Primer Calculator: Gallons, Wall Area & Coat Count
Size primer the way you size paint — net wall area over coverage — except primer needs more gallons per coat since it covers less on bare drywall.
How it works
Primer is sized exactly like paint — net wall area divided by the coverage rate — but with one important difference: the coverage number is lower. Bare drywall, fresh joint compound, and patched repairs are porous, so the first coat soaks in rather than sitting on the surface. Rust-Oleum Zinsser and KILZ both rate their primers around 200 to 400 sq ft per gallon, and the low end is what raw gypsum and mud actually deliver.
The calculator takes your room perimeter times ceiling height, subtracts roughly 21 sq ft per door and 15 sq ft per window, and divides by the coverage you set. Primer is almost always a single coat, so the default is 1; the gallon count rounds up because you buy whole cans.
Prime whenever you are covering new drywall, sealing stains, or making a big color change. A good primer coat means the finish paint covers evenly and often lets you get away with one finish coat instead of two — saving a gallon or two on the color that costs more.
perimeter = 2 × (14 + 12) = 52 ft → gross wall = 52 × 8 = 416 sq ft
openings = 21 + 2 × 15 = 51 sq ft → net wall = 365 sq ft
gallons = 365 ÷ 300 = 1.22 → 2 gallons (≈ 5 L)