The arithmetic of a paint order is not where people go wrong. It is a few multiplications and one division, and if you want it without the thinking, wall square feet to paint cans is a table you can read the answer off.
What goes wrong is around the arithmetic — three decisions, two of them made before a number is typed and one made partway through the arithmetic, each worth real paint. This guide measures all three on one room so you can see the size of them: a gallon, up to a gallon, and a quart. None of the three is a calculation error. Two of them are reading errors and one is a habit.
The room is a 12-by-12 bedroom with 8-foot walls, one door and two windows. It is laid out in "The room, for reference," near the end, with its numbers, if you want to check anything here.
Mistake one: buying for one coat
This is the expensive one, and it usually is not a decision at all. Someone works out 337 square feet of wall, sees that a gallon is advertised around 350, and buys one gallon.
The manufacturers do not plan for that. Benjamin Moore's calculator page states outright that its results "assume 2 coats of paint." Glidden's says a "drastic color change" typically needs a second coat. Two coats is what these manufacturers plan for, and the one-coat case is the exception: a repaint in a similar color on a sound wall.
For this room at 350 square feet per gallon, one coat is 1 gallon and 1 quart. Two coats is 2 gallons and 1 quart. The mistake is worth a gallon.
The part that does not show up in any arithmetic is what happens next. The second coat then comes from a second trip, and a can tinted on another day may come out slightly different. Buying the second gallon on the first trip keeps both cans from the same visit to the tinting counter.
If one coat really is your case, two rooms and the coat question is where that decision gets worked through properly.
Mistake two: reading the top of the range
None of the three makers quoted here gives a single number. Sherwin-Williams puts a gallon at "about 350-400 square feet". Behr's Premium Plus interior page prints "250-400 Sq. Ft. Coverage per Gallon". Glidden says "up to 400 sq. ft. on primed, smooth, non-porous surfaces".
Read those three carefully and they are not really three estimates. Two of them are a span, and the third has "up to" in front of it, with the conditions attached: primed, smooth, non-porous. The top of a printed range is the best case that manufacturer is willing to claim.
For this room, the span is not academic:
| Coverage figure | What you buy |
|---|---|
| 250 sq ft/gal | 3 gallons |
| 300 sq ft/gal | 2 gallons + 2 quarts |
| 350 sq ft/gal | 2 gallons + 1 quart |
| 400 sq ft/gal | 2 gallons |
Same walls, same two coats, same allowance. Bottom to top, a full gallon.
So which one is yours? The honest answer is that the can cannot see your wall, but the direction is knowable. These pull toward the low end:
- new drywall and anything freshly patched or skim-coated, because the surface drinks the first coat before it builds a film
- any texture with relief to it
- a real color change, especially light over dark — strictly a reason for another coat rather than a lower figure, but it points the same way
- old chalky or porous paint
And toward the top: a smooth, previously painted wall going on in a similar color. That is the wall "up to 400" was written about.
The corroborating figure is the primer one. Sherwin-Williams rates primer at only "about 200-300 square feet" a gallon — about two-thirds of its paint figure — and primer's whole job is the porous, unsealed surface. That fits the pattern above, though it does not prove it. Primer is a separate can takes that further.
Treat the top of the range as a ceiling, not an estimate. If your can says 250 to 400 and you genuinely do not know your wall, 300 square feet per gallon is an honest middle, and for this room it buys 2 gallons and 2 quarts rather than 2.
Mistake three: rounding early
The third one is a habit rather than a misreading. The chain has four steps and people round at the first one.
At a 300 figure, 337 square feet "is about a gallon," so two coats "is about two gallons," and the allowance makes it 2.2: two gallons and a quart. Done.
The honest chain keeps the decimals until the end:
337 → × 2 coats = 674 → × 1.10 allowance = 741.4 → ÷ 300 = 2.47 gallons.
2.47 is two gallons and two quarts, not the two gallons and a quart the shortcut landed on. The 0.12 gallon a coat that was rounded away at the first step is a quart by the end. Round at the start and you lose it; round at the end and you buy it.
This is the step the calculator exists for, and it is also why the table shows where the purchase steps: at a 350 figure the order climbs a quart roughly every 40 square feet of wall, so a room measured 40 square feet short is a quart short.
Would I buy a third full gallon here? No. Two gallons and two quarts is what the 300 figure asks for, and the 10 percent allowance is already inside it. A third gallon is the right call for a dark or saturated color, where a third coat is realistic, and that is a decision about the color rather than about the arithmetic.
The one that is not a mistake: the flat deduction
Some online calculators do not ask you to measure your door. They subtract a fixed figure per opening instead. CalculatorSoup, for one, takes off "21 sq ft for each standard 3' x 7' door" and "15 sq ft for each standard 3' x 5' window".
A flat figure sounds crude, so it is worth checking against a measured room. In this room the door is a 30-by-80-inch interior door, which is 16.67 square feet, and the two windows are 36 by 60 inches, 15 each. Measured, the openings total 46.67 square feet. The flat convention gives 51.
A gap of 4.33 square feet. And at 250, 300, 350 and 400 — it changes nothing you buy. Same cans, all four.
So the convention is fine here, and saying otherwise would be false precision. Where it stops being fine is big openings, because the flat figure does not scale:
| Opening | Actual | Flat figure | Missed |
|---|---|---|---|
| 48-by-80-inch bifold closet door | 26.67 sq ft | 21 | 5.67 |
| 72-by-80-inch sliding patio door | 40 sq ft | 21 | 19 |
| An 8-by-8-foot glass wall | 64 sq ft | 21 | 43 |
Measure the big openings; on the small ones, a flat figure rarely changes what you buy. That is the whole rule, and it is worth knowing which half of it you are in before you trust a tool that never asked. What belongs in the openings box works through when it does.
The room, for reference
Perimeter 12 + 12 + 12 + 12 = 48 feet. Times 8 feet of wall height is 384 square feet of gross wall. Take off the 46.67 of measured openings and you are painting 337 square feet.
Everything above was run on that figure at two coats with a 10 percent allowance. If you want the same arithmetic for a wall area that is not 337, it is a lookup rather than a calculation: wall square feet to paint cans runs 100 to 1,000 square feet at four coverage figures, and this room sits between its 300- and 350-square-foot rows.
What this guide does not do
It does not tell you which coverage figure your wall will deliver. No one can promise that in advance; the ranges and the "up to" quoted above are the manufacturers' way of saying it depends on the wall.
It does not cover the ceiling, the trim or the primer. The ceiling is a different product with a different figure, which is a 12-by-15 ceiling, and the primer is a separate can with a separate count.
And it does not name a product or carry a price. The three mistakes happen before the store, not in it.
The whole thing on one line
Gross wall − measured openings, × coats, ÷ the figure on your can, × 1.10, rounded up at the end and not before.
For this room: 384 − 46.67 = 337 · × 2 = 674 · × 1.10 = 741.4 · ÷ 250 to 400 = 1.85 to 2.97 gallons.
One question before you go: what does your can actually say? If the bottom of its range is 250, the "one gallon per bedroom" rule you may have heard is two gallons short.
Sources
Quotes were checked against the pages on the dates given. Every gallon and quart figure was produced by the calculation engine behind this site's paint calculator on 2026-10-17.
- 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". 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 2026-10-06)
- 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." A third-party tool convention, quoted to compare against a measured room, not an endorsement. https://www.calculatorsoup.com/calculators/construction/paint-calculator.php (accessed 2026-09-09)
Numbers are a worked reference 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.