Bra Size Calculator – US, UK & EU Sizes
US/UK Size
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EU Size
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Band Size
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Cup Size
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Sister Sizes (same cup volume)
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How Bra Sizing Works
Bra sizing is a two-component measurement system that combines a band size (the circumference of the torso below the breasts) with a cup size (the volume of the breast relative to the band). According to a 2016 study published in the journal Chiropractic & Manual Therapies, approximately 70-80% of women wear incorrectly sized bras, most commonly wearing bands too large and cups too small. Wearing the wrong bra size can contribute to shoulder pain, back discomfort, poor posture, and breast tissue migration over time.
This calculator determines your US, UK, and EU bra size from two simple measurements: underbust (snugly around your ribcage) and bust (at the fullest point). The band size is derived from the underbust measurement rounded to the nearest even number, while the cup size comes from the difference between bust and band measurements. Each inch of difference corresponds to one cup size. The calculator also provides sister sizes — equivalent cup volumes in adjacent band sizes — for finding the best fit across brands. For related body measurements, try our body fat calculator or waist-to-hip ratio calculator.
How Bra Size Is Calculated
The standard bra sizing method used in US and UK markets:
Band Size = underbust measurement rounded to nearest even number (inches)
Cup Size = bust measurement - band size (each inch = one cup letter)
- 0" difference = AA cup
- 1" difference = A cup
- 2" difference = B cup
- 3" difference = C cup
- 4" difference = D cup
- 5" difference = DD/E cup (US) or DD cup (UK)
Worked example: Underbust = 32 inches, bust = 36 inches. Band = 32 (already even). Difference = 36 - 32 = 4 inches = D cup. US/UK size: 32D. EU size: 32" x 2.54 = 81.3 cm, rounded to nearest 5 = 80D.
Key Terms You Should Know
- Band Size — the number in a bra size (32, 34, 36, etc.) representing the chest circumference below the breasts. The band provides 80-90% of a bra's support.
- Cup Size — the letter(s) in a bra size (A, B, C, D, DD, etc.) representing the breast volume relative to the band. Cup volume is not absolute — a 32D has less volume than a 38D.
- Sister Sizes — bra sizes with the same cup volume but different bands. Going up one band size means going down one cup letter (and vice versa). Example: 32D = 34C = 36B in cup volume.
- Scoop and Swoop — a fitting technique where you lean forward, place the bra, then use your hand to scoop breast tissue from under your arms into the cups. This ensures all tissue is properly contained and gives the most accurate fit assessment.
- Gore — the center front piece of the bra between the cups. In a properly fitting bra, the gore should sit flat against the sternum. If it lifts, the cups are too small.
US, UK, and EU Size Comparison
Bra sizing varies significantly between countries. US and UK sizes share the same band numbering (inches) but diverge in cup lettering after D. EU sizes use centimeter-based bands rounded to the nearest 5 cm. According to international lingerie retailers, size confusion across systems is a major contributor to poor bra fit when shopping internationally or online.
| US Size | UK Size | EU Size | Difference (inches) |
|---|---|---|---|
| 34A | 34A | 75A | 1" |
| 34B | 34B | 75B | 2" |
| 34C | 34C | 75C | 3" |
| 34D | 34D | 75D | 4" |
| 34DD | 34DD | 75E | 5" |
| 34DDD/F | 34E | 75F | 6" |
| 34G | 34F | 75G | 7" |
Practical Examples
Example 1 — Standard fit: Underbust 30", bust 34". Band = 30, difference = 4" = D cup. Size: 30D. Sister sizes: 28DD (band down, cup up) and 32C (band up, cup down). If 30D feels tight in the band, try 32C first before going to 32D, which would increase cup volume.
Example 2 — Metric conversion: Underbust 78 cm, bust 96 cm. Converting to inches: 78/2.54 = 30.7" (rounds to 30 band), 96/2.54 = 37.8". Difference = 37.8 - 30 = 7.8" ≈ 8" = H cup (US). EU band = 78 cm rounded to nearest 5 = 80. Size: US 30H / EU 80H. Use our clothing size converter for other garment conversions.
