Utility July 13, 2026 · 8 Min Read

Bra Cup Size Calculator – Guide & Formulas

Calculate your perfect bra size, band size, cup volume, and alternative sister sizes using underbust and bust measurements.

Wearing the correct bra size is critical for postural support, shoulder comfort, and back health. Studies suggest up to 80% of women wear the incorrect bra size, usually with a band that is too loose and cups that are too small. This Bra Cup Size Calculator uses the industry-standard underbust and overbust measurement method to find your correct size and recommended sister sizes.

Key Takeaway

Use the free Bra Cup Size Calculator to calculate your perfect bra size, band size, cup volume, and alternative sister sizes using underbust and bust measurements. Get instant results with step-by-step explanations.

How to Use the Bra Cup Size Calculator

  1. Measure your underbust (directly beneath your breasts) snug against your skin, in inches or cm.
  2. Measure your full bust (across the fullest part of your breasts) loosely, in inches or cm.
  3. Input the measurements to get your recommended Band Size, Cup Letter, and Sister Sizes.

The Formula

Band Size = Underbust + 4 (if even) or Underbust + 5 (if odd) (traditional method), or direct underbust (modern standard). Cup Size letter increases by one for every inch of difference between Bust and Band.

Variable Definitions

  • Underbust: Ribcage perimeter directly beneath the breast tissue
  • Bust: The maximum perimeter measured around the fullest projection of the bust

Determining Bra and Cup Size

A woman measures her snug underbust at 30 inches and her full bust at 34 inches.

  1. Step 1: Snug Underbust = 30 inches. Her modern band size is 30 or 32.
  2. Step 2: Full Bust = 34 inches.
  3. Step 3: Difference = 34 - 30 = 4 inches.
  4. Step 4: Map 4-inch difference to cup sizes: 1" = A, 2" = B, 3" = C, 4" = D.
  5. Step 5: The resulting starting size is 30D (or 32C as a comfortable sister size).

Frequently Asked Questions

What is a "Sister Size" in bras?

Sister sizes are groups of bra sizes that share the exact same cup volume, even though the band size and cup letter are different. For example, if a 34C feels too tight in the band, a sister size of 36B will offer the same cup volume with a roomier band.

How do I know if my bra band is too small or too loose?

The band should fit firmly and run parallel to the floor. If it rides up your back, the band is too loose. If it digs into your ribcage or leaves painful red marks, it is too tight.