The basic wall area formula
For a rectangular room, wall paintable area equals the perimeter multiplied by ceiling height. Perimeter = 2 × (length + width). Example: a 12×14 ft room with 8 ft ceilings has perimeter 52 ft × 8 ft = 416 sq ft of walls.
Add ceiling area (length × width) if you are painting the ceiling — another 168 sq ft in that example. Subtract doors (~20 sq ft each) and windows (~15 sq ft each) for a closer estimate.
From square footage to gallons
Divide total paintable area (× number of coats) by the coverage number on your paint can label. Most interior latex lists 350–400 sq ft per gallon on smooth, primed walls.
Always round up. A 416 sq ft wall with two coats at 350 sq ft/gallon needs about 2.4 gallons — buy 3 gallons to avoid a mid-project store run.
- Smooth new drywall: often 400 sq ft/gallon
- Textured or porous walls: 300–350 sq ft/gallon
- Dark color over light: plan 2 coats minimum
- Same-color touch-up: 1 coat may suffice
Use the calculator instead of guesswork
Our room paint calculator accepts feet and inches, handles multiple rooms, deducts openings, and outputs gallons plus quarts with a copyable shopping list.