Industry research consistently estimates that 70-80% of women wear sports bras in the wrong size. The bra fits roughly, but not in a way that delivers proper support for training. Sister sizing is the framework that explains why, and how to fix it. This is the guide.
Bra sizing has two variables: the band (number) and the cup (letter). A 32C, 34B, and 36A are all "sister sizes", they have approximately the same cup volume but different band sizes. The volume in the cup is what matters; the band determines how the bra anchors to the body.
Most women guess at the band size and pick a cup that "looks right", leading to a bra that is mathematically the wrong size but feels acceptable until they start training.
Step 1, Measure the band:
Step 2, Measure the bust:
If your "true" size is 34C but the brand only stocks 32C and 36C, sister sizing tells you which to pick:
| If your size is | Smaller sister | Larger sister |
|---|---|---|
| 34C | 32D | 36B |
| 36D | 34E (DD) | 38C |
| 32A | (no smaller) | 34AA |
The cup volume stays similar; the band shifts. Sports bras need a snug band, so going to a smaller band size with a larger cup letter is usually the better trade-off when your true size is out of stock.
A properly-fitted sports bra passes four tests:
| Impact | Activity | Support needed |
|---|---|---|
| Low | Yoga, pilates, walking | Light compression or shelf bra |
| Medium | Strength training, hiking | Compression with structured cup |
| High | Running, HIIT, kickboxing | Encapsulation + compression hybrid |
Quick online sizing quizzes rely on self-reported measurements that are systematically incorrect. The band is usually measured too loose; the cup is usually measured by eye instead of tape. The result: 70%+ of online-sized bras are wrong. A proper sizing session (in-store or with a soft tape at home, following the protocol above) corrects this.
Premium sports bras solve four problems generic versions skip:
Explore the sports bras collection or the full womens collection for premium fit and support.
Sports bras lose support after 6-12 months of regular training, regardless of brand. The elastic degrades through stretch and wash cycles. Replace earlier than you think, most women run a bra 18+ months past its support window because it still looks fine.
Properly-sized, properly-replaced premium sports bras transform training quality. Browse the full womens collection for the standard.
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Want the deep dive? Read our complete guide to seamless activewear covering knitting tech, fabric science, sizing and care.
