Paint Calculator

How much paint a room takes

1. Wall area = perimeter x wall height, less 21 sq ft per door, 15 sq ft per window and any custom openings.
2. Ceiling area = length x width, less the cut-out corner of an L-shaped room.
3. Gallons = area x coats / coverage per gallon, worked out separately for walls and ceiling because they usually take different paint.
4. Primer gallons = wall area plus ceiling area / primer coverage, one coat, when the drywall is new.
5. Round each up to whole cans. Four loose gallons almost always cost more than one five-gallon bucket, so the calculator switches at that point.

Sherwin-Williams puts a typical gallon at 350 to 400 sq ft and a gallon of primer at 200 to 300 sq ft. Some premium paints claim more: Benjamin Moore lists 400 to 450 sq ft for Regal Select matte. The calculator starts at 350 because the low end is the safe end, and you can type the figure from your own can.

New drywall needs primer first

Bare drywall has two different surfaces, paper and joint compound, and they absorb paint at different rates. Painting straight over them leaves the taped seams showing through as dull stripes, a defect called flashing. A coat of drywall primer evens out the absorption so the finish coats look uniform.

Primer coverage on new board is lower than paint coverage because the paper soaks it up. Behr rates its drywall primer at 250 to 400 sq ft per gallon depending on how it is applied and how porous the surface is; the calculator defaults to 250.

One coat, two coats or three

• One coat: refreshing a wall in the same or a very similar color, over a sound existing finish.
• Two coats: the normal answer, and what most manufacturers base their color and sheen on.
• Three coats: large color changes, deep or bright colors over light ones, and some reds and yellows that are low in hiding power.