How to read a paint can label for coverage

Ask what a gallon of paint covers and you will be told 350 to 400 square feet. That figure is real, it is printed by the manufacturer, and it is not a measurement of the paint. It is a division sum, and once you know which division it is, the whole coverage line on a label becomes readable — including why one premium interior wall paint claims 250 and another claims 450 for essentially the same job.

A gallon is 231 cubic inches, and that is the entire trick

One US gallon is 231 cubic inches. A mil is one thousandth of an inch. Spread a gallon out at exactly one mil thick and it goes 231 ÷ 0.001 = 231,000 square inches, which is 1,604 square feet.

Every published spread rate is that constant divided by an assumed film thickness. Divide 1,604 by four and you get 401. That is where 400 sq ft per gallon comes from: it is a gallon spread four thousandths of an inch thick.

You can watch a manufacturer do the arithmetic. Sherwin-Williams’ sheet for its Waterborne Acrylic Dryfall gives volume solids of 41 ± 2% and then prints “Theoretical coverage: 657 sq ft/gal @ 1 mil dry”. One thousand six hundred and four times 0.41 is 657.6. The constant is not a rule of thumb; it is what the sheets are built on.

The label prints the answer, the data sheet prints the assumption

Sherwin-Williams’ SuperPaint Interior Latex Flat sheet gives the coverage line in full:

“Coverage: 350-400 sq. ft. per gallon @ 4 mils wet, 1.4 mils dry”

Three numbers, and only the first usually survives onto marketing copy. Four mils wet is the assumption. And 1.4 mils dry is what is left after the water leaves, which is the wet film multiplied by the volume solids — that sheet gives 36 ± 2%, and 4 × 0.36 = 1.44.

Line four sheets up and the pattern is unmistakable.

Data sheet	Volume solids	Published spread rate	Film assumed
SuperPaint Interior Latex Flat A86	36 ± 2%	350–400 sq ft/gal	4 mils wet, 1.4 dry
SuperPaint Interior Latex Semi-Gloss A88	37 ± 2%	350–400 sq ft/gal	4 mils wet, 1.5 dry
Behr Premium Plus Interior Satin No. 7050	39%	250–400 sq ft/gal	6.4 wet / 2.5 dry at 250; 4.0 wet / 1.6 dry at 400
Benjamin Moore Regal Select Flat N547	46 ± 2%	400–450 sq ft/gal	3.6–4.0 wet, 1.6–1.9 dry

Check the constant against every row. Behr’s low end: 1,604 ÷ 6.4 = 251, and the sheet says 250. Benjamin Moore’s high end: 1,604 ÷ 3.6 = 446, and the sheet says 450. Sherwin-Williams prints one film thickness for a range, so the 350 end is not annotated; 1,604 ÷ 350 works out at 4.6 mils wet, which is our arithmetic rather than theirs.

The range on the can is a thickness range, not an error bar

This is the part that changes how you order. A span like “250–400 sq ft/gal” reads like uncertainty — as though nobody quite knows. Behr’s sheet says otherwise by giving both ends their own film:

“Recommended Film Thickness: Wet: 6.4 mils / Dry: 2.5 mils @ 250 sq ft/gal. Wet: 4.0 mils / Dry: 1.6 mils @ 400 sq ft/gal”

Same gallon, same paint. At the 250 end you have decided to leave a coat 2.5 mils thick; at the 400 end, 1.6 mils. The number moves because you moved, and the range is telling you how much room the product allows.

So when you put a figure into our paint calculator, use the one from the container you are actually buying rather than a generic 375, and know which end of its range you intend to work at. A porous, patched or textured wall pulls you toward the thick end, and that is the end where a gallon covers 250 sq ft, not 400.

Two cans that both say 400 do not leave the same paint on the wall

Read the dry-film column across the table at the same published coverage. At 400 sq ft per gallon — four mils wet in all three cases — the sheets declare:

• SuperPaint Interior Flat: 1.4 mils dry
• Behr Premium Plus Interior Satin: 1.6 mils dry
• Regal Select Flat: 1.9 mils dry

Benjamin Moore leaves about a third more dry film than Sherwin-Williams from the identical spreading action, because its volume solids are 46% against 36%. And Benjamin Moore is simultaneously the one claiming the higher coverage, 400–450 against 350–400, because it also assumes you will spread it thinner.

That is the single most useful thing to take from a coverage figure: it describes how far you are being asked to spread a gallon, and says nothing about how much coating is left behind when it dries. The number that governs that is volume solids, and volume solids is on the data sheet, not on the label.

Volume solids is not measured — it is declared

Which raises the obvious question: who checks it?

