How much paint you actually need
The honest formula is wall area minus what you don't paint, times the number of coats, divided by real coverage. Wall area is the room's perimeter times its height — so a 12×12 room with 8 ft ceilings is 2 × (12 + 12) × 8 = 384 sq ft of wall. Subtract about 21 sq ft per door and 15 per window and you're near 348. Two coats is 696 sq ft of painting, and at 350 sq ft per gallon that's two gallons. The ceiling, if you paint it, adds its own length × width on top.
Where people overspend or come up short is coverage. The "350–400 sq ft per gallon" on the can assumes smooth, sealed drywall. Fresh drywall, patched joints, and textured or porous walls soak up far more — closer to 300 — which is exactly why bare walls should be primed first: one cheap coat of primer seals the surface so your finish color covers in two coats instead of three.
Coverage by surface
| Surface | Sq ft / gal | Notes |
|---|---|---|
| Smooth, previously painted drywall | 350–400 | Best case; same or similar color |
| Light texture (orange peel) | 325–375 | Most homes land here |
| Heavy texture / knockdown | 275–325 | More surface area to cover |
| New / bare drywall | 250–300 | Prime first, then paint |
| Porous masonry, brick, stucco | 150–250 | Prime; expect heavy draw |
Frequently asked
How much paint do I need for a 12x12 room?
A 12×12 room with 8 ft ceilings has about 384 sq ft of wall. After subtracting one door and one window that is roughly 348 sq ft. At two coats and 350 sq ft per gallon coverage you need about 2 gallons for the walls. Add a third gallon if you are also painting the ceiling.
How much does one gallon of paint cover?
A gallon covers about 350–400 sq ft on smooth, previously painted drywall for one coat. Drop to roughly 300–350 sq ft on textured, porous or freshly primed surfaces, which drink more paint. Manufacturers print the exact coverage on the can — this tool lets you match it.
Do I need primer?
Prime when the surface is new/bare drywall, patched, stained, glossy, or when you are making a big color change (especially dark-to-light). One coat of primer seals the surface so your color covers evenly in two coats instead of three. Over clean, previously painted walls in a similar color, you can usually skip it.
How many coats of paint do I need?
Two coats is the standard for an even, durable finish and is what manufacturers assume in their coverage ratings. One coat only works over primer or when repainting the same color. Dark, saturated or drastically different colors can need three — factor that in before buying.
Should I buy extra paint?
Yes — buy about 10% more than the exact number, or at least one extra quart. It covers cut-in touch-ups, a heavier second coat, and future repairs with a guaranteed color match. Custom-tinted colors are hard to re-match later, so the extra is cheap insurance.
Sources
- Manufacturer coverage specifications: Sherwin-Williams, Benjamin Moore, Behr (350–400 sq ft/gal smooth, one coat).
- Standard opening deductions: ~21 sq ft per door, ~15 sq ft per window.
- Two-coat application assumption used in all major brand coverage ratings.