Why the Paint Coverage Number on the Can Is a Best-Case Estimate
Published 2026-10-04
A lab number, not a guarantee
The coverage rate printed on a paint can — commonly 350 to 400 square feet per gallon — comes from manufacturer testing under close to ideal conditions: a smooth, properly primed surface and an even, consistent application. Real walls and real painters rarely match that exactly.
Why textured or porous walls change the math
A textured wall has more actual surface area per square foot of flat wall space than a smooth one does, and a porous, unprimed surface soaks up more paint before it's fully covered — both situations mean the real coverage rate per gallon ends up lower than the number on the can, sometimes substantially.
Why the door and window subtraction is a standard allowance
Subtracting a fixed area per door (about 21 sq ft) and window (about 15 sq ft) rather than measuring each one individually is a standard shortcut in paint calculators — close enough for an average-sized door or window, though an unusually large picture window or French door would make the actual paintable area smaller than this estimate assumes.
Estimate yours, then round up
Our Paint Calculator estimates paintable wall area and gallons needed from your room dimensions, number of coats and your specific paint's coverage rate — buying a bit extra beyond the calculated amount is cheap insurance against running short mid-project.