Estimates for planning only. Always confirm measurements and order a little extra for waste and cuts.
How the paint calculator works
One gallon of paint covers roughly 350 square feet in a single coat. This calculator measures your walls, subtracts doors and windows, multiplies by your number of coats, and tells you how many gallons to buy.
The formula
We start from the paintable wall area, subtract openings, then multiply by coats:
Wall area = perimeter × ceiling height
Paintable = wall area − (doors × 21) − (windows × 15)
Gallons = (paintable × coats) ÷ 350 (rounded up)
We use 21 sq ft for an average door and 15 sq ft for an average window.
How many coats do you need?
Two coats is the standard for a durable, even finish and is what we assume by default. You can sometimes get away with one coat when repainting a similar color over a good existing surface. Plan on two to three coats when going from dark to light, covering stains, or painting fresh drywall (which also needs primer).
Pro tips
- Buy a little extra so touch-ups years later are a perfect match from the same batch.
- Primer is separate, count it as its own coat if you're priming bare drywall or making a big color change.
- Ceilings, trim, and doors are usually a different paint and finish, calculate them separately.
- Textured or porous walls drink more paint, expect closer to 300 sq ft per gallon.
Frequently asked questions
How much does a gallon of paint cover?
About 350 square feet per coat on smooth, primed walls. Rough, porous, or textured surfaces can drop that to around 300 square feet.
How many gallons to paint a 12x12 room?
A 12×12 room with 8-foot walls is roughly 384 sq ft of wall. After openings and two coats, that's about 2 gallons.
Do I need primer?
Prime bare drywall, patches, stains, and big color changes. Over an existing similar color in good shape, a quality paint-and-primer often skips the separate primer step.
Should I subtract doors and windows?
For accuracy, yes, this tool does it for you using average sizes. For a quick high estimate you can skip it and buy slightly extra.
How many coats should I plan for?
Two coats is the reliable standard. Use three when going dark-to-light or covering stains, and remember primer counts as its own coat on bare surfaces.
