Bra Size Calculator

How Our Bra Size Calculator Works

Most commercial fashion brands rely on an outdated 1960s sizing convention known as the "+4 method", in which fitters arbitrarily add 4 or 5 inches to a woman's measured ribcage circumference before assigning a band size. This practice originated when bras were manufactured from stiff, non-stretch cotton and linen twill.

In modern intimates made from elastic elastane and power-mesh, the +4 method is mathematically obsolete: it forces women into oversized, unsupportive bands (e.g. putting a 30-inch ribcage into a 34 band) paired with artificially undersized cups (White & Scurr, 2012).

HerSizeHub uses pure anatomical differential mathematics. Your underband measurement directly determines your band size, while the differential between your fullest bust and snug underbust dictates your cup volume.

Step-by-Step Measurement Protocol

Biomechanical studies in sports medicine confirm that over 85% of women wear the incorrect bra size (McGhee & Steele, 2010). Follow this precise protocol using a flexible, non-stretch fiberglass tailor's tape:

A Concrete Worked Example

How to Handle Borderline & Odd Underbust Measurements

Commercial bra bands are manufactured exclusively in even-numbered inch increments (30, 32, 34, 36, etc.):

• If your snug underbust is an odd number (e.g. 31 or 33 inches): Your comfort depends on ribcage compressibility. If you have minimal subcutaneous ribcage padding, round up to the next even band (e.g. 33" moves to a 34 band). If your ribcage has soft tissue compressibility, size down to the lower even band (e.g. 33" moves to a 32 band fastened on the loosest hook or paired with a temporary band extender).
• If your bust difference falls between whole inches (e.g. 3.5 inches): Try both adjacent cup sizes (e.g. C and D). Always choose the cup that accommodates your breast tissue without quad-boobing or tissue spilling under the arms.

Understanding Cup Differential Calculations

Cup size is determined by subtracting your snug underbust measurement from your fullest bust measurement. Each 1-inch (2.54 cm) increment corresponds to one cup letter progression. Our calculator uses standard continuous midpoint rounding to map your measurement to the nearest whole cup volume:

Note on Boundary Sizing: If your bust differential falls exactly near a half-inch boundary (such as 1.5", 2.5", or 3.5"), you sit between two adjacent cup sizes. Because cup volume and cup projection vary across lingerie brands and cut styles (e.g. shallow t-shirt bras vs. deep plunge bras), we recommend test-fitting both adjacent sizes to determine your optimal fit.

Breast Shape & Asymmetry: The Non-Tape Variables

Two women with identical 32D measurements can fit into the same bra completely differently due to anatomical 3D contours (Chen, LaBat, & Bye, 2010):

• Shallow vs. Projected: Shallow breasts spread tissue across a broad chest wall, requiring open, balconette, or demi-cup styles. Projected breasts extend outward from a compact root, requiring deeper, seamed, multi-part cups to prevent flattening.
• Full on Top vs. Full on Bottom: If fullness is concentrated beneath the nipple, plunge or closed-rim cups will fit smoothly. If fullness is concentrated above the nipple, closed cups will cut into tissue, requiring open half-cup or balconette cuts.
• Breast Asymmetry: More than 50% of women have one breast that is half to a full cup size larger than the other. Always fit the bra to your larger breast to prevent underwire poking, and use a removable insert or slightly tighten the shoulder strap on the smaller side.

Troubleshooting Common Fitting Problems

• Underband rides up your shoulder blades: The band is too loose. Size down one band size and size up one cup letter (e.g. 36C → 34D) to restore 80% underband anchoring. Learn more in our guide: The Sister Size Equation.
• Underwires dig into underarm tissue: The cup diameter is too narrow. Increase cup size (e.g. 34D → 34DD).
• Center gore floats off your sternum: The cups are too shallow or too small, forcing the bridge away from your chest wall. Increase cup volume.
• Shoulder straps dig painful grooves: The band is not doing its job. Tightening straps to compensate for a loose band shifts the weight of breast tissue directly onto your trapezius muscles.

Verified Scientific Sources & Standards

• McGhee, D. E., & Steele, J. R. (2010). Optimising breast support in female patients through correct bra fit: A cross-sectional study. Journal of Science and Medicine in Sport, 13(6), 568–572. [PMID: 20451452]
• White, J., & Scurr, J. (2012). Evaluation of professional bra fitting criteria for bra selection and fitting in the UK. Ergonomics, 55(6), 704–711. [PMID: 22397508]
• Chen, C. M., LaBat, K., & Bye, E. (2010). Physical characteristics related to bra fit. Ergonomics, 53(4), 514–524. [PMID: 20309747]

Frequently Asked Questions