Paint Coverage Calculator
Buying too little paint means a trip back to the store mid-project, risking a different dye lot. Buying too much wastes money and creates disposal problems. This paint coverage calculator helps you estimate gallons needed for walls and ceiling in a rectangular room. Enter the room length, width, and ceiling height in feet, then specify how many standard doors (approximately 21 sq ft each) and windows (approximately 15 sq ft each) to subtract from the paintable area. The calculator applies 350 square feet per gallon as the coverage rate, which is a conservative figure typical of good-quality interior latex paints on smooth, primed surfaces. If you are painting over a porous surface such as bare drywall or bare plaster, or applying a light colour over a dark one, reduce the expected coverage or plan for an extra coat. The tool separates wall gallons from ceiling gallons, as ceilings often use a different product. It also shows the total paintable area before coats so you can check your paint label's stated coverage and adjust if it differs. Always round up to the nearest quart or gallon when purchasing. Many paint stores will match your paint and store the formula on file so you can order more if needed.
Paint calculation formula
Wall area = 2 x (Length + Width) x Ceiling height
Minus: (Doors x 21) + (Windows x 15)
Ceiling area = Length x Width
Total paintable area = Wall area + Ceiling area
Gallons = (Total paintable area x Coats) / 350
Worked example: 14 x 12 ft room, 8 ft ceiling, 1 door, 2 windows, 2 coats
- Wall area: 2 x (14 + 12) x 8 = 416 sq ft
- Subtract doors/windows: 416 - (1 x 21) - (2 x 15) = 365 sq ft
- Ceiling: 14 x 12 = 168 sq ft
- Total paintable: 365 + 168 = 533 sq ft
- Gallons (2 coats): (533 x 2) / 350 = 3.05 gallons (buy 3.25 gal)
Paint coverage calculator: frequently asked questions
How much area does one gallon of paint cover?
Most interior latex paints cover 350 to 400 square feet per gallon on smooth, properly primed surfaces. This calculator uses 350 sq ft per gallon as a conservative estimate to account for surface texture, paint sheen, and colour variance. Textured walls, bare drywall, or dark colour changes may require a primer coat and will yield less coverage per gallon. Always check your specific paint label for the manufacturer's stated coverage rate.
Should I subtract doors and windows from the paint calculation?
Yes. Standard interior doors are approximately 21 square feet (3 ft x 7 ft) and standard windows are approximately 15 square feet (3 ft x 5 ft). This calculator subtracts those typical areas automatically when you enter the number of doors and windows. If your doors or windows are significantly different from those sizes, adjust your room measurements accordingly.
How many coats of paint do I need?
Most interior paint jobs require two coats for even coverage and good washability. When painting over a dark colour with a lighter one, or when painting fresh drywall without a primer, you may need a primer coat plus two finish coats. When repainting with a similar colour on a well-prepped surface, one coat may suffice. This calculator defaults to two coats but allows you to enter any number.
Do I need a separate ceiling paint calculation?
Yes. Ceilings are typically painted with flat-sheen paint and may need fewer or more coats depending on the condition. This calculator uses the ceiling dimensions (length x width of the room) as a separate area and adds it to the total. If you are using a different product or colour for the ceiling, note the ceiling gallons separately from the wall gallons.
How do I measure a room for paint?
Measure the length and width of the room in feet. Multiply the room perimeter (2 x length + 2 x width) by the ceiling height to get wall area. Add the ceiling area (length x width). Subtract 21 sq ft per door and 15 sq ft per window. This calculator does all of those steps automatically. For rooms with vaulted ceilings or arched walls, measure the actual wall height at its tallest point as a rough average.
Sources
- US EPA Architectural Coatings guidance: epa.gov.
- Painting and Decorating Contractors of America: pdca.org.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. Always check your paint label's stated coverage.