How to calculate how much paint you need
The amount of paint comes down to one number: the total area you're covering, in square feet. For a room, that's the perimeter of the walls multiplied by the ceiling height. From that you subtract the openings — doors and windows don't need paint — then multiply by how many coats you plan, and divide by how much area a gallon covers. This calculator runs that whole chain for you and rounds to whole gallons, since that's how paint is sold.
How many square feet does a gallon cover?
A gallon of wall paint covers roughly 350–400 square feet per coat on a smooth, sealed surface. Rough textures, bare drywall, and dark-to-light color changes all drink more paint, so coverage falls toward the low end. The calculator defaults to 350 to stay realistic; lower it if your walls are textured or porous.
How many coats?
Most projects need two coats for a uniform, durable finish. A single coat may work when you're refreshing the same color, while three coats help when going from a very dark wall to a pale one. If you're painting new drywall or covering a stain or bold color, add a primer coat — primer seals the surface so your finish color goes on evenly and you often need less of it.
Don't forget the ceiling and trim
Ceilings are easy to overlook but add real square footage — check the box above to include yours. Trim, doors, and baseboards use a separate, smaller amount of paint (often a quart or two of a different finish), so budget for those on top of the wall total.
Frequently asked questions
Should I round up?
Yes. Always round to the next whole gallon and keep a little extra for touch-ups — paint from the same batch matches best, and running out mid-wall leaves a visible line.
Does paint type change the amount?
Coverage varies by product. Premium paints often cover more per gallon and may need fewer coats, while cheap paint can require an extra coat — which can make the pricier paint cheaper overall. Adjust the coverage figure to match the can's label.
How much does it cost to paint a room?
Enter your price per gallon and the calculator multiplies it by the gallons needed. A typical bedroom runs one to two gallons for the walls, so material cost is usually modest; labor is the bigger expense if you hire it out.
Paint calculation example
A 12-by-12-foot room with 8-foot ceilings has 384 square feet of wall area before subtracting doors and windows. Two coats require about 768 square feet of coverage. At 350 square feet per gallon, plan for three gallons after rounding up.
Related room calculators
Estimate surfaces and improvements with the drywall calculator, tile calculator, or BTU calculator.