A 12-by-12 bedroom with 8-foot ceilings has 384 square feet of wall. That number is easy. The gallon count is not, because the answer swings between 1.7 and 2.7 gallons depending on one figure printed in small type on the can. Here is the whole calculation, done by hand, with the spots where it usually goes wrong.
Wall area first, then subtract what you will not paint
Perimeter is 12 + 12 + 12 + 12 = 48 feet. Times the 8-foot ceiling: 384 square feet of gross wall.
Now the openings. Say the room has one interior door and two windows. Measure them; if your door is a 30-by-80-inch interior door (a common stock size), that is 2.5 ft × 6.67 ft = 16.7 square feet. Two windows at 36 by 60 inches are 15 square feet each, 30 total.
384 − 16.7 − 30 = 337.3. Call it 337 square feet of paintable wall.
Most online calculators skip the tape measure and subtract a flat 20 square feet per door and 15 per window (CalculatorSoup uses 21 and 15, based on a 3-by-7 door and a 3-by-5 window). For one 30-inch door the difference is 3 square feet, which rarely changes the can count. For a closet with 48-inch bifold doors, the flat number undercounts by 7 square feet, and a wall of sliding glass is off by 40 or more. Measure the big openings. Guess the small ones.
Two coats is the default, not the upgrade
Benjamin Moore's calculator states that its results "assume 2 coats of paint." Glidden's says a "drastic color change" typically needs a second coat. Two coats is what the manufacturers themselves plan for.
So the 337 becomes 674 square feet of coverage to buy.
This is slip number one. People compute 337, see "about one gallon," and buy one gallon. The second coat then comes from a second trip, and the second can is sometimes a different tint batch.
The number on the can moves the answer by a full gallon
Here is where the same room produces different shopping lists.
Sherwin-Williams' calculator FAQ puts a gallon at "about 350-400 square feet." Behr's product page for Premium Plus interior paint says "250-400 sq. ft. per gallon depending on the application method and substrate porosity" (the same wording appears on its Ultra and Marquee pages). Glidden says "up to 400 sq. ft. on primed, smooth, non-porous surfaces."
Run 674 square feet through each:
- At 400 sq ft per gallon: 674 ÷ 400 = 1.69 gallons.
- At 350: 674 ÷ 350 = 1.93 gallons.
- At 250: 674 ÷ 250 = 2.70 gallons.
Same walls. Same two coats. One gallon of difference, and it comes entirely from which end of the printed range your walls land on. Fresh drywall, patched plaster and anything with texture pull toward the low end, because the surface drinks the first coat before it has a chance to build a film, which is also why Sherwin-Williams rates primer at only 200 to 300 square feet. Previously painted smooth walls in a similar color sit near the top.
That is slip number two: taking the 400 because it is the biggest number on the label. Treat 400 as the ceiling, not the estimate. If your can says 250-400 and you do not know your walls, 300 is an honest middle.
I built the Wall Math paint calculator to make you type the coverage figure from your own can instead of using a hidden default, for exactly this reason. A default of 400 quietly under-orders for any can whose printed range starts below 400.
Waste, then round to what the store sells
Add an allowance for the roller tray, the cut-in brush and the wall you touch up on Sunday. Ten percent is the usual figure: 674 × 1.10 = 741 square feet.
At 350 sq ft per gallon that is 2.12 gallons. Sherwin-Williams lists a quart at 100 square feet, so 2.12 gallons is two gallons plus one quart, with 0.13 gallon (about half a quart) left over. At 400 it is 1.85 gallons, which still rounds up to two gallons.
Would I buy the third full gallon? No. Two gallons and a quart covers the 350 case with margin, and the quart is cheap insurance for the 400 case. The exception is a dark or saturated color, where a third coat is realistic and a third gallon is the safer buy.
Slip number three is rounding early. 337 rounds to "one gallon per coat," which becomes "two gallons" and feels done. The honest chain is 337 → 674 → 741 → 2.12, and the quart at the end is the part people forget.
The whole thing on one line
Gross wall 384 − openings 47 = 337 · × 2 coats = 674 · × 1.10 waste = 741 · ÷ coverage from your can (250 to 400) = 1.85 to 2.96 gallons.
Ceilings, trim and primer are separate math. Primer in particular covers less: Sherwin-Williams lists "about 200-300 square feet" per gallon, so a primed bedroom is close to another two gallons on its own. The coverage guide walks through reading that label before you get to the register.
One question before you go: what does your can actually say? If the low end of the range is 250, the "one gallon per bedroom" rule you may have heard is a gallon short.
Sources
Quotes were checked against the pages on 2026-09-09. The Behr page returned HTTP 403 to the site's own check on that day; its wording was confirmed by the operator's session.
- Sherwin-Williams, "How to Use Paint Calculator" FAQ: gallon "about 350-400 square feet," quart "100 square feet," primer "about 200-300 square feet." https://www.sherwin-williams.com/en-us/color/color-tools/paint-calculator (accessed 2026-09-09)
- Behr, PREMIUM PLUS Interior product page: "250-400 Sq. Ft. Coverage per Gallon." https://www.behr.com/consumer/products/interior-paint/premium-plus-interior ; same figure on BEHR ULTRA and MARQUEE interior pages (accessed 2026-09-09)
- Glidden Paint Calculator: "1 gallon of paint typically covers up to 400 sq. ft. on primed, smooth, non-porous surfaces"; "Drastic color changes typically require a second coat." https://www.glidden.com/paint-calculator (accessed 2026-09-09)
- Benjamin Moore Paint Calculator: "These calculations assume 2 coats of paint and do not include the ceiling." https://www.benjaminmoore.com/en-us/paint-calculator (accessed 2026-09-09)
- CalculatorSoup Paint Calculator: "21 sq ft for each standard 3' x 7' door," "15 sq ft for each standard 3' x 5' window." https://www.calculatorsoup.com/calculators/construction/paint-calculator.php (accessed 2026-09-09)
- Wall Math paint calculator assumptions (2 coats default, coverage typed from the can, allowance user-set, 10% example) — own site, checked 2026-09-09. Every gallon figure in this article was recomputed with the site's calculation code on 2026-09-09 (2 gal + 1 qt at 350 sq ft/gal with 10% allowance; 2 gal at 400; 3 gal at 250).
Numbers are worked examples for one room. Coverage ranges are quoted from the sources above as of the access date; the can you buy is the source of truth.