Do I subtract the window? The paint math says one quart

A 12-by-14 room with 8-foot walls has 416 square feet of wall before the door and the windows come off. They should come off. The open question is how carefully, so I ran this room every way the calculator allows, and the answer has two halves: subtracting the openings saves one quart of paint, and measuring them exactly, instead of accepting a door-sized guess, saves nothing at all. The cans are identical. I swept the whole range to find where that stops being true, and it does stop, at a sliding door.

So this guide is about one input box. What it does, what it's worth, and when the tape measure earns its keep.

The box, as it sits on the page

On this site's paint calculator the field lives inside a collapsed section called "Openings, extra paint & can sizes". Its label is "Area of doors & windows", the unit is square feet, and it starts at 20. The help line under it reads "Subtract only openings you measured."

Twenty square feet is a door-shaped number. A door 3 feet wide and 6 feet 9 inches tall is 3 × 6.75 = 20.25, so the default is one ordinary door, rounded down a little.

The calculator has three ways to take your walls. Room dimensions, where the help says "Room mode adds all four walls." One wall, where it says "Add openings you measured under “Openings”." And total area, where it says "Openings are subtracted under “Openings”." All three subtract whatever is in that box. It is easy to think of the openings field as a piece of room mode, because that is where people meet it first, but a single accent wall and a hand-added total both pass through it too.

That has a consequence, and it is slip number one. If the wall you are entering has no door and no window in it, the 20 is still sitting in the box, and the calculator will take a door off a wall that doesn't have one. Type 0.

The example room and four openings

The room is an example, not a source. It is 12 by 14 feet with walls 8 feet high, so the perimeter is 12 + 14 + 12 + 14 = 52 feet, and 52 × 8 = 416 square feet of gross wall. It is the same living room as in the two-room repaint guide.

Two coats, because the can says so: Benjamin Moore's data sheet for Regal Select reads "two coats are required; previously painted surfaces can be finished with 1 or 2 coats." The allowance is 10 percent, the figure the calculator offers as its own example. The shop sells gallons and quarts.

I compared four values in the openings box.

• 0, which subtracts nothing.
• 20, the default. One door, as a guess.
• 44.25, measured. The 20.25 door plus two windows at 3 by 4 feet, 12 square feet each.
• 64.5, measured. A sliding door 6 feet wide and 6 feet 9 inches tall, which is 40.5, plus the same two windows.

The formula is the one every paint guide here uses. Wall area minus openings, times coats, divided by the coverage on the can, times 1.10. Then round up to the next quart.

Subtracting something is worth one quart

Coverage is the maker's number, and the three makers on file here print different ones. Sherwin-Williams' calculator page gives 350 to 400 square feet a gallon. Glidden's says up to 400. A Behr product page lists 250 to 400. I ran 250, 350, 400 and 450, and since no label on file reaches 450, read that row as a best case.

Read across any row. Between the first column and the second, the purchase drops by one quart. Between the second and the third, nothing moves.

That is the whole finding. Leaving the box at zero costs a quart on every label I ran, so slip number two is skipping the openings because "it's only a door." And on this room the door-sized default buys exactly what the measured 44.25 buys, in all four rows. The extra 24.25 square feet of window changed the decimals and left the cans alone.

Would I still measure? Yes. The next two sections are why.

The sliding door is a step by itself

Swap the hinged door for a sliding one and the openings go from 44.25 to 64.5. At the 350 label the room now needs (416 − 64.5) × 2 ÷ 350 × 1.10 = 2.209 gallons, and the purchase drops to 2 gallons + 1 quart.

So the default 20 was good enough for a door and two windows, and one quart too generous for a sliding door and the same windows. In the first room the guess cost nothing. In the second it sends you home with a quart the walls didn't ask for. That is the safe direction to be wrong in. It is still a quart.

A room with two ordinary doors lands in the same place, because two doors are 20.25 + 20.25 = 40.5, the same area as the slider. Add the two windows and you are at 64.5 again. The default only describes a room with one door in it.

Why the steps are about 40 square feet apart

The count is a staircase because paint is sold in quarts. A quart is 0.25 gallons. With two coats and a 10 percent allowance, every square foot of opening saves 2 × 1.10 ÷ coverage gallons. So the opening that saves exactly one quart is

0.25 × coverage ÷ 2.2

which comes to 28.41 square feet at the 250 label, 39.77 at 350, 45.45 at 400 and 51.14 at 450.

