1. Surface area
Gross walls = 2 × (length + width) × heightNet walls = max(0, gross walls − openings)Ceiling = length × widthDoor and window counts use editable standard areas. “Other openings” lets you deduct fireplaces, arches, built-ins, or unusually large openings. The planner warns when deductions consume the entire wall area.
2. Coats, coverage, and waste
Required gallons = area × coats × (1 + waste) ÷ coverageWall paint defaults to 375 square feet per gallon, ceiling paint to 350, and primer to 250. Sherwin-Williams currently describes a typical range of about 350–400 square feet per gallon for paint and 200–300 for primer. Texture, porosity, repairs, application method, and a dramatic color change can all alter real coverage.
3. Package rounding
Exact volume rounds upward to the nearest quart, shown as a combination of 5-gallon buckets, 1-gallon cans, and quarts. Availability and pricing vary. The rough cost treats those package volumes as proportional to the entered per-gallon price; it is not a retailer quote.
4. Product and color boundary
One project represents one wall product/color and one ceiling product/color. Split the work into separate projects when colors or products differ, otherwise combined package rounding can understate what must be purchased.
5. What is not modeled
Trim, painted doors, cabinets, stairs, vaulted ceilings, exteriors, labor, sprayer loss, and live prices are outside this MVP. Non-rectangular rooms should be broken into measured rectangles or estimated wall-by-wall.
Sources
- Sherwin-Williams paint calculator FAQ
- Sherwin-Williams measurement guidance
- Benjamin Moore paint calculator and disclaimer
Method reviewed July 9, 2026.