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Bra Fit Guide

Sister Size Chart

Every sister size from 28AA to 48J. Find yours in seconds — and finally understand why the same cup can feel totally different in a different band.

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Find my sister size What is sister sizing? When to use it Popular lookups Full chart Limits of sister sizing FAQ

Find Your Sister Sizes

Select your current bra size to instantly see both sister sizes and when to use each one.

What Is Sister Sizing?

Sister sizes are bra sizes that contain exactly the same cup volume but use different band sizes. The rule is simple: go up one band size and down one cup letter, and the space inside the cup stays the same. Go down one band size and up one cup letter — same cup volume again.

This exists because of how bra sizing is calculated. Your cup size is the difference between your bust measurement and your underbust measurement — not an absolute number. As the band gets larger, the letter needed to represent the same difference gets smaller. A 36B and a 34C both represent exactly a 2-inch difference between bust and underbust (for those band sizes) — the cup volume is identical.

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The maths behind it

Cup size = bust measurement minus underbust measurement. A C cup means a 3-inch difference. That 3-inch difference creates a different physical cup on a 34 band vs a 36 band — but the volume of space inside is the same, because both the band and the cup scaled together.

34C: underbust ~34", bust ~37" → 3" = C cup
36B: underbust ~36", bust ~39" → 3" = B cup on a larger frame. Same space inside.
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The sister size rule

One band size up = one cup letter down. One band size down = one cup letter up. The cup volume stays constant. You can travel multiple steps in either direction — 34C, 36B, 38A are all sisters (two steps). 34C, 32D, 30DD are also all sisters.

34C sister family: 30E · 32D · 34C · 36B · 38A
Important: Sister sizes have the same cup volume — but they don't feel identical on your body. The band sits at a different point on your ribcage, the straps sit at a slightly different angle, and the underwire width changes. Sister sizing is a starting-point fix, not a perfect substitute for your true size.

When to Use Sister Sizing

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Band too tight → go up + down a cup

If your cup fits well but the band digs in, go up one band size and down one cup letter. Example: 34C feeling tight in the band → try 36B. The cup volume is preserved; the band gives you more room.

Try: 32D → 34C → 36B → 38A
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Band too loose → go down + up a cup

If the band rides up or gaps away from your back, go down one band size and up one cup letter. Example: 36B band riding up → try 34C. Same cup volume, snugger band that does its job.

Try: 38A → 36B → 34C → 32D
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Your size is out of stock

When a specific style isn't available in your size, a sister size is the closest alternative. It will fit differently, but the cup volume is preserved — better than going to a completely different size or a different style.

34D sold out? Try 36C (same cup volume, slightly larger band).
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Trying a new brand

Brands size differently. If you normally wear a 34C but a specific brand's 34C cups feel too small, the issue may be cut rather than volume — try the sister size down (32D) which has the same volume but a snugger band that sometimes improves cup projection.

Note: This is a fit-adjustment trick, not a true size change.
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When not to use it

Sister sizing doesn't fix a cup that's genuinely too small or too large. If breast tissue is spilling over the cup or the cup is wrinkling, you need a different cup size — not a sister size. Sister sizing only works when the cup volume is right but the band comfort isn't.

Spillage or wrinkling: Change cup size, don't sister-size.
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Sports bras and sister sizing

Sister sizing applies to sports bras too. Sports bra fabric is more compressive, so women often find they need a sister size with a larger band and smaller cup than their everyday bra. If a sports bra band feels restrictive during exercise but the cup is fine, try one band up, one cup down.

Sports bra tip: 34C everyday → try 36B in sports bras first.

Full Sister Size Chart

Each row is a band size. Sizes in the same diagonal column share the same cup volume — they are all sister sizes of each other. Tap any size to look up its sisters.

