How the paint calculator works
It adds the four walls: 2 × (length + width) × wall height. Then it subtracts 21 ft² for each door and 15 ft² for each window, and adds the ceiling (length × width) if you tick the box. That gives the area to paint. Multiply by the number of coats and divide by the coverage on the label to get gallons, then round up to whole gallons.
Example: a 12 × 12 ft room with 8 ft walls has 384 ft² of wall. One door and two windows take off 51 ft², leaving 333 ft². Two coats is 666 ft², which at 350 ft² per gallon is 1.9 gallons, so buy 2.
Walls, ceilings and what is not included
The calculator covers walls and, if you tick the box, the ceiling. It does not include trim, doors, window frames, skirting or the inside of closets. Those are usually painted separately, often in a different colour and finish, so measure them on their own. A ceiling in a 12 × 12 ft room adds 144 ft², which at two coats is 288 ft², close to another gallon.
Doors, windows and coverage
A standard door is about 3 × 7 ft, or 21 ft², and a standard window about 3 × 5 ft, or 15 ft². The calculator uses those figures, so unusual sizes will shift the answer a little. Coverage per gallon depends on the paint and the surface: labels commonly claim 350 to 400 ft², while bare or porous surfaces soak up more. If you are painting a dark colour over a light one, or the wall is new plaster, allow an extra coat rather than extra coverage.
Frequently asked questions
How much paint do I need for a 12 × 12 ft room?
With 8 ft walls, one door and two windows, the wall area is 333 square feet. Two coats is 666 square feet, which is 1.9 gallons at 350 square feet per gallon, so buy 2 gallons.
Does the ceiling add much paint?
A 12 × 12 ft ceiling adds 144 square feet. At two coats that is 288 square feet, or about 0.8 gallons, so it usually means one more gallon.
What if my paint covers 400 square feet per gallon?
Enter 400 in the coverage field. Two coats over 333 square feet is 666 square feet, or 1.67 gallons, which still rounds up to 2 gallons.