Formula
Wall area = 2 × (length + width) × height − openingsGallons = area × coats ÷ coverage per gallon How it works
Add up the area of the four walls, subtract the doors and windows you will not paint, multiply by the number of coats and divide by the coverage printed on the can. A typical door takes out about 20 ft² and an average window about 15 ft².
Two coats are the norm for a color change or fresh drywall (which should also be primed); one coat may do when refreshing the same color. Rough or porous surfaces such as textured plaster, brick or bare wood soak up more paint, so use the low end of the coverage range.
Buy whole gallons and keep the leftover for touch-ups — it is far easier than matching a color later. For large jobs, a 5-gallon bucket is usually cheaper than five single gallons.
Frequently asked questions
How much does a gallon of paint cover?
Most interior wall paints claim 350–400 ft² per gallon on smooth, primed walls. Real-world coverage is often closer to 300–350 ft².
Should I include trim?
Trim, doors and cabinets are usually painted in a different sheen, so estimate them separately. A quart covers the trim of an average room.