Estimate a starting bra size from underbust and full-bust measurements, compare US, UK, EU, French, and AU/NZ labels, and view sister sizes.

Required: snug underbust and full-bust circumferences measured in the same unit.
Measure level around the ribcage directly below the bust.
Measure the fullest point without compressing tissue.
Changing the band also changes the cup letter needed for the same full-bust measurement.
Measurement-based bra sizing is a starting point. Cup labels and fit vary by brand, style, breast shape, fabric, and comfort preference; confirm with the brand chart and an on-body fit check.

Bra Size Formula (US/UK)

The calculator above estimates a US/UK bra size (band + cup) using underbust (UB) and full-bust/overbust (OB) measurements. A common modern approach is:

Band & ≈ nearest even inch of UB \ CupSteps & ≈ round(OB - Band)

Variables:

  • UB is the underbust measurement (circumference) in inches (or convert cm to inches first)
  • OB is the overbust/full-bust measurement (circumference) in inches (or convert cm to inches first)
  • Band is the estimated US/UK band size (typically an even number in inches)
  • CupSteps is the estimated difference (in inches) used to select a cup (for example: 1=A, 2=B, 3=C, 4=D, 5=DD/E, 6=DDD/F, etc.)

Note: Your bust measurement itself is simply the full-bust/overbust circumference (OB). Underbust is used to estimate bra band and cup size, not to “average” into a bust measurement.

What is a Bust Size?

Bust size most commonly refers to the measurement around the fullest part of a person’s bust/chest and around the back (often called the full bust measurement). It is typically measured in inches or centimeters and is often used in determining clothing sizes. In bra sizing, the “bra size” is different: it includes both the band (related to the underbust) and the cup (related to the difference between full bust and band).

How to Calculate Bra Size (Band + Cup)?

The following steps outline how to estimate a US/UK bra size from underbust and full-bust measurements:

  1. Measure your underbust (UB) around the ribcage, under the bust.
  2. Measure your full bust (OB) around the fullest part of the bust/chest.
  3. If you measured in centimeters, convert to inches (1 in = 2.54 cm).
  4. Estimate the band size by rounding UB to the nearest even number of inches (or use the classic +4/+5 method if desired).
  5. Compute the difference in inches: OB − Band, then map that difference to a cup size (each ~1 inch is one cup step in US sizing).
  6. Combine band + cup to get the estimated bra size, then check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge:

Underbust measurement (UB) = 30 inches

Overbust measurement (OB) = 36 inches