A 400-square-foot room has somewhere between 640 and 1,360 square feet of wall, depending on its shape. That is the trouble with being asked how many square feet you have. People answer with the number they know, which is the floor, and the paint goes on the walls.
The problem is that "square feet" is three different numbers in a paint job, and only one of them goes into a calculator. Short answer first: a square 400-square-foot room with 8-foot ceilings takes about 4 gallons at a 350 label in two coats, and a corridor-shaped one takes 5. The rest of this guide separates the three numbers, runs that room three ways to show how far apart they sit, and gives the figure people are actually asking for: what a square foot of wall takes.
The three numbers
Floor area is the number you already know. It is on the listing, the lease, the floor plan. A 400-square-foot room is 400 square feet of floor.
Wall area is the number the paint goes on. It is the perimeter of the room times the ceiling height, minus the openings you measured. It is not derived from the floor area, and that is the whole difficulty.
Coverage is the number on the can: square feet per gallon. It describes what one gallon spreads over in one coat, under conditions the maker usually qualifies. One retailer's calculator page says "a gallon of paint typically covers about 350-400 square feet". Another maker says "1 gallon of paint typically covers up to 400 sq. ft. on primed, smooth, non-porous surfaces". A third prints "250-400 Sq. Ft. Coverage per Gallon" on an interior line. Three sources, three different numbers, all of them per gallon rather than per room.
Mixing the first and the second is the common fault, and it is easy to see why. Both are areas, both are in square feet, and one of them is already written down.
Same floor, different walls
Here are three rooms. Every one has 400 square feet of floor and 8-foot ceilings. The ceiling height is an assumption for this example, not a fact about your house.
- A square room, 20 by 20 feet.
- A slightly long room, 16 by 25.
- A corridor of a room, 10 by 40.
Walk the perimeter of each: 20 + 20 + 20 + 20 = 80 feet. 16 + 25 + 16 + 25 = 82. 10 + 40 + 10 + 40 = 100.
Multiply by the 8-foot height and the gross wall areas are 640, 656 and 800 square feet.
| Room | Floor | Perimeter | Gross wall |
|---|---|---|---|
| 20 × 20 ft | 400 sq ft | 80 ft | 640 sq ft |
| 16 × 25 ft | 400 sq ft | 82 ft | 656 sq ft |
| 10 × 40 ft | 400 sq ft | 100 ft | 800 sq ft |
The corridor has 160 more square feet of wall than the square room, with exactly the same floor. Stretch the shape further and the gap grows, because the area of a rectangle is fixed by multiplication and its perimeter is not. A 5-by-80 foot room is still 400 square feet of floor, and its perimeter is 170 feet: 1,360 square feet of wall, more than double the square room. Nobody lives in a 5-by-80 room, but hallways and galley kitchens lean that way, and the lean is the thing that moves the order.
So the first answer to "how much paint for 400 square feet" is a question. Four hundred square feet of what, and in a room shaped how?
What those three rooms buy
Give each room the same door and window, measured rather than guessed: a door 3 feet by 6 feet 9 inches is 20.25 square feet, and a 3-by-4 window is 12, so 32.25 square feet comes off each. Two coats, because the data sheet for one maker's interior line reads "two coats are required; previously painted surfaces can be finished with 1 or 2 coats". A 10 percent allowance, which is the figure this site uses in every worked example and which the calculator offers as its own example value.
At a 350 square feet per gallon label:
| Room | Net wall | Gallons the math needs | What you buy |
|---|---|---|---|
| 20 × 20 ft | 607.75 sq ft | 3.820 | 4 gallons |
| 16 × 25 ft | 623.75 sq ft | 3.921 | 4 gallons |
| 10 × 40 ft | 767.75 sq ft | 4.826 | 5 gallons |
Two of these rooms land on the same order. The third, with the same floor area as the other two, needs a whole extra gallon. Nothing about the paint changed. The room got longer.
Run the two extremes at the ends of the published ranges and the spread widens again. The square room needs 5 gallons and 2 quarts at a 250 label and 3 gallons at 450. The corridor needs 7 gallons at 250 and 4 at 450. The 450 figure sits above every label quoted here, so read that column as a best case rather than a promise.
The lower bound you can get from floor area
Floor area will not give you wall area. It will give you a floor under it, and that is worth knowing when the listing is all you have.
