A 14 by 12 foot bedroom with 8 foot walls, one door and two windows has 371 square feet of wall. Two coats at 375 square feet per gallon, plus 10% for the tray and roller, come to 2.18 gallons: buy two gallons and a quart. The ceiling adds 168 square feet, which is one gallon of flat ceiling paint.
How the paint calculator works
Paint is sold by the gallon but spread by the square foot, so everything starts with area. For a room the calculator takes the perimeter times the wall height, then subtracts every door and window, because neither gets paint. It multiplies by the number of coats, divides by the spread rate on the can and adds the waste allowance.
Gallons = (2 × (length + width) × height − doors − windows) × coats ÷ sq ft per gallon × (1 + waste)
A door comes off as 21 square feet and a window as 12 square feet, the standard sizes HomeAdvisor uses for painting estimates. The result is then turned into cans you can actually carry home: 5-gallon pails, gallons and quarts. "Least left over" finds the mix with the smallest surplus (a quart is a quarter gallon); "Whole gallons" rounds up to gallons and swaps four or five gallons for a pail.
Worked example: a 14 × 12 ft room
| Step | Numbers | Result |
|---|---|---|
| Gross wall area | 2 × (14 + 12) ft × 8 ft | 416 sq ft |
| Less 1 door and 2 windows | 416 − 21 − 2 × 12 | 371 sq ft |
| Wall paint, 2 coats | 371 × 2 ÷ 375 × 1.10 | 2.18 gal |
| Ceiling, 2 coats | 168 × 2 ÷ 400 × 1.10 | 0.92 gal |
| Primer on the walls, 1 coat | 371 ÷ 250 × 1.10 | 1.63 gal |
That is 2 gallons and 1 quart of wall paint, 1 gallon of ceiling paint and, if the walls are new drywall, 1 gallon and 3 quarts of primer. Turn on the trim and the calculator adds 94 feet of baseboard and casing (the perimeter less the doorway, plus casing around the door and both windows) at 3½ inches tall: one quart of enamel covers it twice.
How many square feet does a gallon of paint cover?
Sherwin-Williams puts a typical gallon at 350 to 400 square feet for one coat, and says a quart covers about 100 square feet. Premium lines go further: the data sheet for Benjamin Moore Regal Select lists 400 to 450 square feet per gallon, "depending on surface texture and porosity". That is why the calculator asks about the wall surface. Smooth, previously painted drywall uses the top of the range, orange-peel or knockdown texture the middle, and heavy texture, bare patches or raw wood the bottom.
| Product | Spread rate per gallon, one coat |
|---|---|
| Typical interior paint (Sherwin-Williams guidance) | 350 to 400 sq ft |
| Benjamin Moore Regal Select, walls | 400 to 450 sq ft |
| Benjamin Moore Waterborne Ceiling Paint | 400 to 450 sq ft |
| Benjamin Moore Advance, trim and doors | 400 to 500 sq ft |
| Primer (Sherwin-Williams guidance) | 200 to 300 sq ft |
The calculator uses the low end for ceiling paint and trim enamel, and the middle of the primer range. If your can prints a different number, pick "From my can's label" and type it in.
Paint calculator for walls, square feet and one wall
Square feet: if you already know the wall area, switch to "Square feet". 400 square feet of wall, two coats, is 2.35 gallons with waste: 2 gallons and 2 quarts. Use the net area, with doors and windows already taken off.
One wall: an accent wall 12 feet wide and 8 feet high with one window is 84 square feet. Two coats need about half a gallon, so buy two quarts rather than a gallon.
By room size: add every room with "Add a room". Rooms of the same size can be counted with "Rooms like this", and the diagram shows whichever room you are editing, unfolded like a box with the ceiling in the middle.
Paint for the ceiling
Ceilings take their own flat paint, so they come out as a separate line. The area is simply length times width. Benjamin Moore recommends one or two coats of its ceiling paint; the calculator uses the same number of coats as the walls, so set one coat if you are refreshing a ceiling that is already white.
Paint for doors, trim and cabinets
Trim is measured in linear feet and converted to area with the trim height, 3½ inches for a 1x4 baseboard. Doors are painted on both sides, 2 × 21 square feet each. For cabinets, add up the width times height of every door, drawer front and face frame; 60 square feet of fronts takes two quarts for two coats. Trim and cabinet paint is bought in quarts or gallons, never pails.
How many coats, and when to prime
Benjamin Moore recommends two coats for full color, and two coats on bare surfaces; a repaint in a similar color can be one. Prime new drywall, patched areas and big color changes. Sherwin-Williams gives primer 200 to 300 square feet per gallon, less than paint, so a primer coat on a whole room often needs as much product as one finish coat.
If your home was built before 1978, the EPA warns it may have lead-based paint. Test before you sand or scrape, and follow lead-safe work practices.
Next steps
- Hang new board first? The drywall calculator counts sheets, and the drywall mud calculator the compound to finish them.
- Price the job with the painting cost calculator, hired out or done yourself.
- Painting the outside of the house? Use the exterior paint calculator, which adds gable ends and siding texture.
- Papering an accent wall instead: the wallpaper calculator counts rolls with the pattern repeat.
- Need the floor area of a room for other materials? Try the room square footage calculator.
More wall and ceiling tools are in drywall, paint and walls.