Paint Calculator
Wall perimeter times height, minus doors and windows, times the number of coats, divided by the coverage on your can — that chain, run live, tells you how many gallons to carry to the register.
Walls, openings, and coats
Example: 40 ft of walls at 8 ft high, 20 sq ft of openings, 2 coats at 350 sq ft/gal → buy 2 gallons.
Gallon counts are purchasing estimates only — surface preparation, lead-safe practices in older homes, and code questions call for qualified professionals.
The chain, written out
gallons = (perimeter × height − openings) × coats ÷ coverage, rounded up to whole
cans. The worked room: 40 × 8 = 320 sq ft gross, minus 20 sq
ft of openings leaves 300 sq ft. Two coats make the demand 600 sq ft; at 350 sq ft/gal that is 1.71 gallons, so you
buy 2 and finish with about 0.29 gallons for touch-ups. Engine-computed at build time, same code as the form.
Coats multiply — they don’t add a little
The most common misjudgment here is mental, not typographical: a second coat is not “a bit more paint”, it is the entire wall again. Halve your coverage expectations for bold color changes and you will stop making mid-project store runs.
Frequently asked questions
How do I get the wall perimeter?
Walk the room adding wall lengths: a 12 × 8 ft room has a perimeter of 40 ft. Painting only some walls? Add just those lengths. The tool multiplies the perimeter by the wall height to get gross area, so it works for any subset of walls.
What should I subtract for doors and windows?
A rough convention many painters use is about 20 sq ft for a door and 15 sq ft for an average window — but they are your openings, so measure or adjust as needed. Small openings are often ignored, which simply pads the estimate slightly.
Why did my gallon count double when I changed one field?
Almost certainly the coats field: coats multiply the entire area. Two coats of coverage on 300 sq ft of wall is 600 sq ft of paint demand. Most color changes need two coats; deep reds and drastic changes sometimes need more.
Which coverage number should I trust?
The one on your can. Behr publishes 250-400 sq ft per gallon for its Premium Plus interior lines and Sherwin-Williams cites 350-400; this tool defaults to 350 but the field is fully adjustable because porous, textured, or primed surfaces land at different points in that range.
Does the tool handle primer or ceiling paint?
Run it once per product: primer coverage differs from topcoat coverage, and a ceiling is its own area (length × width, not perimeter × height). Keeping the runs separate keeps each purchase honest.
All figures compute in your browser and are never sent anywhere. Coverage sources and the full pipeline live on the methodology page.