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.

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Put these formulas into practice with our instant, step-by-step Bra Cup Size Calculator.

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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