Paint coverage depends on surface area, not floor area. This calculator works out the wall area from the room perimeter and height, subtracts standard door and window sizes, optionally adds the ceiling, and multiplies by the number of coats.
Formula
Wall area = 2 × (length + width) × height − openings. Paint = area × coats ÷ coverage. A 14 × 12 ft room with 8 ft walls has 416 ft² of wall; less one door and two windows leaves 344 ft². Two coats at 350 ft²/gal needs just under 2 gallons.
Tips
- Prime new drywall, patched areas and strong colour changes.
- Buy all paint for one room at once so the tint matches.
- Dark or saturated colours often need a third coat.
Frequently asked questions
How much does a gallon of paint cover?
Most interior paints list 350–400 ft² per gallon (about 9–10 m² per litre) for one coat on smooth, primed walls.
Should I paint the ceiling the same colour?
Usually ceilings get flat white ceiling paint. Set 'Include ceiling' to yes only if you are using the same paint.
Results are planning estimates. For structural work, confirm requirements with your local building authority and a qualified professional. See our disclaimer.