Estimate wall paint from the surfaces you actually plan to coat. This room calculator starts with a rectangular room, subtracts your combined door and window area, and uses the paint coverage and coat count you enter. It is a planning estimate for walls, not a guarantee that a particular paint will cover a particular surface.
Measure before choosing gallons
Measure the room’s length, width, and wall height in feet. The wall area of a rectangular room is its perimeter multiplied by height: 2 × (length + width) × height. If the room is irregular, split the walls into simple rectangles and calculate them separately. Sloped ceilings, alcoves, stairwells, and vaulted walls need their own measurements.
Measure the width and height of each opening you want to exclude, multiply them, and add the results. Enter that combined area in square feet. Subtracting a doorway assumes you are not painting that part of the wall. Door leaves, window trim, baseboards, and ceilings are separate surfaces and are not included in this model.
Use the product’s coverage figure
Enter the coverage shown on the paint label in square feet per US liquid gallon. One US liquid gallon is 3.785411784 liters; it is not an Imperial gallon or a dry-volume measure. Lowe’s notes that typical paint coverage is around 350–400 square feet per gallon, but the correct input depends on the actual product and surface. Rough, porous, previously unpainted, or heavily repaired walls may use more paint than smooth, sealed walls.
The estimate is net wall area × number of coats ÷ coverage per gallon. The waste allowance then multiplies that quantity by 1 + waste percentage ÷ 100. It allows for ordinary handling losses and an estimate buffer; it cannot make an unsuitable product cover well. Primer has its own coverage and purpose and should be estimated separately.
Worked example: a two-coat room
A room is 12 feet long, 10 feet wide, and 8 feet high. Its perimeter is 44 feet and gross wall area is 352 square feet. Subtract 40 square feet of openings to leave 312 square feet. Two coats create 624 square feet of coat coverage. At 350 square feet per gallon, the base estimate is about 1.78 gallons.
Adding a 10% allowance gives approximately 1.96 gallons. Two one-gallon cans would meet that mathematical estimate. Check the result against the label, surface condition, and available container sizes before purchasing. A strong color change or low-hiding paint may require another coat, which changes the result much more than a small rounding difference.
Keep the estimate useful
Keep a note of the paint name, finish, tint code, room, and purchase date. If you expect future touch-ups, decide how much spare paint you want to retain rather than assuming the default allowance will provide it. Coverage is not an instruction about thinning paint, and the calculator does not estimate drying time or when a surface can be recoated. Use the manufacturer’s directions for preparation and application.
Common questions
Does this include the ceiling?
No. Calculate ceiling area separately as length × width for a flat rectangular ceiling, then apply its own coats and product coverage.
Should I always subtract every opening?
Use the actual areas you do not intend to paint. For small openings, leaving them in can provide a modest buffer, but it should be a deliberate choice rather than an unnoticed input error.