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. A gallon of PVA primer costs less than the extra finish coat it saves. If you’re still in the estimating phase and haven’t ordered drywall yet, see the drywall calculator for sheet counts and joint compound.

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, because you’re brushing narrow surfaces. Trussly starts at 200 sq ft per gallon for brushed trim; check the label.

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. A single bedroom with typical trim usually needs about a quart.

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 brushed on trim: 200-300 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)

Spraying uses more paint than rolling because some of it ends up as overspray. Budget extra.

The Quart Trap

A quart covers about 87 sq ft at 350 sq ft/gal. Per square foot, quarts usually cost a lot more than gallons. Quarts make sense for accent walls under 80 sq ft and small trim jobs. Everything else, buy the gallon.

Bathroom and kitchen repaints often overlap with tile work. If you’re painting above a tile backsplash or wainscot, subtract the tiled area from your paintable wall area before ordering.

The Quick Version

Trussly has separate calculators for wall area, paint, primer, trim, and exterior paint. Each one lets you enter the coverage from your can, picks the number of coats, and rounds to gallons and quarts. The budget calculator runs the math backward too: tell it you’ve got $150 for paint, and at your price per gallon it tells you about how many square feet two coats will cover. Useful when you’re pricing a job and need to work from a budget, not a floor plan.

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