How Much Paint Do You Actually Need? A Room-by-Room Guide

Paint coverage is usually measured in square feet per gallon, and most cans say 350–400 sq ft. To find your wall area, measure the perimeter of the room (add up all four walls) and multiply by the ceiling height. That gives you total wall area before any deductions.

A 12×10 room with 9-foot ceilings has a perimeter of 44 feet, so the gross wall area is 44 × 9 = 396 sq ft. Divide that by 350 and you get about 1.1 gallons for one coat — but almost nobody stops at one coat.

That 350–400 sq ft figure assumes a smooth, previously painted wall and a single thin, even coat applied with a roller. Textured walls, unprimed drywall, and darker colors going over lighter ones all reduce real coverage — sometimes by 20% or more.

The safest habit is to plan for two coats on any wall that's changing color, and treat the can's coverage number as a best-case scenario rather than a guarantee.

A standard interior door opening removes about 21 square feet from your total, and a typical window removes about 15. For a room with two doors and two windows, that's roughly 72 sq ft you don't need to paint — enough to change whether you need one gallon or two.

If you'd rather skip the manual arithmetic, our Paint Calculator does this exact calculation for you — enter your room dimensions, door and window count, and how many coats you're planning, and it converts straight to gallons needed.