I swept the box one square foot at a time at 350 to watch the steps land. At 0 the answer is 2 gallons + 3 quarts. At 19 it becomes 2 gallons + 2 quarts. At 59, 2 gallons + 1 quart. At 98, 2 gallons even. The steps sit about 40 apart, as the formula says they should.

Now put real openings on that staircase. One 3-by-4 window is 12 square feet, under a third of a step. A door is half a step. The door and two windows together, 44.25, clear the first step at 19 and stop well short of the second at 59. That is why the default and the measurement bought the same cans: both numbers were standing on the same stair. The sliding-door room, at 64.5, is past the second step.

Where the first step falls depends on the room. It fell at 19 here because 416 square feet at the 350 label happens to sit 19 square feet above a quart boundary. Your room sits somewhere else on its own stair, so the 19 doesn't travel. The width of the step does, and it changes with the coats: on a one-coat job each square foot saves half as much paint, so the step doubles to about 80 square feet at the same label, and the openings matter half as much.

Here is how I'd use that. Measure the door, because it is one measurement and it is half a step. Measure anything large, whether that's a sliding door, a picture window or a wide cased opening, since any one of those is a step or most of one. Measure the small windows too. The help line asks for measured openings, and a window takes a minute. But the difference between 5.5 and 6 square feet on a bathroom window moves the count only if the room happens to sit on the edge of a step, and the way to find that out is to run both numbers and look.

With gallons only, the box changes nothing you buy

Set "Available can sizes" to "Gallons only" and run the same room at 350. Openings 0 gives 3 gallons. Openings 20 gives 3 gallons. Openings 44.25 gives 3 gallons. What is left in the last can is 0.385, 0.511 and 0.663 of a gallon.

When a gallon is the smallest thing on the shelf, the step is four times as wide, about 159 square feet at this label, and the first drop does not come until 98 square feet of openings, close to a quarter of this room's wall. The openings box still changes the answer on the page. It changes the leftover, which is paint in the garage, and leaves the receipt alone.

The bathroom guide found the same thing from the other direction. There, at the 350 label, taking quarts off the shelf moved the purchase from 1 gallon + 1 quart to 2 gallons. Here it flattens three different openings into the same three cans. In both rooms, what the shop stocks decided more than what the tape measure said.

What goes in the box, and what stays out

The help line, "Subtract only openings you measured.", works as a limit in both directions.

Into the box go the door, slab and frame together, and each window. No wall paint lands on any of that.

Out of the box, and this is slip number three, stay the things that cover the wall without being a hole in it. A bookcase, a mirror, a radiator, the switch plates. The wall behind them is still wall and most of it gets painted, now or the day the furniture moves.

Also out: a doorway you paint straight through into a hallway you are painting too. That opening is inside the job. Measure the hallway as its own room and add it.

And outside the number entirely are trim, ceiling and primer. The calculator says so under every result, in the same words each time: "Ceiling, primer and trim are not included unless measured separately."

Every slip in this list runs the same way. Anything you subtract without measuring is a guess that lowers what you buy, and if the guess is too big you find out on the last wall.

Checking it on the calculator

Use room mode, 14 by 12 by 8, coverage 350, two coats, allowance 10, "Gallons + quarts". With 0 in the openings box the page prints 2 one-gallon cans + 3 quarts and under it 2 gal + 3 qt · 2 coats · 416 sq ft. With 20 it prints 2 gal + 2 qt on 396 square feet. With 44.25, 2 gal + 2 qt on 371.75. With 64.5, 2 gal + 1 qt on 351.5.

Those four runs were made on the live page on September 28, and every figure in the tables above matches them.

The count in one line

Paint wall = gross wall − measured openings. Gallons = paint wall × coats ÷ label × 1.10. Cans = gallons rounded up to the next quart, or to the next gallon if that is all the shop has.

For the 416-square-foot room at the 350 label, that is 2 gallons + 3 quarts with nothing subtracted, 2 gallons + 2 quarts with the door and windows out, and 2 gallons + 1 quart if the door is a slider.

Put the door in the box first. Then look around the room for the one big opening, a slider or a picture window, because that is the measurement that can change a can. Does your room have one?

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 checked on this site's live calculator on 2026-09-28.

• 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 field "Area of doors & windows", its default of 20 and its help "Subtract only openings you measured."; the help lines of the three measuring modes; the setting "Available can sizes" with "Gallons + quarts" and "Gallons only"; and the note printed under every result, "Ceiling, primer and trim are not included unless measured separately." https://www.wallmath.com/paint-calculator/ (own site, checked 2026-09-28)

Numbers are a worked example for one room. 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.

Use the paint calculator →