, same coverage math.
- Buy all gallons at once for one colour, so the tint matches across the room.
Frequently asked questions
Does one gallon really cover 350 sq ft?
On smooth, primed walls, yes — that is the figure most interior paints state. Rough, textured, or bare surfaces absorb more and cover less.
How much paint for a 12×12 room?
Four 8-foot walls total 48 feet of wall, or 384 sq ft. Minus a door and two windows, about 334 sq ft — one gallon per coat, so two gallons for two coats.