Paint Calculator: Gallons & Cost to Paint a Room

How to measure a room for paint

Measure the length of each wall and the floor-to-ceiling height in feet. The calculator multiplies the room’s perimeter by the wall height, then deducts standard estimating allowances: 21 sq ft per door (a 3 × 7 ft opening) and 15 sq ft per window (3 × 5 ft). If you have a wide patio slider, a large picture window or an open archway, count it as two or three openings, or subtract its actual area by shortening a wall dimension.

Count a door only if you are not painting it with wall paint. Doors and trim usually get a semi-gloss or satin enamel, bought separately.

Worked example: a 12 × 12 ft bedroom

These are the calculator’s defaults: 8 ft walls, one door, two windows, two coats, 350 sq ft per gallon and $40 per gallon, walls only.

Adding the ceiling brings in another 144 sq ft: one more gallon of ceiling paint, and $477–$1,431 for a painter covering walls and ceiling together.

Coverage is the number to get right

Manufacturers list a coverage range on the can, often 350–400 sq ft per gallon per coat on smooth, previously painted surfaces. The calculator defaults to the low end, 350, because real walls are rarely perfect. Use a lower figure for:

• Textured walls such as knockdown or orange peel, which have more surface area.
• New drywall or fresh patches, which soak up paint unless primed.
• Deep or bright colors, which may need a third coat to look even.

The coats table shows the gallons for one, two and three coats so you can see where a third coat tips you into another can.

When to prime

Turn on the primer option for bare drywall, large patched areas, water or smoke stains, glossy surfaces, or big color changes such as going from dark red to off-white. Primer is calculated as one coat over the same area and priced separately, defaulting to the low end of the paint price range. On clean, previously painted walls in a similar color, two coats of a quality paint usually cover without primer. If you are hanging new board first, the drywall calculator estimates sheets, tape and mud.

DIY or hire a painter?

Painting is the most common DIY project for good reason: in the default example, paint costs $80 while a painter’s estimate runs $333–$999. Most of a painter’s price is labor and prep: moving furniture, filling holes, caulking, masking, cutting in clean lines and cleaning up. The pro estimate here uses a national range per sq ft of wall area and assumes paint and standard prep are included. Use the local cost level setting to shift it for your area.

Hiring makes more sense for tall stairwells and vaulted ceilings that need scaffolding, extensive drywall repair, lead-paint homes built before 1978 (which require certified renovators for many jobs), or when your time is worth more than the savings.

What’s not included

Trim, baseboards, door and window casings, and the doors themselves; wall repair beyond light patching; wallpaper removal; and DIY supplies such as rollers, brushes, trays, painter’s tape, drop cloths and patching compound. Plan the order of work too: paint before new flooring goes in. The flooring calculator can price the floor once the walls are done.

How this calculator works

• Wall area = 2 × (length + width) × height − doors × 21 − windows × 15 sq ft. Ceiling area = length × width.
• Gallons = area × coats ÷ coverage per gallon, rounded up to whole gallons, calculated separately for walls, ceiling and primer.
• DIY cost = gallons × your price per gallon. Painter cost = painted area × the national per-sq-ft range × local cost factor.

Assumptions and limitations

• Each door deducts 21 sq ft and each window 15 sq ft, standard estimating allowances for a 3 × 7 ft door and a 3 × 5 ft window.
• Paint and primer are bought in whole gallons. Coverage defaults to 350 sq ft per gallon per coat, the low end of a typical manufacturer range for smooth, previously painted walls.
• Painter pricing uses a national range per sq ft of wall area, applied to the ceiling area as well, and is assumed to include paint and standard prep. Priming is not priced separately for the pro estimate.
• Not included: trim, doors and window casings, wall repair beyond light patching, wallpaper removal, and DIY supplies such as rollers, brushes, tape and drop cloths.

These results are planning estimates, not quotes. Local prices, site conditions and building codes vary, so get written estimates from licensed contractors before you commit to a budget.

Frequently asked questions

How much paint do I need for a 12 × 12 room?

With 8 ft walls, one door and two windows, there are 333 sq ft of wall. Two coats at 350 sq ft per gallon need 1.9 gallons, so buy 2 gallons. Add 1 gallon for the ceiling.

Do I need primer?

Prime bare drywall, patched areas, stains, glossy surfaces and dramatic color changes. On clean, previously painted walls in a similar color, two coats of good paint usually cover without primer.

Should I buy one extra gallon?

Usually not if the math already rounds up, but keep any leftover for touch-ups. If your result is very close to a whole gallon, or the walls are textured or freshly patched, an extra quart or gallon avoids a mid-job trip and color mismatch between batches.

Sources and data

• Sherwin-Williams product data sheet — 035777309247
• HomeGuide (2026) — how much does a gallon of paint cost
• HomeGuide (2026) — cost to paint interior of house
• Angi (2026) — how much does it cost paint room

Prices are US national ranges reviewed October 2026. A few values are editorial estimates where no single published source exists; theCost Index marks each one. Read our methodology and editorial policy.