Paint calculator
Walk the room with a tape measure, add up the perimeter, and enter it with your ceiling height. Doors and windows come off automatically.
Where the numbers come from
Perimeter times height gives total wall area — easier and more accurate than measuring each wall separately. A standard interior door removes about 21 square feet and an average window about 15, so both are subtracted rather than painted over.
The default coverage of 350 square feet per gallon is the middle of the range printed on most cans. Reach for the lower end of your can's stated range if you are painting bare drywall, rough plaster, or anything textured, because porous surfaces drink far more than smooth ones. Going from a dark colour to a light one usually means a third coat, or a primer coat first.
Buy in whole gallons and expect a little left over. Running out mid-wall is the one genuinely expensive mistake here: a fresh can mixed later can differ enough in tint to leave a visible seam, so it is worth having the extra rather than matching a second batch.
Questions
Should I paint the ceiling with the same amount?
No — ceilings are a separate calculation. Multiply room length by width for ceiling area and run it through as a separate job, since ceiling paint is usually a different product anyway.
Do I need to subtract every window?
Only if they are large or numerous. The 15 square feet used here is an average; a wall of glass deserves a real measurement, while a small bathroom window barely moves the total.
How much extra for texture?
Textured or unprimed surfaces can cut coverage by a third or more. Enter a lower coverage figure — around 250 square feet per gallon — if the surface is rough or has never been painted.