The short answer is one gallon per 350 to 400 square feet per coat, but that number doesn’t account for doors, windows, or whether you’re making a big color change. Let me walk through the actual math.
How much paint do I need: the formula
Start with your wall area. Measure the perimeter of the room (the total of all four walls’ widths) and multiply by the ceiling height. A 12 by 14 foot room with 9-foot ceilings has a perimeter of 52 feet. Multiply by 9 and you get 468 square feet of wall.
Subtract the areas you’re not painting. I use 21 square feet for a standard door (roughly 3 x 7 feet) and 15 square feet for a standard window (roughly 3 x 5 feet). If you have one door and two windows in that room, subtract 21 + 30 = 51 square feet. That leaves 417 square feet of actual paintable wall.
Divide by your paint’s coverage rate. Most interior paint covers 350 to 400 square feet per gallon per coat. I use 350 in my own calculations because real walls rarely hit the top number: texture, porosity, and applicator loss all eat into it. So 417 / 350 = 1.19 gallons per coat.
Multiply by the number of coats. For two coats, that’s 1.19 x 2 = 2.38 gallons.
Add a 10 percent waste allowance for roller loss, edge work, and touch-ups. 2.38 x 1.10 = 2.62 gallons. Round up to 3 gallons.
If you’re also painting the ceiling, add floor area (length x width), in this case 168 square feet. Ceilings usually only need one coat of ceiling paint if the existing ceiling is already white, so add 168 / 350 = 0.48 gallons, round up to 1 quart of ceiling paint.
Coverage rate: what the can actually means
Paint manufacturers print coverage on the label, typically 350 to 400 square feet per gallon. Sherwin-Williams, Benjamin Moore, and most premium brands assume one coat over a smooth, previously painted surface of similar color. That assumption holds if you’re touching up or repainting the same color. It does not hold if you’re going dark-to-light, painting fresh drywall, or rolling over a textured surface.
On bare or heavily textured walls, expect coverage closer to 250 to 300 square feet per gallon. On a big color change (navy to white, for instance) plan on three coats rather than two, or use a tinted primer to close the gap first. A tinted primer (one matched to your topcoat color) is almost always the more efficient approach. It reduces the number of paint coats needed and usually costs less than an extra coat of the finish color.
One coat vs. two coats: when it matters
Most interior jobs need two coats. The exception is repainting the same color (or very close to it) over a surface in good condition. In that case, a single coat of good-quality paint is usually fine and there’s no point buying a second gallon you won’t use.
New drywall always needs primer plus two coats of paint, sometimes three. The drywall paper and joint compound absorb the first coat heavily, and what you see after the first coat dries will look patchy. That’s normal, not a quality problem. Don’t skip the second coat.
Dark-to-light color changes are where most DIY paint jobs go wrong. People buy enough paint for two coats, apply them, and then stand there looking at their old navy walls bleeding through the new white. The fix is a stain-blocking primer first, or committing to three coats. I’d go primer: it’s cheaper and faster than a third coat of finish paint.
A room-by-room guide
Living room
A typical living room runs 15 x 18 feet with 9-foot ceilings. That’s about 594 square feet of wall. Subtract two doors and three windows (42 + 45 = 87 sq ft) and you’re at 507 square feet. Two coats at 350 sq ft/gal coverage = about 2.9 gallons; add 10 percent and round up: 4 gallons. Living rooms often have larger windows, so measure yours rather than using the average.
Bedroom
A standard bedroom at 12 x 12 feet with 8-foot ceilings gives you 384 square feet of wall. One door, two windows = 51 sq ft gone. That leaves 333 square feet per coat, or 666 for two coats. Divide by 350 = 1.9 gallons; add waste and round up: 2 gallons. Most bedrooms take 2 gallons comfortably.
