A pork carcass is divided into the major primals tracked by the USDA Pork Carcass Cutout: loin, Boston butt, picnic shoulder, ham, belly, and spareribs. Yield shares vary across packers and trim styles, and different sources report slightly different proportions, but the ranges below are reasonable working numbers.
Loin
The loin runs along the top of the carcass and is one of the two largest primals, accounting for somewhere in the 17 to 24 percent range of carcass weight depending on fabrication and trim. The major sub-primals are the bone-in pork loin (IMPS 410), the boneless pork loin (413), the center-cut loin (412), the tenderloin (415), and the country-style ribs (which technically come off the loin's blade end rather than the rib primal). Loin is the structural backbone of mainstream pork retail. Boneless loins flow into chops and roasts, country-style ribs flow into BBQ programs, and tenderloins flow into a smaller but premium value-added segment. Loin pricing is closely tied to retail feature timing, with major chains running loin features on a roughly six-week cadence.
Boston butt
The Boston butt comes off the upper front shoulder and carries roughly 7 to 10 percent of the carcass. The bone-in butt (IMPS 406) and the boneless butt (406A) are the main lines. Butts are the workhorse of the pulled-pork BBQ segment and a major input into sausage formulations. Demand for butts firms predictably ahead of grilling holidays and softens through the winter. Butt pricing is also responsive to chemical lean trim values, since butts can be deboned and trimmed into pork trim if the price relationship favors it.
Picnic
The picnic is the lower portion of the front shoulder and carries somewhere in the 8 to 11 percent range. It yields the cushion-out picnic combo, the cushion-in picnic combo, smoker-trim picnic, and various trim and ham material. Picnics flow primarily into further-processed deli ham programs, smoked product, and sausage formulations. Picnic pricing is less retail-driven than butt pricing because picnics rarely appear as a center-of-plate retail item; the demand comes from processors.
Ham
The ham primal comes off the back leg and is comparable in size to the loin, typically 22 to 25 percent of the carcass. It is one of the two largest primals on a pork carcass alongside the loin. Major sub-primals include the inside ham (IMPS 402F), the outside ham (402E), the various weight-banded boneless ham combos (17-20 lb, 23-27 lb), and the ham trim items (insides combo and outsides red combo). Hams flow heavily into bone-in cured ham programs, deli ham further-processing, and export to markets that consume bone-in cured product (notably parts of Asia for the inside ham and Europe for muscle ham used in prosciutto and speck production). Ham pricing has a strong seasonal pattern around Easter and Christmas, when retail features pull bone-in cured hams.
Belly
The belly carries roughly 16 to 21 percent of the carcass and is one of the more economically interesting primals because of its concentrated end use. Most belly volume becomes bacon. Bacon demand is structurally strong but with notable seasonal and inventory-driven swings, and the belly market is famously the most volatile segment of pork. Bellies trade in weight bands (broadly grouped as light, medium, and heavy, with specific cutoff weights varying by packer and customer) because thicker bellies render bacon differently than thinner bellies and the slicing operations care about the difference. The seasonality and the inventory dynamics of the belly are large enough to warrant their own discussion (see the article on why pork bellies move).
Spareribs
The sparerib primal comes off the belly side of the carcass and carries something in the range of 5 to 7 percent. Three formats dominate: full spareribs (IMPS 416), St. Louis style spareribs (a trimmed format), and back ribs (IMPS 422, which come off the loin primal rather than the spareribs primal). Sparerib demand has a clear summer peak driven by BBQ and grilling, and the price spread between full spares and St. Louis style varies with the labor cost of trimming. Light sparerib weights (3-down basis) trade differently than heavy sparerib weights (5-up basis) because the end-use ratios differ between bone-in retail and trimmed foodservice programs.
Reading them together
A working pork buyer typically watches butts and bellies as the two highest-volatility primals, loins and hams as the two largest demand-led primals, and picnics as the steady processor demand floor. When the pork cutout moves on butts and bellies together, the read is usually a broader retail or BBQ-segment demand shift. When it moves on loins alone, the read is typically a feature pull at a specific large chain.