A 12-by-15 ceiling is 180 square feet, and that is the whole measurement. No doors to subtract. No windows. The gallon count still moves, because the number on a ceiling-paint can is not the number on a wall-paint can, and the calculator on this site leaves the ceiling out unless you put it in on purpose.
The area is one multiplication, so do it once
Twelve feet by fifteen feet is 180 square feet. Ceilings do not have the deductions that walls have, so there is nothing to take away. If a ceiling has a boxed-in beam or a soffit that changes level, measure that face separately and add it; a flat rectangle is just length times width.
Write the number down. Everything after this is a division.
Ceiling paint publishes its own coverage, and it is higher than you expect
Two Benjamin Moore ceiling products print the same range. The technical data sheet for Waterborne Ceiling Paint (508) says "Coverage per Gallon at Recommended Film Thickness 400 – 450 Sq. Ft." The sheet for Muresco Ceiling White (258) says "Spread Rate Per Gallon 400 – 450 Sq. Ft." Both add the same qualifier: the figure depends on surface texture and porosity.
That is the number to use for these two products. It is not a number for ceiling paint in general, and it is not the number on the can you bought. Your can wins. If the label says 350, the arithmetic below changes, and the last section shows by how much.
Two coats is the default here as well
The 508 sheet lists the finish as "1 or 2 coats Benjamin Moore® Waterborne Ceiling Paint (508)." The 258 sheet is firmer: "Benjamin Moore® recommends two coats to achieve full color development and to optimize performance. On bare substrates, two coats are recommended."
Plan on two. A ceiling is one uninterrupted plane under raking light from the windows, so a thin spot shows in the afternoon even when it hid in the morning. One coat is the case you calculate to know what a quart buys you, not the case you paint.
The division, with the 10 percent that every calculator adds
Take the lower label figure, 400 square feet per gallon, because a low estimate of coverage is the safe direction.
180 square feet × 2 coats = 360 square feet of paint to lay down.
360 ÷ 400 = 0.900 gallons.
Add 10 percent for the roller tray, the cut-in brush and the second dip that goes back in the can: 0.900 × 1.10 = 0.990 gallons.
So one gallon. Buy one can, and you have 0.010 gallons left over, which is about 1.3 fluid ounces. Slip number one is reading that as a comfortable answer. It is the tightest fit in this whole guide, and it only holds if your ceiling is smooth and already painted. The data sheets said "depending on surface texture and porosity" for a reason: a popcorn or knock-down texture drinks paint, and bare drywall or a fresh skim coat drinks more. On any of those, 400 is optimistic, and the 1.3 ounces disappear on the first wall of cut-in.
At the top of the label range the picture is easier. 360 ÷ 450 × 1.10 = 0.880 gallons. Still one gallon, now with a real margin.
And without the allowance at all: 360 ÷ 400 = 0.900. One gallon on paper. Nobody paints without an allowance.
I built the Wall Math paint calculator to make you type the coverage figure from your own can instead of guessing it, and to keep two coats as the default. For a ceiling, use the paint calculator in its "Total area" mode: enter 180 square feet, 2 coats, 400 coverage, and it returns 0.990 gallons and a one-gallon purchase, the same chain as above. Slip number two is using the "Room dimensions" mode for a ceiling. That mode multiplies the perimeter by the wall height and gives you the four walls, not the lid on top of them. The calculator says so on its own page: "Ceiling, primer and trim are not included unless measured separately."
Would I buy the quart anyway? Yes
Here is the honest version of the shopping list. The math says one gallon with 1.3 ounces to spare. A quart costs a fraction of a second gallon, and running dry with a two-foot strip of ceiling left is the one paint mistake you cannot hide, because the lap line from the next day sits right over the dining table.
So: one gallon plus one quart for the two-coat ceiling. Open the quart only if the gallon runs out. An unopened quart returns to the store, or waits in the garage for the touch-up you will need after the first ladder scrape.
What a quart alone buys, for the record: 180 ÷ 400 × 1.10 = 0.495 gallons, which is two quarts, one coat. Useful for a quick refresh of a ceiling that is already the right white. Not a plan for a ceiling that has changed color.
The wall-paint number is where the estimate breaks
Slip number three, and the reason this guide exists separately from the bedroom one. The bedroom guide on this site used 350 square feet per gallon as its middle case for wall paint, because that is where many wall labels land. Run the ceiling with that figure:
360 ÷ 350 × 1.10 = 1.131 gallons.
That is one gallon plus one quart as a purchase, and the extra quart is now a requirement, not a hedge. Same ceiling, same two coats, 14 percent more paint, only because a wall number was pasted onto a ceiling product. The whole point of "ceiling is a separate can" is that its coverage figure is different, and the difference is exactly the size of the can you were deciding about.
If your ceiling paint's label says 350, then 1.131 is your answer and the quart is not optional. If it says 400 to 450, one gallon does it with the quart as insurance.
The chain in one line: 12 × 15 = 180 · × 2 coats = 360 · ÷ your label figure · × 1.10 · round up to a can.
If you painted the walls first, the bedroom paint math guide already has your wall figure; the ceiling is a second, smaller calculation with its own can. What does the ceiling can in your cart actually say?
Sources
Both data sheets were read on 2026-09-17 and are quoted as printed. The PDFs are served as compressed streams that the fetch tool could not parse, so each one was downloaded and its text extracted locally; the quotes below come from that text.
- Benjamin Moore, Technical Data Sheet, Waterborne Ceiling Paint 508: "Coverage per Gallon at Recommended Film Thickness 400 – 450 Sq. Ft."; "Recommended Film Thickness – Wet 3.8 mils – Dry 1.4 mils"; "Depending on surface texture and porosity."; finish system "1 or 2 coats Benjamin Moore® Waterborne Ceiling Paint (508)". Volume solids 36.5%. https://media.benjaminmoore.com/WebServices/prod/assets/stage/datasheets/TDS_0508/20200210%20508%20TDS%20US%20OKF.pdf (accessed 2026-09-17)
- Benjamin Moore, Technical Data Sheet, Muresco Interior Acrylic Paint & Primer Ceiling White 258: "Spread Rate Per Gallon 400 – 450 Sq. Ft."; "Wet: 3.6 – 4.0 mils Dry: 1.0 – 1.2 mils"; "Depending on surface texture and porosity."; "Benjamin Moore® recommends two coats to achieve full color development and to optimize performance. On bare substrates, two coats are recommended". Volume solids 30 ± 2%. https://media.benjaminmoore.com/WebServices/prod/assets/production/datasheets/TDS_0258/258_TDS_US.pdf (accessed 2026-09-17)
- A third ceiling product read the same day, Benjamin Moore Color-Change Ceiling Paint PC-1200, prints the same "Coverage per Gallon at Recommended Film Thickness 400 – 450 Sq. Ft." Three products agreeing is not a figure for ceiling paint as a category. (accessed 2026-09-17)
- Wall Math paint calculator, "Total area" mode, and the note it prints with every result: "Ceiling, primer and trim are not included unless measured separately." — own site, checked 2026-09-18. Every gallon figure in this article was recomputed with the site's own calculation code on 2026-09-18.
Numbers are a worked example for one ceiling. Coverage is quoted from the sheets above as of the access date; the can you buy is the source of truth.