The bottom sirloin butt, IMPS 185, is three cuts held together by seams, and the market prices them as three cuts. Split it along the natural seams and you get the flap (185A), the ball tip (185B) and the tri-tip (185C, or 185D when it is defatted). All four are on USDA's daily report, and on its prints the flap has outpriced the other two on every single trading day for five years.
What each piece is
| IMPS | Cut | Weights (lb) |
|---|---|---|
| 185 | Bottom sirloin butt, whole | 5-6, 6-7, 7-8, 8-up |
| 185A | Flap | 1-3, 3-up |
| 185B | Ball tip | 1.5-3, 3-up |
| 185C | Tri-tip | 1.5-3, 3-up |
| 185D | Tri-tip, defatted | 1.5-3, 3-up |
The flap is the obliquus abdominis internus, the thin, coarse-grained muscle the West Coast and France call bavette, and the one that goes to carne asada and fajitas. The tri-tip is the tensor fasciae latae, the triangle of muscle that gives it its name and the Santa Maria barbecue its roast. The ball tip is three muscles from the sirloin tip end (the vastus medialis, vastus lateralis and rectus femoris), a lean, plainer piece that retail sells as sirloin tip steaks and roasts. A whole bottom sirloin with the flap already off is a different item, IMPS 186.
What USDA prints
Four lines on the daily boxed beef report, each with its own trim:
Loin, bottom sirloin, flap is 185A at fat option 4, practically free of fat.
Loin, ball-tip, bnls, heavy is 185B at fat option 1, the commodity trim. USDA's guide to the report sets heavy at 2 lb and up, which does not line up with the IMPS ranges above. There has been no light ball tip on the report since 2009.
Loin, sirloin, tri-tip is 185C at fat option 1, up to ¾ inch of fat on average.
Loin, sirloin, tri-tip, pld is 185D at fat option 4. USDA's "pld" reads like peeled, but the spec is defatted: practically free of fat, not the peeled and denuded trim, which is fat option 5. The Select version prints only now and then.
How they price
Over the five years to August 2026, Choice:
The flap printed above the tri-tip every day, by a median 76 percent, about $3.24 a pound, and above the ball tip by a median 93 percent. It is also the most traded of the four: 43 percent of the family's reported pounds, against 28 for the ball tip, 22 for the tri-tip and 7 for the defatted tri-tip. The flap prices with the carne asada and fajita trade, not with the rest of the sirloin.
The defatted tri-tip printed above the regular one every day, by a median 46 percent, about $2.05 a pound. That gap is what the plant charges for the trimming and for the fat you no longer pay freight on. Compare it to what the same trim costs on your own bench before paying it.
The tri-tip printed above the ball tip on 71 percent of days, by a median of about 43 cents a pound. Grade matters most on the tri-tip: Choice ran a median 10 percent over Select, against 5 for the flap and 2 for the ball tip.
When each one is strong
Measured against the Choice cutout, so a move in the whole market cannot pass for a season, over 2016 to 2025:
The tri-tip is a June cut. It ran above its own yearly average in June in all 10 years and in July in 9, and in no November or December. The defatted tri-tip has the same shape.
The ball tip is a spring piece, above its yearly average in March and in May in 9 of the 10 years, and never in November, December or January.
The flap is strong from April, above its yearly average that month in 9 of the 10 years, and weak in the fall: never above in October, and only once in December.
Keeping the quote straight
Sirloin alone is not a spec: the top sirloin butt (184) and the bottom sirloin butt (185) are different subprimals, and their cap and flap names cross over. Tri-tip alone is not a spec either, because the defatted one has run nearly half again the price of the regular. And flap without a number can mean the chuck flap, a different cut from the other end of the carcass. The IMPS number settles all three.