The primary difference between a porterhouse and a T-bone steak is the size of the tenderloin section, which is determined by where the cut is made in the short loin. According to USDA regulations, a steak must have a tenderloin at least 1.25 inches wide to be classified as a porterhouse, whereas a T-bone has a tenderloin between 0.5 and 1.24 inches wide.

Key Distinctions
Cut Location: Porterhouses are cut from the rear of the short loin where the tenderloin is thickest, while T-bones are cut from the front where the tenderloin tapers off.
Size and Weight: Because of the larger tenderloin, porterhouses are generally heavier and thicker (often 24 ounces or more) compared to T-bones, making them better suited for sharing.
Composition: Both cuts feature a T-shaped bone separating a New York strip on one side and a tenderloin (filet mignon) on the other; essentially, every porterhouse is technically a T-bone, but not every T-bone qualifies as a porterhouse.
Price: Porterhouses typically cost more overall due to their larger size and higher proportion of tenderloin, though the per-pound price is often similar.