How Much Paint Do I Need? (Calculator + Rules of Thumb)

Most rooms need about 2 gallons of wall paint. That's the short answer for an average bedroom or living room getting two coats. The math behind it is simple: one gallon of interior paint covers roughly 350 to 400 square feet per coat on a smooth, primed wall, according to Sherwin-Williams, so you measure the wall area, subtract doors and windows, multiply by the number of coats, and divide by the coverage on the can.

The things that throw the estimate off are predictable: a big color change, a dark color, bare drywall, texture, and forgetting the ceiling and trim. This guide covers each one with a worked example, so you leave the store with the right number of cans and a small, useful leftover instead of a garage shelf of half-empty gallons.

How much does a gallon of paint cover?

Coverage is printed on every can and on each product's data sheet, and it varies more than people expect. Here's what the major brands publish for common interior lines:

Two things to notice. First, these are best-case numbers on a smooth, sealed surface at the film thickness the manufacturer specifies. Rolling thin stretches paint further but leaves weak, streaky coverage, so don't treat the top of the range as a target. Second, coverage is per coat. When a can says 400 square feet and you're doing two coats, it really covers 200 square feet of wall.

For estimating, 350 sq ft per gallon is a sensible planning number. It sits at the low end of the premium-paint range and leaves a small cushion for the corners, cut-ins, and roller loading that eat paint in real life. If your product lists a different figure, use that one.

The wall-area formula

You only need a tape measure and a calculator:

1. Measure the perimeter. Add up the length of every wall. For a rectangular room, that's 2 x (length + width).
2. Multiply by the wall height (floor to ceiling, not counting the ceiling itself). That's your gross wall area.
3. Subtract openings. A common estimating rule is 20 sq ft for each standard door and 15 sq ft for each average window. Measure anything unusual, like a sliding patio door or a wall of windows.
4. Multiply by the number of coats.
5. Divide by the coverage per gallon and round up.

Wall area to paint = (perimeter x height) - (doors x 20) - (windows x 15)

Gallons = (wall area x coats) / coverage per gallon

For non-rectangular rooms, measure each wall and add them. Vaulted or sloped walls are a rectangle plus a triangle: the triangle's area is half the base times the height.

Worked example: a 12 x 14 room with 8-foot ceilings

Say you're repainting a 12 x 14 ft living room with 8-ft ceilings, one door and two windows, moving from beige to a soft white.

What to buy: two gallons will cover it if your paint performs near the top of its range. At the more conservative 350 figure you're slightly short, so two gallons plus a quart is the safe order. A quart covers about 100 square feet, which also becomes your touch-up reserve.

If the same room were going from white to a deep navy, plan on three coats or a tinted primer plus two coats (see below), which pushes the wall paint to about 3 gallons.

Add the ceiling and trim to the same room

• Ceiling: 12 x 14 = 168 sq ft. One coat of ceiling paint needs about half a gallon; two coats take about a gallon. Buy one gallon.
• Baseboards: a common shortcut is to treat trim as half a foot wide. The 52-ft perimeter minus about 3 ft for the doorway is 49 ft, x 0.5 = about 25 sq ft.
• Door and window casings: one estimating guide suggests figuring about half the window area for window trim and sash, so about 15 sq ft for two windows. Add about 20 sq ft for one side of the door slab.
• Trim total: about 60 sq ft, or about 120 sq ft for two coats. That's slightly more than one quart, so buy two quarts or a gallon of semi-gloss, depending on the shelf price and whether you'll paint more trim later.

How many coats do you actually need?

Two coats is the default for walls, and it's what manufacturers assume when they recommend a finish system. Sherwin-Williams' Emerald Interior data sheet, for example, specifies two topcoats. One coat can work when you're refreshing the same color on sound, clean walls, but a single coat usually shows roller marks and thin spots near cut-in lines.

Plan for extra coats or primer in these situations:

• New or bare drywall. Joint compound and paper face absorb paint unevenly. Prime first, then two coats.
• Heavily patched walls. Patches flash (show as dull spots) through a single coat. Spot-prime the patches.
• Dark-to-light changes. Covering a deep red or navy with a white often takes a primer coat plus two topcoats.
• Light-to-dark or saturated colors. Deep and vivid colors are built on clearer bases and hide poorly. Sherwin-Williams recommends a gray-tinted primer for about 20 percent of its color palette, and says the right shade of gray basecoat gets you to the true color in fewer coats.
• Glossy existing finishes. Scuff-sand and use a bonding primer so the new paint grips.

When you add primer, estimate it separately. With primer covering about 200-300 sq ft per gallon, the 366 sq ft example room needs about 1.5 gallons of primer for one coat, so buy two gallons, or one gallon if you're only spot-priming patches.

Ceilings, trim, and doors

Ceilings are calculated the same way as floors: length x width. Most people use a flat ceiling paint because it hides surface flaws and roller lap marks. Since you're working overhead, two coats is normal; a well-hiding ceiling paint over the same white may need only one.

Trim is where people over-buy. Trim is narrow, so even a lot of it adds up to modest square footage. For a single room, a quart or two is usually right. Use the half-foot-wide shortcut above for baseboards and crown molding.

