A small bathroom is the room where people are most sure they don't need to calculate. Four short walls, a door, maybe a window, "one gallon, obviously." And one gallon is very often the right answer. What I want to show you is how narrow the margin is, because in this room the difference between one can and two has almost nothing to do with the paint you choose. It has almost everything to do with whether the shop will sell you a quart.
I'll use a 5-by-8 foot bathroom with 8-foot walls, two coats, and I'll do the whole thing by hand before I let the calculator on this site check it.
Measure the walls, then take the openings out
Four walls at 8 feet high: the perimeter is 5 + 8 + 5 + 8 = 26 feet, so 26 × 8 = 208 square feet of wall before anything comes off.
Now the openings. The paint calculator here has one box for this, "Area of doors & windows," and it starts at 20 square feet, which is about one standard door. Its help text is blunt: "Subtract only openings you measured." I would take that literally, so for this example the openings are measured rather than guessed. The door is 3 feet by 6 feet 9 inches, which is 3 × 6.75 = 20.25 square feet. The window is a small one, 2 feet by 3, so 6 square feet. Together that's 26.25, and the wall you actually paint is 208 − 26.25 = 181.75 square feet.
That's a plain number, and it's the last plain number you'll get from me. Everything after it is a division that has to round.
Two coats, and why I'm not arguing about it
Every worked example on this site uses two coats, and the reason is on the can rather than in my head, so I'd rather quote it to you. One maker's data sheet for its interior wall paint says "two coats are required; previously painted surfaces can be finished with 1 or 2 coats." A bathroom that's changing color, or that has a patch of new drywall behind the old towel bar, is the "required" case. A bathroom that's the same white it was, and just tired, might get away with one. I'm going to calculate for two, but I'll tell you at the end what one coat would have bought you.
Two coats of 181.75 square feet is 363.5 square feet of paint to put on the wall.
The label number, and the four I ran
Coverage per gallon isn't one number. One big retailer's own calculator page says "a gallon of paint typically covers about 350-400 square feet," and a few lines down, "a quart of paint covers 100 square feet." Another maker's page says "1 gallon of paint typically covers up to 400 sq. ft. on primed, smooth, non-porous surfaces," which is a best case with the conditions spelled out. A third lists "250-400 Sq. Ft. Coverage per Gallon" on one of its interior lines, and 250 is the number a bathroom with old, chalky, or patched walls can actually see.
So instead of trying to pick one, I ran the same room at four labels: 250, 350, 400 and 450. If you are not sure where that figure is printed on your own can, the coverage guide shows where to look and why it is a range. The formula the calculator uses, and the one I did by hand, is the wall area times the coats, divided by the coverage, times 1.10 for the 10 percent it adds for waste and touch-up. For 250:
181.75 × 2 ÷ 250 × 1.10 = 1.599 gallons.
Then the calculator turns gallons into cans. If you leave "Available can sizes" on its default, "Gallons + quarts," it rounds up to the next quarter gallon: 1.599 × 4 = 6.4, round up to 7 quarts, which is 1 gallon and 3 quarts. Here are all four:
- 250 square feet a gallon: 1.599 gallons → 1 gallon + 3 quarts.
- 350: 1.142 gallons → 1 gallon + 1 quart.
- 400: 1.000 gallons → 1 gallon, and nothing to spare. The raw figure is 363.5 × 1.10 ÷ 400 = 0.9996, so what's left in the can is about a quarter of a teaspoon.
- 450: 0.889 gallons → 1 gallon.
Read down the right-hand column and notice what isn't there. I'll wait. There is no two-gallon answer anywhere from 250 to 450. The worst label on the list, the one that assumes thirsty walls, still comes in at one gallon and three quarts. The label moved the answer by three quarts, top to bottom. On a bedroom or a living room the same spread would have moved it by whole gallons, and that's why those guides are about the label. This one isn't, because in a room this small the label can only push you around inside the second can. It can't get you to a third.
The one that lands exactly on a gallon
The 400 row deserves a second look. 1.000 gallons, and the calculator's leftover reads 0.000. That looks like the tidy answer, the one where you buy one can and walk out. But it is also the only row on the table with no margin at all. The 10 percent is already in that 1.000; it's the reason the number isn't 0.909. So if your walls drink a little more than the label promised, or the second coat goes on heavier than the first, you run out with a strip of wall left, and the calculator told you in advance that this was the row with nothing behind it.
I don't think the answer to that is "buy two gallons." I think it's to look at the leftover line before you decide whether the quart goes in the basket. At 450 you have 0.111 of a gallon in hand, a bit under half a quart. At 400 you have none. Same purchase, one gallon, very different afternoon if the paint runs thin.
Now take the quarts off the shelf
Here's the part I actually wanted to write about. Switch "Available can sizes" from "Gallons + quarts" to "Gallons only." The calculator's note for that setting is short: "Rounds by volume, not by price." What it does is round the gallon figure up to a whole can.
- 250: 1.599 → 2 gallons.
- 350: 1.142 → 2 gallons, with 0.858 of a gallon left in the second can. That's more than three quarts you paid for and don't need.
- 400: 1.000 → 1 gallon.
- 450: 0.889 → 1 gallon.
Compare the 350 row across the two settings. Quarts available: one gallon and one quart, 1.25 gallons bought. Gallons only: two gallons bought, for a room that needs 1.142. Nothing about the room or the paint changed between those two lines. The only thing that changed is what sizes the shop happens to stock, and that one fact moved the purchase by three quarters of a gallon, which is more than any label change managed to do in the table above.
This is the whole argument of the guide in one sentence: in a small room, the can sizes on the shelf are a bigger variable than the coverage number on the can. On a 300-square-foot living room the quart is a rounding detail. On 181.75 square feet of bathroom wall, whether a quart exists is the difference between one can and two.
