Bra Size Calculator
Estimate a US bra size from underbust and bust measurements. Returns band size and cup letter in the form '34B'.
Last updated: September 2026
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About this calculator
US bra sizing pairs a band size in inches (measured around the ribcage just under the bust) with a cup letter based on the difference between the bust measurement and the band measurement. A 1-inch difference is roughly an A cup, 2 inches a B, 3 inches a C, 4 inches a D, and so on up through DD, DDD/E, F, G, and beyond.
This calculator uses the modern 'true fit' rule: band inches ≈ underbust inches (rounded to the nearest whole number). Older '+4' or '+5' methods added inches to the underbust to derive the band and are now considered inaccurate — they systematically produce a too-loose band, which fit specialists say is the root cause of most 'my bras don't fit' complaints.
The output is a US size (34B, 36D, and so on). UK sizes use the same band inches but different cup labels above D (UK DD ≈ US DD; UK E ≈ US DDD; UK F ≈ US G). EU sizes use centimetres for the band (US 34 ≈ EU 75) and letters that don't fully align with US letters. This calculator does not convert between size systems — measure in inches and read the US size directly.
How to use
Example — a fitting session. Underbust 32 in, bust 36 in. Enter underbust = 32, bust = 36. Band = 32 (rounded); cup difference = 36 − 32 = 4 → D. Result: 32D. Try that size on. If the band rides up in back, size down one band and up one cup (30DD) — those 'sister sizes' have the same cup volume in a tighter band. Example — larger frame. Underbust 38, bust 44. Band = 38; cup diff = 6 → DDD. Result: 38DDD. Sister sizes in either direction (36G/40DD) may also fit if the band comfort is off.
Frequently asked questions
How do I measure correctly?
Wear a well-fitting non-padded bra. For underbust, run the tape snugly under the bust and around the ribcage, level all the way around, and note the number of inches. For bust, run the tape across the fullest part of the bust, keeping it level. Both measurements should be taken with a normal breath. If your bust measurement changes by more than an inch when you inhale, take the exhale reading.
What are 'sister sizes'?
Sister sizes have the same cup volume but a different band. Going down one band and up one cup preserves cup volume: 34B, 32C, 30D, 28DD all have the same-size cup. If a 34B fits the cup but the band feels tight, try 36A. If it fits the cup but the band is too loose, try 32C. Sister sizing is the fastest way to iterate on fit without changing brands.
Why do brands fit so differently?
There is no legally binding US bra sizing standard. Each brand's cup shape, wire width, and gore height (the piece between the cups) differs, so a 34C in one brand can feel like a 32D or 34B in another. Use the calculator's number as a starting point and iterate. Sister sizing (see above) is your friend.
How often should I remeasure?
Whenever weight or torso size changes materially — pregnancy, post-partum, weight loss/gain of more than 10 lb, hormonal changes — because both band and cup can shift. Fit specialists also recommend a fresh fitting every 6–12 months even without body changes because bras stretch out and lose support with wear.