Flat Iron vs Sirloin Flap — What's the Difference?
Quick Answer
Flat Iron (flat iron steak) and Sirloin Flap (sirloin flap) are not the same cut: Flat Iron is chuck primal (Infraspinatus muscle, top blade of the shoulder clod); Sirloin Flap is sirloin primal (sirloin flap / bottom sirloin flap).
Canonical entities: Flat Iron · Sirloin Flap
Side-by-side
| flat iron | sirloin flap | |
|---|---|---|
| Primal | chuck | sirloin |
| Muscle / location | Infraspinatus muscle, top blade of the shoulder clod | sirloin flap / bottom sirloin flap |
| Character | The second most tender muscle on the entire animal, after the tenderloin. A flat, rectangular steak extracted from the top blade by splitting it along the central connective tissue. Uniform thickness makes it ideal for grilling. Also known as butler's steak in the UK. | Thin, loose-grained flap from the bottom sirloin; overlaps skirt/flank in some regional breakdowns. |
Key differences
- Different primals: chuck vs sirloin.
- Texture and slicing: compare fibrous, grain-heavy cuts vs more tender steak-style muscles based on each cut’s description.
- Retail naming diverges by country—always map through a canonical cut when translating menus or labels.
When to use each
Flat Iron
Pick Flat Iron when you want its specific marbling/texture profile: The second most tender muscle on the entire animal, after the tenderloin. A flat, rectangular steak extracted from the top blade by splitting it along the central connective tissue. Uniform thickness makes it ideal for grilling. Also known as butler's steak in the UK.
Sirloin Flap
Pick Sirloin Flap when its primal/muscle traits fit the dish: Thin, loose-grained flap from the bottom sirloin; overlaps skirt/flank in some regional breakdowns.
Flat Iron and Sirloin Flap are different canonical muscles/primals: Flat Iron is chuck (Infraspinatus muscle, top blade of the shoulder clod); Sirloin Flap is sirloin (sirloin flap / bottom sirloin flap).
Choose based on tenderness, marbling, grain direction, and how you plan to cook (sear vs braise vs slice thin).
Read the full guides: flat iron (what-is) · sirloin flap (what-is) · flat iron hub · sirloin flap hub