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How Much Paint Do I Need for a Room?
The Formula
(Wall Height x Room Perimeter) - Window and Door Area = Paintable Wall Area
Paintable Wall Area / Coverage Rate per Gallon = Gallons Needed
One gallon of interior latex covers about 350-400 square feet on smooth drywall. That’s the manufacturer number printed on the can. In practice, 350 is closer to reality. Textured walls, fresh drywall, and dark-to-light color changes eat more paint.
A Real Room
Standard bedroom: 12 x 14 feet, 8-foot ceilings. One door (21 sq ft) and two windows (15 sq ft each).
Perimeter: 12 + 14 + 12 + 14 = 52 linear feet
Wall area: 52 x 8 = 416 sq ft
Subtract openings: 416 - 21 - 15 - 15 = 365 sq ft
At 350 sq ft per gallon, that’s 1.04 gallons for one coat. Buy a gallon and you’ll have just barely enough. Buy two and you’ll have leftovers for touch-ups down the road, which you’ll want.
Two coats: 2.08 gallons. Two gallons.
When You Need Two Coats
Always, basically. One coat works if you’re repainting the same color or going from a lighter shade to a very similar lighter shade. Every other situation needs two.
Going light over dark (covering a red accent wall with off-white) might take three coats even with a tinted primer. That red accent wall from 2019 that seemed like a good idea is going to cost you an extra gallon and an extra afternoon.
Fresh drywall soaks up paint. Prime it first. Unprimed drywall will drink your first coat and look blotchy no matter how careful you are. PVA primer is $15 a gallon and saves you $40 worth of finish paint.
Ceilings
Same formula, simpler math.
Length x Width = Ceiling Area
12 x 14 = 168 sq ft. About half a gallon. Ceiling paint is thicker than wall paint and covers slightly less, maybe 300-350 sq ft per gallon. You’d still get away with one gallon for that bedroom ceiling.
Ceilings usually get one coat unless they’re stained. Water stains need a shellac-based primer first (Zinsser BIN or Kilz Original, not the latex version). The latex primer won’t block water stains no matter how many coats you apply.
Trim, Doors, and Baseboards
Trim paint covers less. Figure 300 sq ft per gallon for semi-gloss on woodwork.
An interior door is about 21 sq ft per side. Both sides of one door: 42 sq ft.
Baseboards: measure the linear footage, multiply by the height. Standard baseboard is 3.25 inches tall. 52 linear feet of baseboard in that 12x14 bedroom: 52 x 0.27 = 14 sq ft. Two coats: 28 sq ft.
Window casing: roughly 10-12 sq ft per window for the frame and sill, both sides. Two windows: 24 sq ft.
Crown molding, chair rail, and door casing add up. For a single bedroom with typical trim, you’ll use about a quart of trim paint. A whole house with 40 doors, 25 windows, and 800 linear feet of baseboard needs 5-7 gallons of trim paint.
Coverage Rates That Actually Matter
These are real-world numbers, not the optimistic label:
- Flat/matte latex on smooth drywall: 350 sq ft/gal
- Eggshell on smooth drywall: 350 sq ft/gal
- Semi-gloss on trim: 300 sq ft/gal
- Flat on textured walls (orange peel, knockdown): 250-300 sq ft/gal
- Exterior latex on wood siding: 250-350 sq ft/gal depending on weathering
- Stain on deck boards: 150-200 sq ft/gal (it soaks in)
Spray application uses 20-30% more paint than rolling. You cover faster, but a lot of it ends up as overspray. Budget accordingly.
The Quart Trap
A quart covers about 87 sq ft at 350 sq ft/gal. Quarts cost $12-18. Gallons cost $30-50. The price per square foot from a quart is roughly double what you’d pay from a gallon. Quarts make sense for accent walls under 80 sq ft and small trim jobs. Everything else, buy the gallon.
The Quick Version
SiteCalc has a paint calculator that takes your room dimensions, subtracts doors and windows, and tells you gallons needed for walls, ceiling, and trim separately. It accounts for number of coats and different coverage rates. The budget calculator runs the math backward too: tell it you’ve got $150 for paint, and it tells you how many square feet that covers. Useful when you’re pricing a job and need to work from a budget, not a floor plan.