The underbust measurement is the circumference of the torso taken directly beneath the breasts, where a bra band rests against the ribcage. This measurement is a foundational metric for determining the correct size of garments that require precise upper-body fit. It serves two primary functions: establishing the band size for supportive undergarments and acting as a specific design line in clothing construction.
The Role of Underbust in Bra Sizing
The underbust measurement is directly correlated with the numerical portion of a bra size, known as the band size. For example, an underbust measurement of 34 inches typically translates to a size 34 bra band, though some sizing methods may require adding a few inches to the raw measurement. This band is the most important component of a bra’s structure, providing the majority of the support.
The bra band is responsible for carrying approximately 80% of the breast weight, not the shoulder straps as is often assumed. A band that is too loose will ride up the back, shifting the weight onto the straps and causing discomfort or poor posture. The snug fit determined by the underbust measurement ensures the band remains level and anchored around the ribcage, effectively lifting and supporting the breast tissue.
The underbust measurement is also used in conjunction with the overbust measurement—the circumference taken at the fullest part of the chest—to calculate the cup size. The difference between these two measurements determines the cup letter, with each inch of difference corresponding to a specific cup size. This two-part system ensures the bra provides both foundational support from the band and the correct volume containment from the cups.
How to Take an Accurate Underbust Measurement
Taking an accurate underbust measurement requires a soft, flexible tape measure. The measurement should be taken while wearing a non-padded bra or no bra at all, ensuring the tape is placed directly against the skin of the ribcage. The tape must be positioned horizontally, parallel to the floor, and directly beneath the breast tissue.
Pull the tape measure snug against the body, but not so tight that it digs into the skin or restricts breathing. The person being measured should exhale fully before noting the number, as this ensures the measurement is taken when the ribcage is at its smallest point.
Once the raw measurement is recorded, it may need to be adjusted depending on the specific sizing system being used. Some methods use the raw number directly, while others, particularly in US sizing, instruct the wearer to round to the nearest whole number and then add four or five inches to determine the final band size. Following the specific instructions of the garment manufacturer is necessary to ensure the correct size is selected.
Underbust in Garment Design and Style
Beyond functional undergarments, the term “underbust” is frequently used in clothing design to describe a specific style line or garment type.
Underbust Seams
An underbust seam is a horizontal seam placed directly below the bust line on a dress or top. This design element is often used to create an empire waist silhouette, which visually lengthens the body and provides shaping to the bodice.
Underbust Corsets
The underbust corset is a distinct garment that begins just below the breasts and extends down to the waist or hips. Unlike an overbust corset, it leaves the bust exposed, allowing it to be worn over a variety of tops and dresses. This type of corset is designed to cinch the waist and provide torso shaping, emphasizing the natural curves of the body for a defined aesthetic.
Pattern Drafting
In pattern drafting, the underbust line is a reference point for creating fitted bodices and structured outerwear. Designers use this measurement to ensure that the garment’s shaping elements, such as darts or princess seams, align correctly with the wearer’s anatomy.