Primer Coverage Calculator
Buying too little primer means a second trip to the store mid-project; buying too much wastes money. This calculator divides your net surface area by the primer's spread rate (square feet per gallon, found on the label) and multiplies by the number of coats and a waste factor. Always use the spread rate from your specific product's label since coverage varies by brand, formulation, and surface porosity. Subtract door and window openings from the total wall area for accuracy.
Primer coverage formula
Gallons = Area x Coats x (1 + waste%) / Spread rate (sq ft/gal)
Quarts = Gallons x 4
Round up to the nearest quart or gallon when purchasing. Primer is more economical in gallon containers; quarts are useful for small touch-up areas or accent coats.
Typical primer coverage rates
- Interior latex primer on previously painted walls: 350 to 400 sq ft per gallon.
- Primer-sealer on new drywall: 200 to 300 sq ft per gallon (first coat penetrates paper).
- Exterior primer on wood: 250 to 350 sq ft per gallon.
- Masonry primer or block filler: 100 to 200 sq ft per gallon (fills pores).
- Shellac-based stain-blocking primer: 300 to 400 sq ft per gallon.
Primer coverage calculator: frequently asked questions
How do I calculate how much primer I need?
Divide the total area to be primed by the primer's spread rate (coverage per gallon, listed on the label). Add 10 percent for waste and uneven surfaces. Gallons = area / coverage per gallon x 1.1.
What is the typical coverage rate for primer?
Most interior primers cover 350 to 400 square feet per gallon on smooth surfaces. Exterior primers cover 250 to 350 square feet per gallon. Rough or porous surfaces (bare drywall, bare wood, masonry) reduce coverage by 20 to 30 percent.
Do I need two coats of primer?
One coat is sufficient for previously painted surfaces in good condition. Use two coats on new drywall (seal the paper), bare wood, dark colors being switched to light, or stained surfaces. Multiply gallons by the number of coats.
Do I subtract openings for primer?
Yes. Subtract door and window areas from the total wall area before calculating primer. Standard interior door opening is approximately 21 square feet (3 x 7 ft) and a standard window is approximately 15 square feet (3 x 5 ft).
What is the difference between primer and paint coverage?
Primers are formulated for penetration and adhesion rather than hide, so their coverage rate on porous surfaces is often lower than finish paint. Sealer-primers on new drywall can drop to 200 to 250 square feet per gallon.
Official sources
- EPA National Volatile Organic Compound Emission Standards for Architectural Coatings: EPA AIM Rule.
- USDA Forest Products Laboratory: Wood Finishing and Coating guidelines.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.