For a fixed area, the rectangle with the smallest perimeter is the square. Our 400-square-foot rooms show it: 80 feet of perimeter for the 20 by 20, 82 for the 16 by 25, 100 for the 10 by 40, 116 for an 8 by 50, 170 for the 5 by 80. The square is the floor, and every other shape is above it.
So for any 400-square-foot room with 8-foot ceilings, the gross wall is at least 4 × √400 × 8 = 640 square feet. With the same door, window and 10 percent allowance as the rooms above, that puts the order at a 350 label at no less than 4 gallons. Not about 4 gallons. At least. Larger openings pull it back: take 60 square feet off instead of 32.25 and the same square room comes to 3 gallons and 3 quarts.
That is the honest use of a floor area: a number you cannot go under. If a shortcut tells you one gallon and a quart, you now know it is wrong without measuring anything, because no room of that size with openings like these comes anywhere near as low.
Ceiling height moves it too
Shape is one multiplier on the perimeter; height is the other. The square room at three common ceiling heights, everything else held still:
| Ceiling | Gross wall | Net wall | Gallons the math needs | What you buy |
|---|---|---|---|---|
| 8 ft | 640 sq ft | 607.75 | 3.820 | 4 gallons |
| 9 ft | 720 sq ft | 687.75 | 4.323 | 4 gallons + 2 quarts |
| 10 ft | 800 sq ft | 767.75 | 4.826 | 5 gallons |
A foot of ceiling is worth two quarts here, and two feet is worth a gallon. Notice where the 10-foot row lands: 800 square feet of gross wall, 5 gallons. That is the same answer as the 10-by-40 corridor with 8-foot ceilings. A long room and a tall room arrive at the same place, which is another way of saying that the only thing the paint responds to is the area of the walls themselves.
Older houses and converted spaces are where this bites. A 9-foot ceiling is not unusual, and nothing on a floor plan announces it.
The shortcut that fails
The arithmetic people reach for is floor area divided by coverage. For our 400-square-foot room at a 350 label, that is:
400 ÷ 350 = 1.143 gallons, which rounds to 1 gallon and 1 quart.
The square room actually needs 4 gallons. The shortcut is off by a factor of about three and a third, and it is wrong in the direction that leaves you at the store on a Sunday afternoon with half a wall done.
Four things went missing in that one division:
- Floor instead of wall. 400 square feet of floor is 640 of wall in this room.
- One coat instead of two. Two coats doubles the paint.
- No allowance. Nothing for the roller tray, the cut-in, the touch-up a week later.
- No openings. Subtracting the door and window pulls 32.25 square feet back off, which is the only one of the four that works in your favour.
Each of those is a factor of roughly 1.6, 2, 1.1 and 0.95. Multiply them together and you have that factor of 3.3.
What a square foot of wall actually takes
This is the number the question is reaching for, so here it is plainly. At a 350 label, two coats and a 10 percent allowance, one square foot of wall takes
2 × 1.10 ÷ 350 = 0.00629 gallons, which is 0.80 fluid ounces.
About four fifths of an ounce. A shot glass is an ounce and a half, so a square foot of wall is roughly half a shot glass of paint for both coats together.
| Coverage on the can | Per square foot of wall, two coats | Wall one gallon covers |
|---|---|---|
| 250 sq ft/gal | 1.13 fl oz | 113.6 sq ft |
| 350 sq ft/gal | 0.80 fl oz | 159.1 sq ft |
| 400 sq ft/gal | 0.70 fl oz | 181.8 sq ft |
| 450 sq ft/gal | 0.63 fl oz | 204.5 sq ft |
The right-hand column is worth sitting with. A can that says 350 does not cover 350 square feet of your wall. Once the second coat and the allowance are in, it covers 159. The label is telling you about one coat of paint on a surface; your wall needs two coats and a margin, so the useful figure is less than half the printed one.
For a single coat, halve the fluid ounces: 0.40 per square foot at a 350 label. One coat is a real option on a repaint in the same color, which is what the "1 or 2 coats" in that data sheet is describing, and it is not an option when the color changes.
Getting your own wall number
One line, and it needs a tape measure rather than a listing.
Wall area = (the four wall widths added up) × ceiling height − the openings you measured.
