Step by step
Wall area is twice the sum of the length and width, times the ceiling height. Subtract the openings. A standard door is roughly 21 square feet and a typical window around 15. Multiply the remaining area by the number of coats, then divide by the coverage printed on the can.
Coverage is often around 350 to 400 square feet per gallon, but read the label for the product you buy, since it varies.
Worked example
A room 14 by 12 feet with 9-foot ceilings has 2 x (14 + 12) x 9 = 468 square feet of wall. Two doors and three windows remove 87 square feet, leaving 381. Two coats need 762 square feet of coverage. At 400 square feet per gallon that is 1.91 gallons, so buy 2.
What changes the amount
A big color change, such as dark to light, can need more coats or a primer. Rough, porous or freshly patched surfaces soak up more paint. Trim, ceilings and closets are extra. Keep a little left over for touch-ups, and label the can with the room and date.
Mistakes to avoid
Forgetting to count both coats. Using a coverage figure from a different product. Mixing cans from different batches on one wall, which can show slight color differences; mix them together in a larger bucket if you have to use more than one can.
Try the calculator
Run your own numbers in the free paint calculator. It works in your browser and nothing you enter is saved or sent.
Common questions
Does this include the ceiling?
No. Use the square footage calculator for the ceiling and add it.
Do I need primer?
It depends on the surface and the color change. The paint's instructions will say.
What if a wall has big built-ins?
Measure them as extra openings and subtract that area.
This guide is general information, not financial, tax or legal advice. Rates, fees and rules change, so confirm current terms with the lender, store or agency involved. All dollar figures are invented examples.