Bathroom
Small bathrooms (6 x 8 feet, 8-foot ceilings) have only about 224 square feet of wall, minus one door and one window. You’re looking at roughly 188 paintable square feet per coat. Two coats = 376 square feet total, just a little more than one gallon at 350 square feet per gallon, so budget 2 gallons to cover the math and touch-ups. If you’re painting above the tile line only, measure just those sections and you may get away with 1 gallon.
Kitchen
Kitchen walls are complicated by upper cabinets, backsplash tile, and windows above the sink. I’d measure the actual open wall area rather than using room perimeter. Sketch the walls and mark what gets painted; it’s worth the five minutes. Otherwise you’ll either overbuy or find yourself short at the worst moment.
How to measure correctly
Use a tape measure, not steps or guesses. Measure each wall’s length at floor level and record ceiling height. If your ceiling is taller in one section (cathedral ceiling, for example) measure the tallest point. For L-shaped or irregular rooms, break the room into rectangles, calculate each section, and add them. I wrote a full guide on how to measure a room if you want the step-by-step. It’s worth reading before you head to the paint counter.
Primer: do you actually need it?
Primer is not optional in these situations: new drywall, bare plaster, major stain coverage (water stains, marker, smoke), significant color change, or painting over glossy surfaces. In every other case (repainting a room in good condition with a similar color), primer is usually optional if you’re using a quality paint-and-primer product.
Primer coverage runs about 200 to 300 square feet per gallon (it soaks into surfaces more than finish paint does). Calculate primer separately from your topcoats. If you need one coat of primer over 500 square feet, buy two gallons and expect to use them both.
Tips I actually follow
Buy all your gallons from the same batch. The batch number (dye lot) is printed on the lid. Different batches can have slight color variation that shows up as a subtle line on your wall where one gallon ended and the next began. This is especially obvious in low-angle light.
Keep a record of the color formula. Paint stores can reproduce any color from the formula code stored in their system, but if you go back after a year the formula should be saved under your name. Ask for a printout and tape it inside a cabinet door near where the paint was used.
A 4-inch brush at edges before rolling is worth every minute. It’s slower than trying to cut in with the roller, but the result is a cleaner line and fewer roller marks in corners. I’d estimate cutting in properly saves one coat’s worth of touch-ups.
If you want to skip the math entirely, the paint calculator on this site does everything above: room dimensions, door and window deductions, coat count, and waste factor, with a full breakdown you can check against your own numbers.
FAQ
How many gallons of paint do I need for a 12×12 room? Usually 2 gallons for two coats on the walls. A 12 x 12 room with 8-foot ceilings has about 384 square feet of wall; subtract a door and two windows and you’re around 333 square feet per coat. Two coats = 666 square feet total, which is just under 2 gallons at 350 sq ft/gal coverage.
How much paint do I need for a 10×10 room? One to 2 gallons for walls, depending on ceiling height and the number of openings. A 10 x 10 room with 8-foot ceilings has about 320 square feet of wall before subtracting doors and windows, so one gallon handles one coat and two gallons cover two coats with some left over.
Can I get away with one coat of paint? If you’re repainting the same color over a clean, intact surface, yes. For any color change or new surface, plan on two. “Self-hiding” and “one-coat” claims on cans are marketing; in practice, one coat of even the best paint shows brush lines and thin spots on most walls.
Does the calculator account for primer? No. Calculate primer separately using the same area math, with a coverage rate of 200 to 300 square feet per gallon (rather than 350 for finish paint). Primer gets its own gallons; don’t fold it in with your topcoat count.
What if I have vaulted or cathedral ceilings? Measure the wall height at the tallest point and use that number. You’ll slightly overestimate for the lower sections, but the extra paint is better than running short.
Built and maintained by Sam Walsh. Last updated July 2026. Paint coverage varies by surface condition, applicator, and brand. These figures are conservative estimates for planning purposes.
Ready to run the numbers for your room? Use the paint calculator: it does the math above automatically and shows every step. Not sure how to measure the room correctly? See how to measure a room.