For the three rooms above that is 80, 82 and 100 feet of perimeter, times 8 feet, minus 32.25.
Two details decide whether this number is any good:
Measure the walls, not the floor. Walk the perimeter with the tape against the baseboard. In a room with an alcove or a chimney breast, the perimeter is longer than the floor plan suggests, and the extra is wall you will paint.
Subtract only what you measured. The calculator on this site starts its openings field at 20 square feet, which is about one door, and the help line under it says "Subtract only openings you measured." A door is worth measuring. A picture window or a sliding door is worth measuring twice, because a large opening is the one that moves the purchase.
The one place floor area is the right number
There is a surface in the room whose area really is the floor area, and it is the ceiling. A 400-square-foot floor sits under roughly 400 square feet of ceiling, give or take the chimney breast. If you came here with a floor area in hand, that number is not useless. It is simply pointing at the wrong surface for wall paint and the right one for ceiling paint.
Ceiling paint carries its own coverage figure, usually higher than wall paint. At a 400 square feet per gallon ceiling label, two coats on 400 square feet with the same 10 percent allowance is 400 × 2 × 1.10 ÷ 400 = 2.200 gallons: 2 gallons and 1 quart. Drop the label to 350 and it is 2 gallons and 3 quarts; lift it to 450 and it is 2 gallons flat.
So a 400-square-foot room with 8-foot ceilings, painted walls and ceiling both, is around 4 gallons of wall paint and 2 and a quarter of ceiling paint, in two different products. The calculator prints the warning under every result for exactly this reason: "Ceiling, primer and trim are not included unless measured separately."
Where the calculator comes in
The paint calculator does the perimeter arithmetic for you in room mode: give it the room's length, width and wall height and it adds the four walls. If you have already measured the walls yourself, including an alcove the room dimensions would miss, use total area mode and type the number straight in.
Either way the three numbers from the top of this guide go in different boxes. Wall measurements go in the room or area fields. Coverage goes in the field that says "Coverage on the label", which is the number from the can you are buying rather than a rule of thumb. Floor area goes nowhere at all.
Under every result the page prints the same line, and it is the boundary of the answer: "Ceiling, primer and trim are not included unless measured separately." The ceiling is a separate surface with its own coverage figure, and a 12-by-15 ceiling takes its own gallon. Primer is a separate can. Trim is a separate can and usually a different finish.
What this does not answer
It does not tell you cost per square foot. This site does not handle prices, and the honest reason is that a price is a local and dated thing in a way that a coverage figure is not.
It does not tell you what your walls will actually drink. Coverage is printed as a range because texture and porosity move it. A wall with knock-down texture, a fresh skim coat, or a color change underneath sits at the low end of whatever range your can prints, and the low end is where the range guide shows the purchase changing by whole gallons.
And it does not transfer to the other materials. Wallpaper is counted in strips, not square feet, and flooring is counted in whole cartons. Both of those have their own arithmetic and neither is a square-foot division.
The count in one line
Wall area = perimeter × height − measured openings. Gallons = wall area × coats ÷ the coverage on your can × 1.10. Cans = that figure rounded up to what the shop sells.
For 400 square feet of floor at a 350 label, two coats: 4 gallons if the room is square, 5 if it is a corridor, and 1 gallon plus 1 quart if you divide the floor area by the label and hope.
Measure the perimeter before you buy. What shape is your 400 square feet?
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 ounce figure in this article was produced by the calculation engine behind this site's paint calculator on 2026-10-02.
- Sherwin-Williams, paint calculator page: "A gallon of paint typically covers about 350-400 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". https://www.behr.com/products/paint/behr-premium-plus-interior (accessed 2026-09-09)
- Benjamin Moore, Regal Select 550 technical data sheet: "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)
- Wall Math paint calculator: the room and total-area modes, the "Area of doors & windows" field with its default of 20 and its help "Subtract only openings you measured.", and the line printed under every result, "Ceiling, primer and trim are not included unless measured separately." https://www.wallmath.com/paint-calculator/ (own site, checked 2026-10-02)
Numbers are a worked example for three rooms of one floor area at one ceiling height. Coverage is quoted from the pages above as of the access date; the can you buy is the source of truth. The 450 column is above every label quoted here and is shown as a best case only.