Formula
Room wall area = 2 × (length + width) × height. Subtract doors and windows, optionally add the ceiling, then divide coats by coverage.
Paint Calculator
Measure a room or enter direct wall area, subtract openings, and get gallons and whole-gallon purchasing quantity.
How this paint estimate works
Room mode turns four wall dimensions into a paintable area, then subtracts repeated opening sizes. Direct-area mode is useful when another measure already gives you the net wall square footage.
Worked example
A 12 × 10 room with 8 ft walls has 352 sq ft of gross walls. Subtract a 21 sq ft door and two 15 sq ft windows for 301 sq ft; two coats at 350 sq ft/gal need about 1.72 gallons, or 2 gallons to buy.
Before you buy
Keep a small reserve for touch-ups, but confirm whether primer is needed and whether the product’s coverage assumes one coat over a similar color.
Assumptions
Defaults are starting points. Replace coverage, density, package yield, spacing, and allowance values with the product or plan you are actually using.
Limitations
Coverage varies with surface texture, color change, primer, sheen, and application. Trim, cabinets, and accent walls need their own measured areas.