Bra Size Calculator

Bra sizing is based on the mathematical relationship between two key body measurements: the underbust (ribcage circumference directly beneath the bust) and the bust (circumference around the fullest point of the chest).

First, the band size is determined by converting and rounding the underbust measurement. In US and UK sizing systems, the underbust in inches is adjusted to the nearest even number. In EU sizing, the band size directly matches the underbust measurement rounded to the nearest multiple of 5 centimeters.

Second, the cup size is calculated using the difference between the full bust and underbust measurements:

$$\text{Difference} = \text{Bust} - \text{Underbust}$$

Each 1-inch (2.54 cm) increase in this difference represents a step up in cup letter volume:

• 1 inch difference = A Cup
• 2 inches difference = B Cup
• 3 inches difference = C Cup
• 4 inches difference = D Cup
• 5 inches difference = DD (US) / E (UK)

Consider a practical scenario with the following body measurements:

• Underbust: 31 inches (78.7 cm)
• Bust: 36 inches (91.4 cm)

Step 1: Determine the Band Size
Rounding an underbust measurement of 31 inches to the nearest even number yields a band size of 32 in US and UK sizing. In EU sizing, 78.7 cm rounds to 80 cm, giving an EU band size of 80.

Step 2: Calculate the Cup Difference
Subtract the underbust from the bust measurement:
$$\text{Difference} = 36\text{ in} - 31\text{ in} = 5\text{ inches}$$

Step 3: Map Cup Difference to Cup Letter
A 5-inch difference maps to a DD cup in the US system, an E cup in the UK system, and an F cup in the EU system.

Result: The primary size for these measurements is 32DD (US), 32E (UK), or 80F (EU).

Follow these expert tips to ensure your measurements deliver the best possible garment fit:

• Wear an unpadded bra while measuring: Unpadded contour bras hold tissue in a natural position without adding artificial volume, ensuring realistic numbers.
• Keep the measuring tape level: Make sure the tape is parallel to the floor around your back. A drooping tape measure will inflate your underbust reading and alter your cup calculation.
• Understand sister sizes: If a band feels tight but the cups fit well, try a sister size. Moving up one band size requires going down one cup size (e.g., from 32DD to 34D) to maintain the exact same cup volume.