Paint is sold by the gallon, so the job is turning your room into a gallon count. Three things drive it: the wall area, the number of coats, and how far a gallon covers on your particular surface. Get the third one wrong and you make a second trip to the store mid-job with a wet cut line on the wall.
Step 1 — wall area
Add up the wall lengths (the perimeter) and multiply by the ceiling height:
wall area (ft²) = 2 × (length + width) × ceiling heightA 12 × 14 ft room with 8 ft ceilings has 2 × (12 + 14) × 8 = 416 ft² of wall.
Ceiling height matters more than people expect. The same 12 × 14 room goes to 468 ft² at 9 ft and 520 ft² at 10 ft — a 25% jump for two extra feet, because the perimeter is doing the multiplying.
For an L-shaped room, split it into rectangles and add the wall runs; don’t try to average it.
Step 2 — coats and coverage
Two coats is the standard for an even finish. A gallon covers about 350–400 ft² per coat on smooth, primed drywall — use 350 to be safe.
gallons = wall area × coats ÷ coverage (round up)For our room: 416 × 2 ÷ 350 = 2.38, so buy 3 gallons.
That 350–400 figure isn’t marketing. A US gallon is 231 cubic inches; spread over 400 ft² it’s a wet film 4.0 mils thick, and over 350 ft² it’s 4.6 mils — right in the band manufacturers target for a coat that levels, hides, and cures properly. Stretch a gallon to 500 ft² and you’re applying about 3.2 mils, which is why over-spread paint looks patchy no matter how many times you go back over it.
Coverage by surface
| Surface | Coverage per gallon per coat | Notes |
|---|---|---|
| Smooth, primed drywall | 350–400 ft² | The default case |
| Previously painted, same sheen | ~400 ft² | The easiest surface there is |
| Bare drywall or new joint compound | ~250 ft² | Prime first or it drinks the finish coat |
| Knockdown / orange-peel texture | ~250 ft² | More surface area than the tape measure sees |
| Bare wood, masonry, textured exterior | ~250 ft² or less | Prime, and expect to lose more |
Textured and bare walls drink paint. Dropping from 350 to 250 ft²/gal turns our 12 × 14 room from 3 gallons into 4 — a 33% increase from one setting. If the wall is unpainted, budget a primer coat on top of that, not instead of a finish coat.
Should you subtract doors and windows?
The site leaves deductions off by default, and there’s a real reason. Take the same 12 × 14 room with two doors and two windows:
- Gross wall area: 416 ft²
- Deductions:
2 × 20 + 2 × 15 = 70 ft² - Net area: 346 ft² →
346 × 2 ÷ 350 = 1.98→ 2 gallons
Two gallons buys 700 ft² of theoretical coverage against 692 ft² of need. Your entire margin for error is 8 square feet — less than one door. And every opening you deducted is an edge you now have to cut in by brush, which uses paint faster per square foot than rolling does, not slower.
So: leave the openings in unless the room is dominated by glass. A wall of sliders or a stairwell window genuinely worth 60+ ft² should come out. Two standard doors and a couple of windows should stay in as your buffer.
If you do deduct, use 20 ft² per door and 15 ft² per window.
Ceilings and trim are separate purchases
Ceiling paint and trim enamel are different products in different sheens — don’t fold them into the wall gallon count.
- Ceiling: add
length × width. A 12 × 14 ceiling is 168 ft²; two coats is 336 ft², so 1 gallon covers it. Most single rooms need exactly one gallon of ceiling paint, and flat ceiling white is usually the cheapest can in the store. - Trim: baseboard area is roughly
perimeter × baseboard height. In our room that’s52 ft × (3.5 ÷ 12) = 15 ft², or about 30 ft² at two coats. Add door casings and a door face or two and you’re still well inside one quart (~100 ft²). Buy a quart of enamel, not a gallon, unless you’re doing the whole floor. - Doors: a standard door is ~20 ft² per side. Two coats on both sides of three doors is ~240 ft² — that’s when a gallon of enamel starts making sense.
