underbust

The underbust is the horizontal circumference of a woman's torso measured directly beneath the breast tissue, typically just below the inframammary fold where the breast meets the chest wall. This measurement is fundamental to garment sizing in corsetry, bra manufacturing, and fitted bodywear, serving as the primary dimension for band size.

In bra sizing, the underbust determines the band size directly: a 30-inch underbust produces a 30 band, a 32-inch underbust a 32 band, and so on. The measurement must be taken with a soft tape measure held snugly but not compressed against the skin, typically with the subject standing upright and breathing normally. Most sizing systems use whole inches or whole centimeters, though some manufacturers accommodate half-size increments. The underbust differs from the overbust (measured across the fullest part of the breast), and the difference between these two measurements determines cup size.

In historical corsetry and modern corset manufacturing, the underbust measurement defines the core circumference the garment must achieve when laced. A corset designed for a 24-inch underbust will be constructed and boned to close to that dimension under tension. The measurement is also critical in foundation garment construction, where the underbust controls how a corset or waist cincher sits on the body and how much reduction or support it provides.

The underbust can change with weight fluctuation, posture, and breathing; a full breath can increase the measurement by a half inch or more. Band fit problems in bras, where the band rides up at the back, often stem from miscalculation of underbust rather than cup size errors. Similarly, corsets that gap at the front often indicate that the underbust measurement was not taken correctly or that the corset was not constructed to the specified dimension.

The term itself is straightforward: the bust measured under the breast. In technical garment specifications and pattern grading, underbust is abbreviated UB and is one of the foundational dimensions in any system that grades sizes from XS to XXXL or in numeric band sizing. This single measurement anchors all subsequent sizing calculations for fitted upper-body garments.