Room Paint Calculator
A room is two jobs in one: the walls and the ceiling. This tool sizes both at once — walls from the net paintable area at your chosen coats, ceiling from length × width — so you leave the store with the right number of cans for the whole room.
This room is about 3 gallons for the walls and 1 gallon for the ceiling. A room is the walls (perimeter × height − openings) plus optionally the ceiling; the standard door (~21 sq ft) and window (~15 sq ft) deductions are labeled typicals — adjust them to your actual openings.
Calculator inputs
Most rooms get walls in one color and the ceiling in a flat white, so it pays to size them together and buy in one trip. The walls follow the signature quantity formula — net paintable area (walls minus openings) × coats ÷ coverage. The ceiling is simply length × width at one coat, which is plenty for a same-color refresh.
Order of operations: dimensions, then openings, then coats. Trim, doors and baseboard are usually a separate can of enamel in a different sheen — size those with the trim & baseboard linear-feet calculator.
Formula
net wall = 2 × (length + width) × height − openingswall gallons = ceil( net wall × coats ÷ coverage )ceiling = length × widthceiling gallons = ceil( ceiling × 1 coat ÷ coverage )
Worked example
A 12 ft × 15 ft room, 8 ft walls, one door and two windows, two wall coats, 350 sq ft/gal:
- Net wall = 432 − 51 = 381 sq ft; wall gallons = ceil(381 × 2 ÷ 350) = 3 gallons.
- Ceiling = 12 × 15 = 180 sq ft; ceiling gallons = ceil(180 ÷ 350) = 1 gallon.
Add roughly 2 gallons of primer if the drywall is bare or you are going dark-to-light — size it with the primer calculator.
Walls, ceiling and trim are separate lines
Keep walls and ceiling on separate lines so nothing gets lost:
- Different paints, different cans. Wall color, ceiling flat and trim enamel do not share a bucket — plan a can for each.
- Textured ceilings drink paint. Popcorn and knockdown ceilings cover closer to 250–300 sq ft/gal; lower the coverage input for those.
- Adjust the openings. A patio door is ~40 sq ft; a room with a wall of glass needs far less wall paint than the defaults assume.
- Buy once, from one batch. A mid-project re-order risks a visible seam in color.
Reference table
| Paint × surface | Coverage (sq ft/gal, one coat) |
|---|---|
| Smooth / previously-painted drywall (latex) | 350–400 |
| New / primed drywall | 300–350 |
| Textured or porous interior wall | 250–300 |
| Smooth wood / trim (enamel) | 350–400 |
| Bare / rough wood | 200–300 |
| Exterior lap / vinyl siding (smooth) | 300–400 |
| Stucco / rough masonry | 150–250 |
| Brick (unpainted) | 100–200 |
| Concrete / block | 200–300 |
| Deck / fence (semi-transparent stain) | 200–300 |
| Primer (drywall / general) | 200–300 |
Labeled published planning snapshot — the can’s stated spread rate wins. Rough, porous or thirsty surfaces sit at the low end; a big color change or a light coat pushes you toward the high end. Full matrix: paint coverage by surface.
Frequently asked questions
How much paint for a whole room?
Size the walls at two coats (net area × 2 ÷ coverage) and the ceiling at one coat (length × width ÷ coverage), then round each up. A typical 12 × 15 room lands near 3 gallons for the walls and 1 for the ceiling.
Is the ceiling really only one coat?
For a same-color refresh on a smooth ceiling, one good coat usually does it, so that is the default here. Go to two coats for a color change, a textured ceiling, or covering stains — bump the ceiling into the ceiling paint calculator and set two coats.
What about the trim, doors and baseboard?
Those are measured by the linear foot, not the square foot, and painted in a separate enamel. Use the trim & baseboard linear-feet calculator; a quart of trim enamel goes a long way.
Do I need primer as well?
Yes if the drywall is bare, patched, stained, or you are going from a dark color to a light one. A tinted primer can even save a finish coat. Size it with the primer calculator.