About this calculator
Estimating paint needs depends on three numbers: total paintable surface area, coverage per gallon (label says 350–400 sq ft, real-world 250–350), and number of coats. This calculator computes from room dimensions, with adjustments for doors and windows.
The math
Wall area = perimeter × ceiling height. Perimeter = 2 × (length + width). Subtract ~20 sq ft per door and ~15 sq ft per window. Multiply by coats. Divide by coverage. Round up to whole gallons.
When you need more than one coat
- Color change — going from dark to light or light to dark usually needs 2 coats, sometimes 3.
- Going over primer — fresh drywall or new patches typically need primer + 2 finish coats.
- Tinted-base paints — deeper colors require 2 coats almost always.
- One coat is rarely enough for the same color refresh, even with "one-coat" marketing-grade paints.
Coverage realism
The 400 sq ft/gallon spec assumes smooth flat walls, a roller, and a flat or eggshell finish. Textured walls (knockdown, popcorn) cut coverage to 250–300. Brush work cuts it further. Trim and detail areas waste even more. Add 10–15% for waste and touch-ups always, especially for custom-tinted colors (re-tints rarely match exactly).
Painter cost rule of thumb
DIY: paint cost only ($30–80/gallon). Pro painters: $2–6/sq ft labor, depending on prep work, ceiling height, and trim complexity. A typical 12×14 bedroom is $400–800 to have professionally painted in materials and labor.