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Gallons from your room
Room dimensions minus doors and windows, times coats — the honest gallon count.
REFERENCE TABLE · HOME · CONSTRUCTION
Paint Coverage Chart: How Many Gallons Do You Need?
The trade-standard answer is 350 to 400 square feet per gallon, one coat — which makes the classic 12×12 bedroom a two-gallon job (333 paintable square feet per coat after the door and windows, times two coats, rounded up). The gotchas: primer covers less (200–300 sq ft/gal on bare drywall), two coats is the standard for any color change, and a door eats ~21 sq ft you shouldn’t pay to paint. Below: the coverage-by-surface chart, the deduction table, the coats table, and three live calculators — gallons from your room, square feet from your gallons, and primer.
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Square feet from your gallons
Working backwards: what a can in the garage actually covers.
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Primer gallons
Bare drywall and repairs drink primer — size it separately.
Coverage by surface
| Surface | Sq ft / gallon (one coat) | Notes |
|---|---|---|
| Interior walls (latex) | 350–400 | The headline number; 350 is the safe planning figure |
| Ceilings (flat) | ~350 | Flat paint hides; same math as walls |
| Primer on bare drywall | 200–300 | Thirsty surface — size primer separately (calculator 3) |
| Previously painted walls (recoat) | 350–400 | Sealed surface, full coverage rate |
| Exterior siding | 250–400 | Rough-sawn wood drinks paint; smooth siding sips |
| Trim and doors (satin/semi-gloss) | 400–500 | Smooth, non-porous — a quart often does a room’s trim |
What to deduct
| Opening | Deduct (sq ft) | Assumes |
|---|---|---|
| Interior door | 21 | 3 × 7 ft |
| Window | 15 | 3 × 5 ft |
| Sliding glass door | 40 | 6 × 6.7 ft |
How many coats
| Situation | Coats | Why |
|---|---|---|
| Same color, good condition | 1–2 | One refresh coat often suffices |
| Color change | 2 | The standard — first coat never fully hides |
| Bare drywall / fresh mud | Primer + 2 | Seal first or the finish flashes |
| Dark to light (or reverse) | 2–3 | Tinted primer earns its keep here |
The math behind calculator 1: paintable area = 2×(L×H) + 2×(W×H) − 21×doors − 15×windows; gallons = ceil(paintable ÷ coverage × coats). The 12×12×8 default: 384 − 51 = 333 sq ft per coat × 2 coats ÷ 350 = 1.90 → 2 gallons. Always round up — and buy the 10% extra; touch-ups from a mismatched batch are how walls get haunted.
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Gallons needed | ceil(paintable ÷ coverage × coats) | Buy this many: 2 gallons at the defaults. |
| One-coat coverage | gallons × coverage | What your cans cover in a single pass: 350 sq ft per gallon. |
| Primer gallons | ceil(area ÷ primer coverage) | Size it separately: 2 gallons for 333 sq ft of bare drywall. |
Who uses this
- DIY room painters: the Saturday-morning question — how many gallons before the store run.
- Landlords and flippers: sizing paint for turns across identical units without overbuying.
- New drywall jobs: remembering primer is a separate purchase at a worse coverage rate.
- Researchers & students: the 350-sq-ft figure is the industry standard — cite it with the block below.
Accuracy & limitations
Coverage figures are trade-standard planning rates — actual mileage varies with roller nap, surface porosity, color, and technique; dark colors over light primer can need a third coat. Deductions are standard opening sizes; measure yours if they’re oversized. This sizes material only — not labor, prep, or ceiling paint (same math, flat sheen).
Cite this reference tableBright Side Kit. “Paint Coverage Chart: Gallons Per Room.” brightoolbox.com, 2026.
Bright Side Kit. "Paint Coverage Chart: Gallons Per Room." https://brightoolbox.com/paint-coverage-chart/ Accessed October 23, 2026.Frequently asked questions
How much does one gallon of paint actually cover?
The can says 350–400 square feet per coat for interior latex — and 350 is the honest planning number. That’s roughly one coat of one wall set in a 12×12 room (333 paintable sq ft after the door and windows). Porous or textured surfaces land at the low end; smooth previously-painted walls hit the high end.
Do I really need primer?
On bare drywall, fresh mud, or drastic color changes: yes. Primer seals the thirsty surface so the finish coat doesn’t flash (dry unevenly), and it covers at a worse rate — 200–300 sq ft/gal — so size it separately with calculator 3. On previously painted walls in decent shape, a quality paint-and-primer does the job without a dedicated primer coat.
One coat or two?
Two, for any color change. The first coat never fully hides the old color — the second is where the depth and uniformity come from. Same-color refreshes in good condition can get away with one. Bare drywall is primer + two; dark-to-light jumps sometimes want a tinted primer plus two.
Why buy 10% extra?
Because touch-up paint from a different batch never matches perfectly — tint varies slightly can to can. The extra quart lives in the basement for scuffs, nail holes, and the wall the couch scraped. Running short mid-project and buying a fresh gallon risks a visible shade line down the middle of a wall.
How do I figure ceiling paint?
Same math, flat sheen: ceiling area is just length × width (144 sq ft for the 12×12 default — under half a gallon per coat, so one gallon does two coats with room to spare). No deductions needed unless you have skylights. Most people roll the ceiling first, then cut the walls into it.
Why does coverage vary so much (250 vs 400)?
Three variables: surface porosity (bare drywall and rough-sawn siding drink paint), application (a thick-nap roller lays more than a sprayer), and color (deep bases have less hiding pigment per gallon). Plan at 350 for interior walls; drop to 250–300 for exterior rough surfaces and primer.
Can I mix leftover paints to make enough?
Only to finish the same wall, the same day, mixed thoroughly in one bucket (painters call it boxing). Different sheens, brands, or ages can flash, peel, or cure unevenly — and the color will drift. For touch-ups months later, nothing beats the original can.