Estimates — add extra for waste and confirm with your supplier.
Wall area = perimeter × ceiling height, minus doors and windows. We multiply by your number of coats and divide by 350 ft² per gallon (typical coverage), rounding up to whole gallons.
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Measure wall area, subtract openings, multiply by coats, divide by ~350 ft² per gallon.
Two is standard; dark colors or big color changes may need a primer plus two coats.
Yes — taller walls mean more area and more paint.
About 350 square feet on smooth, primed surfaces.
Keep a little for touch-ups; rounding up to whole gallons usually covers it.