How to Calculate Paintable Wall Area
Estimate wall area, subtract openings, plan coats, and add waste before buying paint.
Measure each wall, then subtract large openings
Multiply wall width by height for each wall, add the walls together, then subtract large doors and windows. Small trim details usually do not change the gallon count much.
Coats are the biggest multiplier
One coat over a similar color uses much less paint than two coats over a strong color change. New drywall, rough surfaces, and deep colors often need more coverage.
Add a small waste allowance
A waste allowance covers roller loading, tray loss, rough surfaces, touch-ups, and measurement error. Ten percent is a common planning value for many DIY rooms.