Worked examples — 2 coats, 350 ft²/gal, walls only
| Room | Ceiling | Wall area | Raw gallons | Buy | Est. cost |
|---|---|---|---|---|---|
| 10 × 10 | 8 ft | 320 ft² | 1.83 | 2 gal | ~$70 |
| 12 × 12 | 8 ft | 384 ft² | 2.19 | 3 gal | ~$105 |
| 12 × 14 | 8 ft | 416 ft² | 2.38 | 3 gal | ~$105 |
| 12 × 16 | 8 ft | 448 ft² | 2.56 | 3 gal | ~$105 |
| 14 × 16 | 9 ft | 540 ft² | 3.09 | 4 gal | ~$140 (price a pail) |
| 16 × 20 | 8 ft | 576 ft² | 3.29 | 4 gal | ~$140 (price a pail) |
| 20 × 20 | 9 ft | 720 ft² | 4.11 | 5 gal | pail |
Costs assume a mid-grade interior paint around $35/gallon, a US-average estimate — premium lines run well above that and contractor-grade below.
When to buy a 5-gallon pail
Once your total hits 4 gallons or more, price the 5-gallon pail. Per gallon it’s almost always cheaper than the equivalent cans, and it solves a second problem: color consistency. Tinted paint varies slightly between mixes, so five separate cans of the same color are not guaranteed to be identical. A pail is one batch.
If you are buying cans and the job spans more than one wall, box the paint — pour all the cans into a clean 5-gallon bucket and stir — before you start. It’s the standard fix for the faint color band that shows up where can two took over from can one.
Where the estimate goes wrong
- Big color changes. Two coats assumes similar tones. Going light over dark,
or applying a saturated red, orange, or yellow, commonly takes three coats.
Three coats on our 12 × 14 room is
416 × 3 ÷ 350 = 3.57→ 4 gallons. - Skipping primer over a dark base. A gray-tinted primer under a strong color is cheaper and faster than a third finish coat.
- New drywall. Bare gypsum and joint compound absorb at wildly different rates. Without a coat of drywall primer you’ll see every seam through two finish coats — this is flashing, and more topcoat doesn’t reliably cure it.
- Sheen changes. Going from gloss to flat, or the reverse, needs the surface scuffed and usually primed for adhesion.
- Paint you never get on the wall. The roller cover, tray, and brush hold paint you’ll never recover, and the last inch of a can is unusable. That’s part of why rounding up matters.
- Wanting touch-up paint later. Rounding up to the next gallon leaves you a labeled can for the inevitable scuff. Buying exactly enough leaves you nothing.
Conditions and timing
Interior latex wants roughly 50–85°F and humidity under about 70%. Cold rooms and damp air both stretch dry time; on a humid day a second coat applied too early will drag and roll up the first. Check the can — most interior latex says recoat after about 4 hours, and that’s a minimum, not a target.
Work in this order: ceiling first, then walls, then trim last. Ceiling spatter lands on unfinished walls where it doesn’t matter, and cutting trim against a finished wall is easier than the reverse. Cut in the edges of one wall and roll it while the cut line is still wet, rather than cutting the whole room first — a dried cut line telegraphs through as a visible band.
Frequently asked questions
How many square feet does a gallon of paint cover? About 350–400 ft² per coat on smooth drywall, and roughly 250 ft² on textured, bare, or porous surfaces. A quart covers about 100 ft².
How much paint for a 12 × 12 room?
384 ft² of wall at 8 ft ceilings, so 384 × 2 ÷ 350 = 2.19 → 3 gallons for
two coats. Add a gallon if you’re painting the ceiling too.
Do I really need two coats? Almost always. One coat rarely hides evenly, and “one-coat” paints still look better with two. Budget three for a major color change.
Is it cheaper to buy a 5-gallon pail? Per gallon, yes — from about 4 gallons up, the pail usually beats individual cans, and it guarantees a single tint batch.
How much paint for a whole house? Add the rooms up individually rather than guessing from square footage — hallways and stairwells have far more wall per floor area than bedrooms do. As a sanity check, 5 gallons at 350 ft²/gal covers about 875 ft² of wall at two coats.
The short answer
2 × (length + width) × ceiling height, multiply by coats, divide by 350, and
round up. Leave the doors and windows in — that’s your cut-in buffer. Drop
coverage to 250 for textured or bare walls, and price the pail at 4 gallons.
Get the exact gallon count for your room — coats, surface, ceiling, and cost — with the Paint Calculator, or see the other material guides.