The federal reference method for coatings is EPA Method 24, titled “Determination of Volatile Matter Content, Water Content, Density, Volume Solids, and Weight Solids of Surface Coatings”. It is the method 40 CFR 59.406 names for architectural coatings. Volatile content, water content and density each get a laboratory procedure, duplicate runs, and a precision statement.

Volume solids gets one sentence:

“11.3.4 Solids Content. Determine the volume fraction (Vs) solids of the coating by calculation using the manufacturer’s formulation.”

So the number that turns a spreading rate into a film thickness is computed from the formula the maker supplies, not weighed out of a dish. That is not a scandal — formulation arithmetic is accurate, and a manufacturer has little to gain by misreporting it — but it does settle what kind of claim a coverage figure is. It is a calculation on a declared recipe, multiplied by a chosen thickness. It is not a test result.

What the label is legally required to carry, and coverage is not on the list

Under 40 CFR 59.405(a), a manufacturer or importer of an architectural coating must put exactly three things on the container:

1. “The date the coating was manufactured, or a date code representing the date shall be indicated on the label, lid, or bottom of the container.”
2. “A statement of the manufacturer’s recommendation regarding thinning of the coating… This requirement does not apply to the thinning of architectural coatings with water. If thinning of the coating prior to use is not necessary, the recommendation must specify that the coating is to be applied without thinning.”
3. The VOC content, in grams per litre or pounds per gallon.

Coverage is absent from that list. It appears because manufacturers choose to publish it, which is also why its form varies between brands while the VOC figure does not.

The thinning statement is the one people walk past. On the Sherwin-Williams sheet it is the line “No reduction needed.” On Behr’s it is longer: “Do not thin if using a roller or brush; however, if using a sprayer and thinning is required, thin with water at a rate of no more than 1/2 pint per gallon.” Half a pint in a gallon is a 6% dilution, and it moves your coverage by about the same 6% while thinning the dry film to match.

The date code is worth finding for a reason the rule does not state. Both Sherwin-Williams sheets give “Shelf Life: 36 months, unopened”. The regulation guarantees the date is somewhere on the label, lid or bottom of the can; the shelf life tells you what to do with it.

The VOC number is per litre of something that is not what is in the can

While you are reading the required three, know what the middle-sized number means. Sherwin-Williams prints “V.O.C. (less exempt solvents): less than 50 grams per litre; 0.42 lbs. per gallon — As per 40 CFR 59.406”.

That parenthesis is doing real work. Section 59.406(a)(1) requires the content to be determined “in grams of VOC per liter of coating thinned to the manufacturer’s maximum recommendation, excluding the volume of any water and exempt compounds.” For a waterborne paint, most of the liquid is water, so the denominator is a good deal smaller than the container. A “less than 50 g/L” latex is not fifty grams in every litre you carry home.

The regulation also runs a separate equation for low-solids products — defined in 40 CFR 59.401 as “containing 0.12 kilogram or less of solids per liter (1 pound or less of solids per gallon) of coating material” — and that one does include the water. Two products can therefore both be honest and not be comparable.

The sheen word on the front is a measurement too

“Flat” is not a marketing adjective. 40 CFR 59.401 defines a flat coating as one that “registers gloss less than 15 on an 85-degree meter or less than 5 on a 60-degree meter according to ASTM Method D 523-89”.

Then look at what the sheets declare. SuperPaint Interior Flat: “Finish: 0-5 units @ 85°”. Regal Select Flat: “Sheen / Gloss 1 – 4 @ 85°”. Both are comfortably inside the federal ceiling, and both are reported on the same instrument at the same angle, so they can be compared directly. The semi-gloss and satin products switch meters — SuperPaint Semi-Gloss reports “25-35 units @ 60°”, Behr’s satin “15–25 @ 60°” — and numbers taken at 85° and 60° are not interchangeable.

What the coverage figure still leaves out

Two exclusions are stated outright by the makers.

Loss. Behr’s coverage line ends “Does not include the loss of material from spraying.” Sherwin-Williams puts it at length: spreading rates “are calculated on volume solids and do not include an application loss factor due to surface profile, roughness, or porosity of the surface, skill, and technique of the applicator, method of application, various surface irregularities, material lost during mixing, spillage, overthinning, climatic conditions, and excessive film build.”

Hide. Covering a wall and hiding what was on it are separate achievements. The SuperPaint sheet carries its own instruction on the point — “To optimize hide and color development, always use the recommended P-Shade primer” — which is a manufacturer saying that its spread rate assumes the colour underneath has already been dealt with.

Neither of those is a caveat buried by anyone. They are printed beside the number, on the document the number comes from, and they are the reason a job estimated from the front of the can runs short at the last wall.