Paint Coverage Without Overbuying Gallons
Paint math fails when people multiply the wrong area. Openings come out once; coats multiply what remains. Label coverage is the bridge from square feet to gallons.
Build Net Area First
Add wall rectangles (and ceiling if painting it), then subtract doors and windows. That net area is what actually receives paint.
Coats Multiply Surface, Not Openings
Two coats on 400 sq ft of net wall is 800 sq ft of coverage demand. You do not subtract openings again for the second coat.
Read the label’s surface assumption
Coverage claims assume a prepared surface. Porous new drywall, drastic color changes, and heavy texture often need more product than the headline number.
Primer Is Its Own Pass
If primer coverage differs from finish paint, estimate primer gallons separately and add them. Treating primer as “free” is a common shortfall.