Room details
ft
ft
ft
20 sq ft each
15 sq ft each
Number of coats
sq ft
350 sq ft is typical for smooth primed drywall. Drop to 275 for textured or porous surfaces.
$
Paint Calculator
Room size in, gallons out. Deducts doors and windows, handles multiple coats and the ceiling, and tells you what to actually carry to the register.
Buy this much
2.34 gallons needed, rounded up
Buy 3 gallons
Two one-gallon cans plus a quart would leave you cutting it too fine.
Surface area
- Wall area (gross)
- 416.00 sq ft
- Openings deducted
- −50.00 sq ft
- Ceiling
- +168.00 sq ft
- Paintable area
- 366.00 sq ft
Paint needed
- Total area, all coats
- 732.00 sq ft
- Exact gallons
- 2.09
- Primer (1 coat)
- 2 gallons
Paint cost
$135.00
Finish paint only — primer, brushes, rollers, and tape are extra.
Saved calculations
How this calculator works
Wall area is the room's perimeter × ceiling height — that is 2 × (length + width) × height. From that, 20 square feet is deducted per door and 15 per window, the standard US allowances for a 3 ft × 6 ft 8 in door and an average double-hung window.
If the ceiling is included, its area is simply length × width, added on top. The result is multiplied by the number of coats, then divided by the coverage rating of your paint. Gallons are rounded up, because paint stores do not sell 2.09 of anything.
Primer is calculated as a single coat at 275 square feet per gallon, the typical rating for a sealing primer, which covers less than finish paint by design.
Worked example
A 14 ft × 12 ft bedroom with 8 ft ceilings, one door, two windows, two coats, walls only:
- Perimeter: 2 × (14 + 12) = 52 ft
- Gross wall area: 52 × 8 = 416 sq ft
- Openings: (1 × 20) + (2 × 15) = 50 sq ft
- Paintable area: 416 − 50 = 366 sq ft
- Two coats: 366 × 2 = 732 sq ft
- At 350 sq ft per gallon: 732 ÷ 350 = 2.09 gallons → buy 3
Two gallons plus a quart would technically cover it, but leaves nothing for touch-ups and no margin if the second coat pulls heavier than the first. The third gallon is cheap insurance.
What changes the number
Surface texture is the biggest variable. Knockdown or orange-peel drywall has substantially more actual surface than its flat dimensions suggest, and coverage can fall by a third. Colour change matters almost as much: covering a deep colour with white, or painting a saturated red or yellow, routinely needs a third coat regardless of what the can promises.
Application method shifts things too. Spraying lays down a heavier film and wastes more to overspray than rolling, so add roughly 20 percent if you are spraying. Trim, baseboards, and doors are a separate calculation and usually a separate product — semi-gloss rather than the eggshell or matte used on walls.
Common questions
How many square feet does a gallon of paint cover?
A gallon of interior wall paint covers roughly 350 to 400 square feet in a single coat on a smooth, primed surface. Textured drywall, raw plaster, and bare wood drink far more, dropping real coverage closer to 250 to 300 square feet. The number printed on the can assumes ideal conditions, so use the lower end of the range when the surface is porous or the colour change is dramatic.
How many coats of paint do I need?
Two coats is the standard for almost every job and is what this calculator defaults to. One coat is only realistic when repainting the same colour on a well-sealed wall. Go to three coats when covering a dark colour with a light one, when painting a saturated red or yellow, which have poor hiding power, or when going over patched drywall without primer.
Should I subtract doors and windows?
Yes, for any room with more than one or two openings. This calculator uses the US standard allowances of 20 square feet per door and 15 square feet per window, which reflect typical 3-by-6-foot 8-inch doors and average double-hung windows. In a small bathroom or a hallway with several doors, skipping the deduction can overstate your paint by half a gallon or more.
Do I need primer, and does it count as a coat?
Primer is a separate product with its own coverage, so it does not count toward your finish coats. You need it on new drywall, bare wood, patched areas, glossy surfaces, and any dramatic colour change. Primer typically covers 200 to 300 square feet per gallon, less than finish paint, because it is formulated to seal rather than to build film.
How much paint do I need for the ceiling?
The ceiling is simply the room's floor area — length times width — treated as one more surface. A 12-by-14-foot room has a 168-square-foot ceiling, which needs about half a gallon per coat. Ceilings are usually painted flat white and often take only one coat if the existing surface is clean and the colour is unchanged.
Is it better to buy one extra gallon or run short?
Buy the extra. Paint is mixed to a formula at the store, and while it can be re-mixed, small batch-to-batch variation is real, particularly in deep custom colours. A leftover gallon is also your touch-up supply for the next few years, which is worth more than the twenty or thirty dollars saved by cutting the order fine.
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