A pork belly's weight class is its weight in pounds, and USDA prices every class as its own line. The daily pork report carries derind bellies (skin off) in four classes, 7-9, 9-13, 13-17 and 17-19 lb, and skin-on bellies in two-pound steps from 8-10 up to 20-25. Most classes have a combo line and a boxed line, and those are separate prices too. There is no derind class above 19 lb, so a heavier quote has no USDA line to check against.
Which lines actually trade
Over the five years to August 2026, the 13-17 derind carried 67 percent of all the belly pounds USDA reported, the 9-13 derind 15 percent, and the boxed 10-12 skin-on 9 percent. Everything else is small. The 17-19 derind printed on about one day in seven, and the 7-9 in combo almost never.
The lighter belly usually costs more
Over 2016 to 2025 the 9-13 derind printed above the 13-17 on 80 percent of trading days, and on at least two days in three in every one of those years. The gap has widened: a median of 2 to 3 cents a pound a year through 2018, and over 6 cents in every year since 2021. Over the last five years the median was about 7 cents.
The prints do not say why buyers pay up for the lighter belly. What they do show is scarcity: the 9-13 is about 15 percent of the pounds against the 13-17's 67.
The spread has a calendar
Measured as the gap between the two classes, so a move in the whole belly market cannot fake it: the 9-13 averaged above the 13-17 in every February through July of the ten years. The gap is narrowest in August and September, about 4 cents a pound on average, and the 9-13 still led in 8 of the 10 years in both months. How the 13-17 itself moves through the year is covered in the bacon season article.
Heavy bellies trade at par
The 17-19 derind prints thinly, and on the days it did over the last five years it was a coin flip against the 13-17, at a median about half a cent a pound over. In practice a heavy belly prices off the 13-17 line.
Boxed is a different market
The boxed 9-13 derind printed above the combo 9-13 on all but a handful of days over the last five years, by a median 55 percent, about 94 cents a pound. That is more than the cost of a box, and USDA does not say what separates the two trades. The boxed lines are small next to the combo ones. Price a boxed quote against the boxed line, never the combo one.
What the pork cutout uses
The belly value in USDA's pork cutout is built from the derind 9-13 and 13-17 only, and the belly is 16.28 percent of the 215 lb carcass USDA models. Cutting a skin-on belly in that model gives 81.29 percent derind belly, 10.86 percent skin, 4.10 percent 42 percent trim and 1.76 percent 72 percent trim. The skin is about a tenth of a skin-on belly's weight, which is what a skin-on quote carries that a derind one does not.
The spec sheet uses other sizes
IMPS sizes the skinless belly, item 409, at 9-12, 12-15 and 15-up lb, and the skin-on belly, item 408, at 12-16, 16-20 and 20-up. None of those line up with USDA's classes. A 12-15 skinless quote straddles USDA's 9-13 and 13-17 lines, so ask for the weight USDA would print it under.