Example 3 — Post-pregnancy fitting: A mother previously wearing 34C now measures 36" underbust and 42" bust. New band = 36, difference = 6" = DDD/F. New size: 36DDD. This 2-3 cup size increase during and after pregnancy is common, and breast size may continue to fluctuate during breastfeeding. Use our breastfeeding calorie calculator for nutrition guidance.
Tips for Getting the Best Bra Fit
- The band is the foundation. The band provides 80-90% of support. It should be firm on the loosest hook (new bras stretch over time, and you tighten by moving to inner hooks). You should be able to fit two fingers underneath comfortably — no more.
- Always scoop and swoop. After putting on a bra, lean forward and use your hand to scoop all breast tissue from under your arms and ribcage into the cups. This ensures accurate cup fill and prevents tissue migration over time.
- Check the gore. The center gore (bridge between cups) should sit flat against your sternum. If it floats or pushes away, the cups are too small. Try going up one cup size before changing the band.
- Remeasure annually. Hormonal changes, weight fluctuations, aging, and pregnancy all affect bra size. According to a University of Portsmouth study, breast size can change by 1-2 cup sizes over a woman's adult life independently of weight changes.
- Try before committing. Bra sizing varies by brand and style. A 34C in one brand may fit like a 32D or 36B in another. Always try on or verify fit before bulk purchasing.
Frequently Asked Questions
How do I measure my bra size at home?
You need a soft measuring tape. For the underbust, wrap the tape snugly around your ribcage directly under your breasts and note the measurement in inches or centimeters. For the bust, measure at the fullest point of your breasts (usually across the nipples) while wearing a non-padded bra or no bra. Stand straight with arms at your sides. The band size is the underbust rounded to the nearest even number, and the cup size is determined by the difference between bust and band measurements, where each inch equals one cup letter (1" = A, 2" = B, 3" = C, etc.).
What are sister sizes and when should I use them?
Sister sizes are bra sizes with the same cup volume but different band sizes. For example, 32D, 34C, and 36B all hold the same cup volume. Going up one band size means going down one cup letter, and vice versa. Use sister sizes when your exact size is unavailable, when a brand runs tight or loose in the band, or when you fall between sizes. If a 34C band feels too tight but the cups fit well, try 36B rather than 36C, which would increase both band and cup volume.
How do US, UK, and EU bra sizes differ?
US and UK sizes use the same inch-based band numbering (30, 32, 34, etc.) but differ in cup lettering after D. US uses DD, DDD (or F), G, H, I, J. UK uses DD, E, F, FF, G, GG, H. EU uses centimeter-based bands rounded to the nearest 5 (70, 75, 80, 85) and a simpler alphabetical cup sequence (A through K without doubling). A US 34DDD is equivalent to a UK 34E and an EU 75F. Always check brand-specific size charts when shopping internationally.
How often should I get resized for a bra?
Remeasure at least once a year, or after any significant change: weight fluctuation of 10+ pounds, pregnancy, breastfeeding, starting or stopping hormonal birth control, or menopause. Bras also stretch with wear — a well-worn bra may be 1-2 inches looser than when new. According to research from the University of Portsmouth, breast size can change by 1-2 cup sizes over a woman's lifetime independently of body weight changes, making regular fitting checks important for comfort and support.
What signs indicate I am wearing the wrong bra size?
Common signs of a poorly fitting bra include: the band rides up in the back (band too loose — try a smaller band), the center gore does not sit flat (cups too small — try a larger cup), straps dig into shoulders (band too loose, forcing straps to compensate), cups wrinkle or gap (cups too large — try a smaller cup), breast tissue spills over the top or sides (cups too small), and red marks or pain. According to the British Journal of Sports Medicine, approximately 70% of women in their study wore bands that were too large and cups that were too small.