Doors are about 20 sq ft per side. A quart will do two coats on both sides of one door with room to spare.

For exteriors, Behr offers a simple ratio: about one gallon of trim paint for every eight gallons of body paint. That ratio is a reasonable sanity check if you're also planning a house repaint. Our exterior color combinations pages help with picking the body and trim colors themselves.

Textured walls, bare surfaces, and other paint eaters

Coverage numbers assume smooth, sealed drywall. Real walls often aren't.

• Orange peel and knockdown texture. More surface area means more paint. Behr's guidance is that textured walls take more paint, which puts you toward the lower end of its 250-400 sq ft range. Using 300 sq ft per gallon for planning is a reasonable hedge.
• Popcorn and heavy textured ceilings. This Old House suggests buying 15-20 percent more than for a smooth ceiling; Bob Vila goes further and suggests planning for double. Popcorn soaks up paint, so the cautious number is safer. If the house predates the late 1970s, have the texture tested for asbestos before you disturb it.
• Bare drywall, plaster, wood, or masonry. Porous surfaces drink the first coat. Prime them, and use the primer's coverage figure, not the paint's.
• Spraying. Airless sprayers lose material to overspray. Behr notes its listed coverage doesn't account for paint lost to spraying.
• Rough-sawn wood, brick, and stucco. Exterior texture can cut coverage dramatically. Check the specific product's data sheet for those surfaces.

Gallon, quart, or 5-gallon bucket?

Once you have a number, buy in the containers that waste the least:

Mixing for consistency. When you're using more than one can of the same custom color, painters often "box" the paint: pour the cans into one 5-gallon bucket and stir them together. Dunn-Edwards demonstrates this in its how-to videos. Any small tint difference between cans would otherwise show up where one can ends mid-wall; boxing removes the risk.

Buy a little extra, not a lot. The Northeast Waste Management Officials' Association's paint-waste guide notes that professional painters recommend finishing with about one quart left over for touch-ups. Paint stores mix colors from standard formulas, so you can always buy more of the same color later. Write the formula sticker info down anyway, because a later batch can be slightly off.

Storing and dealing with leftovers

A sealed, labeled quart of the exact wall color is worth more than any patch kit. Benjamin Moore's storage guidance boils down to:

• Seal it right away. Clean the rim, lay plastic wrap over the opening, set the lid, and tap it closed with a rubber mallet.
• Keep it between 60 and 80°F, out of direct sun, and away from garages or sheds that freeze.
• Downsize the container. If only a little is left, move it to a smaller lined metal, glass, or plastic container so there's less air in it.
• Label it with brand, color name and number, product line, sheen, room, and date. Benjamin Moore even suggests writing it on the back of a light switch plate in the painted room.
• Expect about two years of shelf life once opened, according to Benjamin Moore, though temperature swings shorten that.

If you truly have too much, don't pour it down a drain or put liquid latex in the trash. In states with paint stewardship programs (including California, Colorado, Connecticut, Illinois, Maryland, Minnesota, New York, Oregon, Washington, and several others), PaintCare runs drop-off sites that take leftover paint for recycling.

Before you buy: get the color right first

The math only matters if the color is the one you'll keep. Repainting because the color was wrong doubles everything above. A cheap order of operations:

1. Shortlist colors and preview them on a photo of your actual room with the paint colors tool. It applies real brand colors to your walls so you can rule out wrong directions in minutes.
2. Buy samples of your top two and test them on the wall, following the routine in how to test paint colors before buying.
3. Only then calculate and buy your gallons.

If you're still choosing a color for the main living space, our living room paint color ideas covers real colors by mood. On-screen colors are approximate, so always confirm with a physical sample in your own light.

Sources

• Sherwin-Williams: Paint Calculator (accessed 2026-10-12)
• Sherwin-Williams: Emerald Interior Acrylic Latex Satin, product data sheet (accessed 2026-10-12)
• Sherwin-Williams: Cover Your Bases (gray-tinted primer and the Color Prime System) (accessed 2026-10-12)
• Benjamin Moore: Regal Select Waterborne Interior Paint, Matte (N548) (accessed 2026-10-12)
• Benjamin Moore: Regal Select Interior brochure (accessed 2026-10-12)
• Benjamin Moore: 6 Tips for Storing Leftover Paint and Shelf Life (accessed 2026-10-12)
• Behr: Determine How Much Interior Paint to Buy (accessed 2026-10-12)
• Behr: How Much Exterior Paint to Buy (accessed 2026-10-12)
• Behr: Behr Dynasty Interior Matte (Pro product page) (accessed 2026-10-12)
• Dummies: Estimating How Much Paint to Buy (accessed 2026-10-12)
• This Old House: How to Paint a Popcorn Ceiling (accessed 2026-10-12)
• Bob Vila: How to Paint a Popcorn Ceiling (accessed 2026-10-12)
• Dunn-Edwards: How to Mix Paint for Color Consistency (accessed 2026-10-12)
• NEWMOA: Tips for Minimizing Paint Waste (accessed 2026-10-12)
• PaintCare: Paint recycling program and drop-off sites (accessed 2026-10-12)