Measure first. Buy the right amount.
Measure the room length, width and wall height. This tool deducts 20 sq ft per door and 15 sq ft per window; adjust your measurements if your openings differ substantially. Include the ceiling only if you plan to paint it with the same product.
Why rounding matters
A 12-by-12 room with 8-foot walls, one door and one window has 349 sq ft of paintable walls. Two coats at 350 sq ft per gallon use about 1.994 gallons. Buying whole gallons means two cans before an extra allowance, or three with 10% extra.
Check the surface and supplies
Coverage depends on the paint and surface. Follow your chosen product’s preparation, primer and coat instructions. Add rollers, brushes, protection and preparation materials to the shopping budget.
Build a room-painting budget · Explore the 12-by-12 example
Show the original worked calculation
Gross walls = 2 × (12 + 12) × 8 = 384 sq ft
Openings = 20 + 15 = 35 sq ft (Behr how-to)
Net = 384 − 35 = 349 sq ft
Film = 349 × 2 coats = 698 sq ft
Exact gal = 698 ÷ 350 = 1.994 gal
Buy = 2 gal (round up, 0% waste)
With 10% = 1.994 × 1.10 = 2.194 gal → buy 3 gal
Cost @ $32.98 Behr Premium Plus eggshell 205001
= 2 × $32.98 = $65.96 or 3 × $32.98 = $98.94Formulas and manufacturer sources · Dated price references
Check a 12-by-12 room before buying
At 8 feet high, its walls total 384 square feet. Deducting the example door and window gives 349. Two coats require 698 square feet of coverage. At 350 square feet per gallon, two gallons cover the calculated area with only 2 square feet of theoretical capacity left. That is a very small margin for measurement or application differences.
Adding 10% raises demand to 767.8 square feet and the whole-gallon order to three. The extra gallon is a package-rounding consequence, not evidence that every room wastes a gallon. Check your selected product's coverage and package sizes before ordering.