Paint Wall Calculator
Estimate the gallons of paint for a whole room’s walls from the total wall length, height, coats and openings.
How it works
gallons = ceil(((perimeter × height) − openings) × coats ÷ 350)
Unlike a single-wall estimate, this takes the total perimeter of the room so all four walls are covered in one pass — add up the length of every wall and enter that. Multiplying by height gives the gross wall area; a typical door removes about 21 sq ft and a window about 15. The paintable area times the number of coats, divided by 350 sq ft per gallon, gives the gallons to buy.
Worked example
A room with 120 ft of total wall at 9 ft high is 1,080 sq ft. Two doors (42 sq ft) and four windows (60 sq ft) leave 978 sq ft; two coats is 1,956 sq ft ÷ 350 = 5.6, rounded up to 6 gallons.
Frequently asked questions
How is this different from a single-room paint calculator?
It works from the total perimeter — the sum of all wall lengths — rather than one wall or a single length × width. Enter every wall added together and you cover the whole room’s walls in one calculation.
How many coats do I really need?
Two is standard over primer or a similar color. Plan on a third when going light over dark, painting raw drywall, or using deep or red-based colors, which hide poorly.
Does 350 sq ft per gallon always hold?
It is the common average for smooth, primed drywall. Rough, porous or textured surfaces drink more — closer to 250–300 sq ft per gallon — so round up on anything but smooth walls.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.