How much paint do I need?

Where 400 square feet per gallon actually comes from

This is the one estimate in the house you can check from first principles, and once you have, nobody can move it on you again.

A United States gallon is 231 cubic inches exactly. That is not a manufacturer's opinion, it is in NIST Handbook 44, Appendix C, under Units of Capacity or Volume, and it has been settled for two centuries. Sherwin-Williams specifies ProMar 200 Zero V.O.C. Interior Latex at 4 mils wet, which is four thousandths of an inch of liquid paint sitting on the wall before it dries. Spread 231 cubic inches at a thickness of 0.004 inches and you get 57,750 square inches, which is 401 square feet. That is the whole derivation. The 400 on the label is geometry with nothing lost.

The same sheet prints 350 as the low end. The distance between 350 and 401 is the paint that stayed in the tray, soaked into the roller cover, went on thick at the cut line and thin in the middle of the wall. Call it roughly twelve percent, and call it the only loss in painting that has a physical explanation. There is no cutting waste in paint. Nothing is trimmed off and thrown away.

One more check you can do without leaving the page. The sheet lists volume solids at 38 percent and a dry film of 1.5 mils. Four mils wet times 0.38 is 1.52. The document agrees with itself, which is more than can be said for most coverage claims you will hear across a counter.

How much extra is honest, and how much is somebody selling you a gallon

Because there is no cut waste, the extra percentage field on this page starts at zero, and that is deliberate. The reasons to buy beyond the arithmetic are real but they are yours, not technical: a quart's worth of touch up for the next five years, a second cut of a difficult color, a wall that has been patched and will drink unevenly. Each of those is a decision you can name.

What is not a reason is a round number. A bid that quietly assumes 250 or 200 square feet to the gallon on primed drywall is not being careful, it is using a spread rate that this manufacturer does not publish for this product on this surface.

The point where a paint number stops adding up

Divide. Take the gallons somebody quoted, and divide your painted area times the number of coats by that gallon count. You get the spread rate they assumed, in square feet per gallon, and you can hold it against 350 to 400.

Work the room this page opens with. Twelve by ten with an eight foot ceiling gives a wall band of 2 times 22 times 8, which is 352 square feet, less 40 square feet of door and window, so 312. Two coats is 624 square feet of paint applied. At 400 that is 1.56 gallons, at 350 it is 1.78, so you are buying 2 gallons and keeping the rest. If somebody quotes four gallons of finish for that room, they have assumed 156 square feet per gallon, well under half the published minimum, and they owe you a sentence explaining why.

Quarts, gallons and pails are three prices for one color

Four quarts make one gallon, also from Handbook 44, and that conversion is the entire trick. Before you compare anything, put both containers on the same footing: price divided by the gallons inside it. A quart bought four times is almost never the price of a gallon bought once, and a multi gallon pail exists precisely because the price per gallon inside it is meant to be lower.

So the rule for this page is smaller than the rule for bagged materials, and it is this. Work out your gallons first, from the arithmetic above. Then find the container arrangement that covers that number with the least money, and remember that leftover paint in a sealed can is not waste. The sheet gives an unopened shelf life of 36 months, so a spare gallon is a spare gallon for three years, while a spare gallon of the wrong sheen is a spare gallon you will never open.

The one place the pail can lose is tinting. The same sheet gives Deep Base 4 to 12 ounces of colorant per gallon and Ultradeep Base 10 to 12, against 0 to 7 for Extra White, and Dover White is marked do not tint. A dark color and a light color are not the same purchase even in the same line and the same size.

The coats the data sheet specifies and the bid sometimes forgets

Under Specifications, Sherwin-Williams does not describe a paint job as coats of paint. It describes a system, surface by surface, and the systems are not the same:

Every row has a primer in it. This page counts finish coats on your walls and nothing else, so the primer is a line you add yourself: your wall area at one coat, at the coverage printed on that primer's own can. On the room above that is another gallon. A bid written for two coats and no primer over new drywall is not a cheaper version of the same job, it is a different job, and the sheet says why: priming and two coats at the recommended film thickness is what helps when hiding a previous coating or painting new drywall. It also says that a drastic color change is a reason to prime, not a reason to add a third finish coat.

The same document quietly lists the rest of the receipt. Caulk for the gaps between walls, ceiling and crown molding, applied after priming. Patching paste or spackle for the cracks and holes, sanded smooth, with the sanding dust removed. A nylon polyester brush. A roller cover with a 3/8 to 3/4 inch nap. If you are spraying, an airless setup at 2000 psi with a 0.017 to 0.021 inch tip. None of that is paint and all of it is on the receipt.

On price, and where the range above was read

Coverage comes from a manufacturer data sheet. The dollar range comes from somewhere else, and it is worth being precise about where, because a gallon's shelf price really does depend on the line, the base, the sheen, the store and the week.

Sutherlands is a home improvement and lumber chain out of Kansas City, Missouri, and its product pages publish a price for every one of its stores rather than one national number. So a single public page states a low and a high on the same day. Three gallons were read on 2026-08-23, picked to span the shelf from contractor grade to premium rather than to flatter the total:

• Davis Paint Pro interior wall paint, flat off-white, one gallon, item 7052350: 23.99 to 25.99 dollars across the 5 stores with it in stock. Open the product page.
• PPG Wonder-Tones eggshell acrylic latex, white and pastel base, one gallon, item 3771714: 30.00 to 32.00 dollars across the 2 stores with it in stock. Open the product page.
• Valspar Medallion interior latex paint and primer, eggshell pastel base, one gallon, item 5289509: 40.99 to 43.99 dollars across the 2 stores with it in stock. Open the product page.

Two limits, printed here because a range without them is worse than no range. It is one chain on one day and not a national average. And it is not the same product as the one whose coverage is cited above: the gallon count comes from the Sherwin-Williams sheet, the dollars come from these three pages, and the page never pretends one vouched for the other. Type the tag in front of you into the price field and the reading steps aside.

What you can also do is convert, because cost per square foot is the only number that honestly compares two cans. Price per gallon divided by 350 is the worst case cost per square foot per coat. Divided by 400 it is the best case. Multiply by your painted area and your coat count, and you have the material half of the job in your own numbers, from a shelf tag you are standing in front of. A cheap gallon that covers 280 square feet is not cheap.

Sources for everything above. Sherwin-Williams, ProMar 200 Zero V.O.C. Interior Latex Semi-Gloss, B31 Series, product data sheet 101.84, dated 6/2023, paintdocs.com, opened 2026-08-23. NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement, nvlpubs.nist.gov, opened 2026-08-23, for the gallon of 231 cubic inches and the four quarts to the gallon.