How Much Paint Do I Need? (Coverage per Gallon Guide)
One gallon of paint covers about 350 to 400 square feet in a single coat, so a typical bedroom takes one to two gallons per coat. But almost every wall needs two coats, which is where people run short. To buy the right amount, work out your paintable wall area first, double it for two coats, then divide by 350.
How much does a gallon of paint cover?
A standard gallon covers roughly 350 to 400 square feet with one coat, the range both Sherwin-Williams and Behr quote on their calculators. That assumes a smooth, clean, primed wall. Bare drywall, brick, or heavy texture soak up more and can drop coverage to 200 to 300 square feet, so read the surface before you trust the label.
How do I calculate the paintable area of a room?
Measure the perimeter of the room, multiply by the ceiling height to get gross wall area, then subtract the openings you won't paint. Painters use fixed allowances so the math stays quick: about 21 square feet per door and 15 square feet per average window. What's left is the surface your paint actually has to cover.
- Add up the wall lengths — for a rectangular room that's 2 × (length + width). A 12 × 15 room has a 54-foot perimeter.
- Multiply by ceiling height — 54 ft × 8 ft = 432 square feet of gross wall.
- Subtract the doors — one door at 21 sq ft leaves 411.
- Subtract the windows — two windows at 15 sq ft each is 30 sq ft, leaving 381 square feet of paintable wall.
How many gallons do I need for two coats?
Take your paintable area, multiply by two for the second coat, and divide by 350. Then round up to the next whole gallon. For the 381-square-foot room above, that's 762 square feet of coverage, or 2.18 gallons, which rounds to three gallons (or two gallons plus a quart if you want to trim the cost).
| Room size | Paintable area | Two-coat coverage | Gallons (2 coats) |
|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 538 sq ft | 2 |
| 12 × 12 ft | 333 sq ft | 666 sq ft | 2 |
| 12 × 15 ft | 381 sq ft | 762 sq ft | 3 |
| 15 × 15 ft | 429 sq ft | 858 sq ft | 3 |
| 15 × 20 ft | 509 sq ft | 1,018 sq ft | 3 |
| 20 × 20 ft | 589 sq ft | 1,178 sq ft | 4 |
The table assumes 8-foot ceilings with one door and two windows subtracted, at 350 square feet per gallon. Small rooms round up to two gallons on purpose, because running out halfway through a second coat means a trip to the store and a fresh can that may not match. A leftover quart is cheaper than a visible lap line. If your ceilings run 9 or 10 feet, or you have three or more windows, recheck the math rather than reading straight off the table, since taller walls add area fast: a 9-foot ceiling adds roughly an eighth to every wall.
Do I need primer, and how much?
Prime whenever the surface is bare, patched, stained, glossy, or much darker than your new color. Fresh drywall is the big one: it's porous enough to drink paint unevenly, so it needs a sealing coat first. Most bare walls take one coat of primer, covering a similar 200 to 300 square feet per gallon on thirsty drywall.
What else changes how much paint I need?
Coverage isn't a fixed number. Surface texture, paint quality, sheen, and application method all move it. Knowing which direction each one pushes helps you decide whether to trust the 350 figure or pad your order.
- Texture and porosity. Knockdown, orange peel, brick, and bare drywall all cut coverage. Add about 20% more paint for a textured surface.
- Sheen. High-gloss and semi-gloss go on thinner and reflect flaws, so they often need an extra coat versus a flat or eggshell finish.
- Paint quality. Premium paints carry more pigment (titanium dioxide), which means better hide and sometimes one fewer coat than a budget line.
- Color jump. Bold reds, deep blues, and yellows have weak hiding pigments and can need three coats even over primer.
Frequently asked questions
How much paint do I need for a 12x12 room?
A 12 × 12 room with 8-foot ceilings has about 333 square feet of paintable wall after subtracting a door and two windows. For two coats that's roughly 666 square feet, which fits in two gallons with a little to spare for touch-ups.
Does one gallon really cover 400 square feet?
Only under ideal conditions: a smooth, clean, primed wall with even application. On bare drywall, texture, or with a thin high-gloss paint, real coverage often lands closer to 250 to 300 square feet per gallon. Treat 400 as a ceiling, not a promise.
Do I always need two coats?
Almost always, yes. Two coats give even color and durability, even when the can advertises one-coat coverage. You can sometimes skip the second coat when repainting the same color over a clean, primed wall, but plan for two unless you have a reason not to.
Should I subtract doors and windows when measuring?
Yes, if you're leaving those openings bare. Use about 21 square feet per door and 15 per window. If you're also painting the trim or the door itself, don't deduct that opening, since you'll need paint for it too.
How much extra paint should I buy for touch-ups?
Keep at least a quart of the exact color from the same batch. Colors mixed at different times can vary slightly, so a leftover from your original gallons is the best match for future scuffs and nail holes.
Does primer count as one of my coats?
No. Primer seals and evens the surface but isn't your finish color, so it's separate from your two topcoats. Buy primer based on the same area math, then buy your color paint for the two coats on top.
Once you have your paintable area, the buying decision is simple arithmetic. Enter your wall square footage and coat count into the paint gallons by area calculator to get the exact number of gallons, rounded up and ready for the store, so you finish the room without a mid-job supply run.