Buy too little paint and you're back at the store mid-wall; buy too much and money sits in the garage. This paint calculator estimates how many gallons you need from your room dimensions, number of coats, and the doors and windows it subtracts automatically.
How much paint for a room?
Wall area is the room's perimeter times ceiling height: 2 × (length + width) × height. Worked example: a 12 × 14 ft room with 8 ft ceilings has 2 × 26 × 8 = 416 sq ft of wall. Subtract one door (~21 sq ft) and two windows (~30 sq ft) → 365 sq ft. At ~350 sq ft per gallon and two coats, that's 730 ÷ 350 ≈ 2.1 gallons — buy three, or two plus a quart.
Coverage: what a gallon really paints
A US gallon typically covers about 350 sq ft in one coat on smooth, primed walls. Textured, porous or bare surfaces drink more paint — coverage can drop to 250–300 sq ft. Cheap paint often needs an extra coat, which is why quality paint is frequently cheaper per finished wall.
How many coats?
Two coats is the standard for even color and durability. Going from dark to light (or over bare drywall or patches), add a primer coat first — primer is cheaper than a third coat of finish paint. One-coat claims hold only for same-color refreshes on smooth walls.
Pro tips before you buy
Keep 10% extra for touch-ups (note the batch number — color varies between lots); paint the ceiling first; and don't skimp on prep, which determines the finish more than the topcoat does. Measuring an odd-shaped room first? Get the wall area with the square footage calculator, and if you're tiling a splashback instead, try the tile calculator.