Paint Calculator: How Much Paint Do I Need for a Room?

Standing in the paint aisle doing mental math is how people end up with either three half-empty gallons in the garage or a wall that runs dry two feet from the finish line. The honest answer to how much paint do I need is a quick bit of arithmetic: measure the room, subtract the doors and windows, multiply by your number of coats, and divide by how far a gallon goes. The calculator below does all of that instantly. The rest of this guide explains where the numbers come from so you can trust the result and adjust it for your real walls.

Quick answer: To find how much paint you need, multiply the room's perimeter by its ceiling height to get the wall area, subtract about 21 square feet per door and 15 per window, multiply by your number of coats (usually two), and divide by 350 square feet per gallon. A typical 12-by-12-foot room with 8-foot ceilings needs about 2 gallons for two coats of wall color, plus roughly a quart each for the ceiling and trim. Always buy one extra quart for touch-ups.

How the paint calculator works

Behind the scenes, the calculator runs the same formula a pro estimator uses, just faster. Here's each piece.

Step 1: the wall area

A room's wall area is its perimeter times its height. For a rectangular room that's:

Wall area = 2 × (length + width) × height

A 12-by-12-foot room with 8-foot ceilings gives 2 × (12 + 12) × 8 = 384 square feet of gross wall. That's before you take anything off for the parts you won't paint.

Step 2: subtract doors and windows

You don't paint doors and windows, so they come out of the total. The standard trade allowances are:

One door and one window take our 384-square-foot room down to about 348 square feet of paintable wall. For a small room this adjustment is minor, but in a room with several large windows or sliding doors it can save you a whole gallon — so it's worth doing.

Step 3: multiply by coats

Almost every paint job needs two coats for an even, durable finish with full color depth. So the surface you're actually covering is the wall area times the number of coats: 348 × 2 = 696 square feet. Use a single coat only when you're repainting the exact same color over a sound, clean wall. Dramatic changes — a bold color, or light over dark — sometimes need a third coat or a tinted primer.

Step 4: divide by the spread rate

This is where the famous number comes in. A gallon of wall paint covers about 350 square feet per coat on smooth, primed, previously painted drywall. That's the spread rate, and it's printed on every can. Divide your total surface by 350:

696 ÷ 350 = 1.99 gallons → buy 2 gallons

The calculator rounds up to real can sizes (gallons and quarts), because paint isn't sold by the ounce and running short mid-wall is the one outcome you want to avoid.

A worked example

Take that 12-by-12-foot bedroom with 8-foot ceilings, one door, and one window, painted two coats:

1. Gross wall: 2 × (12 + 12) × 8 = 384 sq ft
2. Subtract openings: 384 − 21 (door) − 15 (window) = 348 sq ft
3. Two coats: 348 × 2 = 696 sq ft
4. Divide by 350: 696 ÷ 350 ≈ 1.99 → 2 gallons of wall paint
5. Ceiling: 12 × 12 = 144 sq ft, one coat → about 2 quarts
6. Trim and baseboards: roughly 1 quart

So the whole room is about two gallons of wall color, a quart or two of ceiling white, and a quart of trim paint — plus a spare quart for touch-ups. Bump the ceiling height to 9 or 10 feet and the wall paint climbs; enter your real numbers above to see exactly how much.

How much paint for common room sizes

If you just want a fast ballpark, here's the wall paint for typical room sizes — all assuming 8-foot ceilings, one door, one window, and two coats. These come straight from the calculator above, so they match it exactly; enter your real height and openings for an exact figure.

Notice how close the 10-by-12 and 12-by-12 rooms are — both land at two gallons — while taller ceilings or extra windows can tip a room into the next quart. That's exactly why the calculator subtracts your real openings instead of guessing: it's the difference between buying a clean two gallons and over-buying a third.

Why a gallon doesn't always cover 350 square feet

The 350-square-foot figure is a best case. It assumes a smooth, sealed, previously painted surface. Several real-world things eat into it, and ignoring them is how people run short:

• Texture. Knockdown, orange-peel, or popcorn surfaces have far more area than a flat wall and can drop coverage to 200–250 square feet per gallon.
• Bare or porous surfaces. New drywall, fresh joint compound, raw wood, and masonry soak up the first coat. Prime them first or expect heavy coverage loss.
• Color changes. Hiding a dark wall with a light color — or getting a bold red or deep blue to look even — often needs an extra coat regardless of the math.
• Application method. Spraying lays paint down beautifully but wastes a lot to overspray, so sprayed jobs use noticeably more than rolled ones.

When any of these apply, round up or add a coat to your estimate. Over-buying by a quart costs a few dollars; under-buying costs a second trip and risks a visible seam where a new batch meets the old one.

Don't forget primer, ceiling, and trim

Walls are only part of a room. Budget for the rest:

• Primer is a separate undercoat on bare drywall, big patches, stains, or drastic color changes. It seals the surface so your finish coat goes on evenly. "Paint-and-primer-in-one" products are fine for similar-color repaints, but a dedicated primer still wins on raw or stained surfaces. Primer counts as a coat when you estimate quantity.
• Ceiling paint is usually a flat white and is figured separately — length times width, one coat. A 12-by-12 ceiling needs only a quart or two.
• Trim, doors, and baseboards use a more durable, glossier paint and a surprisingly small amount. A quart of trim paint covers the baseboards and casing of a typical room.

The sheen you choose matters too: flatter finishes hide wall flaws but scuff easily, while satin and semi-gloss wipe clean and suit trim, kitchens, and bathrooms. Buy touch-up paint in the same sheen as the original, or it will dry to a slightly different look even in the identical color.

What it costs to paint a room

Paint itself is the small part of the bill; the supplies and your time (or a pro's) are the rest. Here's how a single average room breaks down.

Doing it yourself, an average room runs roughly $120–250 in paint and supplies — and the reusable tools carry over to the next room. A pro typically charges $350–800 per room depending on prep, ceiling height, and trim detail. For the bigger picture on what home projects cost, see the home maintenance budget calculator.

A simple paint-buying checklist

That last step is the one most people skip and later regret. A wall gets scuffed, you can't remember the color, and a "quick touch-up" turns into repainting the whole wall to hide a mismatch. Recording the color and sheen per room — and keeping the matching quart — is what makes touching up interior paint a five-minute job instead of a project.

Sources & method

• The 350-square-feet-per-gallon spread rate is the long-standing industry rule of thumb for wall paint on smooth, primed surfaces, printed on the coverage label of virtually every can. The calculator uses it as the baseline and recommends rounding up for textured, raw, or porous walls.
• Door (~21 sq ft) and window (~15 sq ft) deductions reflect a standard 3-by-7-foot door and a typical double-hung window; for large or numerous openings, measure and subtract the actual area.
• Two coats is treated as the default for full, durable coverage; single-coat estimates assume an identical-color repaint over a sound surface.
• Cost ranges are 2026 planning estimates for an average room and vary by paint grade, region, prep, and ceiling height. They are guides for budgeting, not quotes.