So before you settle on a paint, look at whether it comes in quarts. Some lines do, some don't, and it varies by color and by finish, which is exactly the kind of thing you tend to find out at the counter rather than on the website. If the paint you want only comes by the gallon, the 350 row says you're buying two, and I'd decide in advance what the second one is for.
Where the count changes
Because the count is a staircase, I ran the label up one square foot at a time to find where the answer steps. These are the calculator's own boundaries under its own 10 percent, not a fact about any product:
- Up to 266 square feet a gallon, the room is 1 gallon + 3 quarts. From 267, it's 1 gallon + 2 quarts.
- Up to 319, 1 gallon + 2 quarts. From 320, 1 gallon + 1 quart.
- Up to 399, 1 gallon + 1 quart. From 400, exactly 1 gallon, with the 0.000 leftover from above.
If your label sits within a few square feet of 267, 320 or 400, the honest reading is that you're standing on a step, and which way you fall depends on your walls, not on the calculator. Buy the extra quart. If your label is somewhere in the middle of a step, the table above is your answer and you can stop.
And with quarts available there is no step to 2 gallons anywhere in the labels above. Not at 250, and not until the label drops under 229. Two gallons only ever appears when the quart is taken away.
The ceiling is a quart
The calculator prints the same note under every result, and people keep missing it: "Ceiling, primer and trim are not included unless measured separately." It is not a footnote on one mode; it sits under every answer the tool gives you. So measure the ceiling separately. A 5-by-8 ceiling is 40 square feet, and it's not going to take long.
Ceiling paint has its own coverage figure, and it is usually a different one from the wall paint — the ceiling guide works a 12-by-15 ceiling through the same chain. One maker's ceiling paint data sheet gives "Coverage per Gallon at Recommended Film Thickness 400 – 450 Sq. Ft." and, for the finish, "1 or 2 coats." Taking two coats at 400: 40 × 2 ÷ 400 × 1.10 = 0.220 gallons. Rounded up to quarters, that's one quart. A single quart does the bathroom ceiling twice, with about four fluid ounces left for the inevitable spot over the shower.
So the shopping list for the 400-label case is one gallon of wall paint and one quart of ceiling paint, 1.25 gallons in all. If the shop has no quarts of ceiling paint, you're buying a gallon of it for 40 square feet, and 0.78 of it goes on the garage shelf. It's the same lesson as the walls, at a smaller scale.
What one coat would have bought
For the record, since I said I'd tell you: one coat at 400 is 181.75 ÷ 400 × 1.10 = 0.500 gallons, which is two quarts. At 350 it's 0.571, which rounds to three quarts. So a bathroom that only needs a refresh in the same color is a quarts-only purchase from top to bottom, walls and ceiling both, and never touches a gallon can. That's a real option for the tired-white case. It isn't an option for a color change, and the data sheet I quoted above is clear about which case you're in.
The count in one line
Wall area = perimeter × height − the openings you measured. Gallons = wall area × coats ÷ label coverage × 1.10. Cans = gallons rounded up to the next quarter if the shop sells quarts, or to the next whole gallon if it doesn't.
For this bathroom: 181.75 square feet, two coats, 1 gallon plus 0 to 3 quarts depending on the label, or 1 to 2 whole gallons if there are no quarts. Ceiling 40 square feet, one quart.
Before you pick a color, ask what sizes it comes in. In a room this size that question is worth more to you than the number on the label. Does yours come in a quart?
Sources
Every product figure was read on the manufacturer's own page or data sheet on the date given and is quoted as printed. Every gallon and quart figure in this article was recomputed with this site's own calculation code on 2026-09-25.
- Sherwin-Williams, paint calculator page: "A gallon of paint typically covers about 350-400 square feet"; "A quart of paint covers 100 square feet". https://www.sherwin-williams.com/en-us/color/color-tools/paint-calculator (accessed 2026-09-09)
- Glidden (PPG), paint calculator page: "1 gallon of paint typically covers up to 400 sq. ft. on primed, smooth, non-porous surfaces." https://www.glidden.com/paint-calculator (accessed 2026-09-09)
- Behr, Premium Plus interior product page: "250-400 Sq. Ft. Coverage per Gallon". This page returns 403 to automated requests; the figure was read on screen by the operations session and recorded as such. https://www.behr.com/products/paint/behr-premium-plus-interior (accessed 2026-09-09)
- Benjamin Moore, technical data sheet 550: "two coats are required; previously painted surfaces can be finished with 1 or 2 coats." https://media.benjaminmoore.com/WebServices/prod/assets/stage/datasheets/TDS_0550/20180731%20550%20TDS%20US%20OKF.pdf (accessed 2026-09-07)
- Benjamin Moore, Waterborne Ceiling Paint 508 technical data sheet: "Coverage per Gallon at Recommended Film Thickness 400 – 450 Sq. Ft."; "Finish: 1 or 2 coats". https://media.benjaminmoore.com/WebServices/prod/assets/stage/datasheets/TDS_0508/20200210%20508%20TDS%20US%20OKF.pdf (accessed 2026-09-17)
- Wall Math paint calculator: the field "Area of doors & windows" and its help "Subtract only openings you measured."; the setting "Available can sizes" with "Gallons + quarts" and "Gallons only" and its help "Rounds by volume, not by price."; and the note printed under every result, "Ceiling, primer and trim are not included unless measured separately." — own site, checked 2026-09-25.
Numbers are a worked example for one bathroom. Coverage is quoted from the pages above as of the access date; the can you buy is the source of truth. The step points are this calculator's output under its own assumptions, not manufacturer statements.