How much paint do I need for a room?

Find the paintable wall and ceiling area, subtract openings, multiply by the number of coats, then divide by the coverage rate (typically 350 to 400 sq ft per gallon). A 12 by 12 foot room with 8 foot walls needs roughly 2 gallons for two coats.

What coverage rate should I use?

Most interior latex paints cover about 350 to 400 square feet per gallon on a smooth, primed surface. Textured walls, deep colors, and bare drywall cover less. Use the figure printed on your can and lower it if the surface is rough or unprimed.

Why round up to a quart or gallon?

You can only buy paint in whole containers, so the tool rounds the raw gallon figure up to the next quart for small jobs or gallon for larger ones. A little extra is also useful for touch-ups later, so a small overage is intentional.

Does primer count as a coat?

Yes, treat primer as its own coat with its own coverage rate, which is often lower than topcoat paint. For a fresh drywall job, plan one primer coat plus two finish coats. Run the tool once for primer and once for the topcoat.

How are doors and windows handled?

Each door is deducted as about 21 square feet and each window as about 15 square feet of unpaintable area. This keeps the estimate from over-counting trim and glass that will not take wall paint.

Paint Coverage Calculator

Buying paint comes down to one ratio: paintable area times coats, divided by how many square feet a gallon covers. This calculator measures the walls and ceiling, deducts the doors and windows, applies your coats and coverage rate, and rounds up to the gallons and quarts you can actually buy.

How it works

Wall area is the perimeter times the height; the ceiling adds the floor footprint when painted. Openings are removed, then coats and coverage decide the gallons:

wall area    = 2 × (length + width) × height
ceiling area = length × width      (if painting ceiling)
paintable    = wall + ceiling − openings
total area   = paintable × coats
gallons raw  = total area / coverage_per_gallon

The raw figure is then rounded up to the next quart (0.25 gal) so you buy a whole container with a little to spare for touch-ups.

Example and tips

A 12 × 12 ft room with 8 ft walls, one door and two windows, ceiling painted, has about 384 sq ft of walls plus 144 sq ft of ceiling, less ~51 sq ft of openings — roughly 477 sq ft paintable. Two coats is 954 sq ft, which at 400 sq ft per gallon is about 2.4 gallons, rounded up to 2.5 gallons to buy. Drop the coverage rate to 300 to 350 for deep accent colors or bare drywall, which drink far more than a primed, light surface.