Band AA A B C D DD DDD DDDD G H I J
28 28AA 28A 28B 28C 28D 28DD 28DDD 28DDDD 28G 28H 28I 28J
30 30AA 30A 30B 30C 30D 30DD 30DDD 30DDDD 30G 30H 30I 30J
32 32AA 32A 32B 32C 32D 32DD 32DDD 32DDDD 32G 32H 32I 32J
34 34AA 34A 34B 34C 34D 34DD 34DDD 34DDDD 34G 34H 34I 34J
36 36AA 36A 36B 36C 36D 36DD 36DDD 36DDDD 36G 36H 36I 36J
38 38AA 38A 38B 38C 38D 38DD 38DDD 38DDDD 38G 38H 38I 38J
40 40AA 40A 40B 40C 40D 40DD 40DDD 40DDDD 40G 40H 40I 40J
42 42AA 42A 42B 42C 42D 42DD 42DDD 42DDDD 42G 42H 42I 42J
44 44AA 44A 44B 44C 44D 44DD 44DDD 44DDDD 44G 44H 44I 44J
46 46AA 46A 46B 46C 46D 46DD 46DDD 46DDDD 46G 46H 46I 46J
48 48AA 48A 48B 48C 48D 48DD 48DDD 48DDDD 48G 48H 48I 48J
How to read this chart: Pick your band row. Your sister sizes sit one cell left (smaller band + larger cup) and one cell right (larger band + smaller cup) in the same row. They share the same cup volume. Moving diagonally down-left or up-right across rows also traces sister size families.

The Limits of Sister Sizing

Sister sizing is a useful tool — not a magic fix. Understanding where it breaks down helps you use it correctly.

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Underwire width changes

As band size increases, underwire width increases too — the wire is designed to wrap around a larger ribcage. A 38B underwire is physically wider than a 34C underwire even though the cup volume is the same. If you have a narrow ribcage or close-set breasts, the wider underwire in a larger sister size may not sit correctly.

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Strap placement shifts

On a wider band, straps sit further apart. If you have narrow or sloping shoulders, sister sizing up to a larger band may cause straps to fall off. Sister sizing down (smaller band, larger cup) moves straps slightly closer together — sometimes helpful for narrow-set shoulders.

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Cup shape differs by brand

Volume is the same in sister sizes — but the cup shape varies by brand. A 36B in one brand may project more or less than a 34C in another brand even if the stated cup volume is identical. Sister sizing between brands requires trial; sister sizing within the same brand is more predictable.

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Three steps is too many

One step is reliable. Two steps sometimes works. Three or more steps away from your true size — e.g. wearing a 40AA instead of a 34C — means the band, wire, straps, and cup shape are so different from your true size that fit will be poor regardless of shared volume. Stick to one sister step when possible.

Rule: Maximum two sister steps from your true size.

Frequently Asked Questions

Is 36B the same as 34C?
Yes — 36B and 34C are sister sizes with the same cup volume. The difference is the band: 36B has a larger, looser band; 34C has a snugger band. Choose 36B if the 34C band feels too tight. Choose 34C if 36B rides up. The space inside the cup is identical in both.
Is 34C the same as 36B?
Yes — this is the same question from the other direction. 34C and 36B share the same cup volume. 34C has a smaller, firmer band. 36B has a larger, softer band. Neither is "better" — it depends on your ribcage measurement and how the band feels on your body.
What are the sister sizes of 34DD?
The sister sizes of 34DD are: 32DDD (smaller band, same cup volume) and 36D (larger band, same cup volume). The full sister family extends to 30DDDD, 32DDD, 34DD, 36D, 38C, 40B, 42A — all share the same cup volume.
What are the sister sizes of 36DD?
The sister sizes of 36DD are: 34DDD (smaller band, same volume) and 38D (larger band, same volume). Full family: 32DDDD, 34DDD, 36DD, 38D, 40C, 42B, 44A.
Can I use sister sizing to go up more than one cup size?
Yes — but the further you go from your true size, the worse the fit will be in other ways (underwire width, strap placement, overall support). One step is reliable. Two steps can work in a pinch. Three or more steps is not recommended — the bra will look and feel wrong even if the cup volume is technically correct.
Does sister sizing work for plus sizes?
Yes, the same mathematical relationship applies across all band and cup sizes. However, for larger cup sizes (DD and above), the differences in underwire width and cup construction between sister sizes become more significant. Our plus size guide covers fit in more detail for larger cups.
My true size isn't available — how many sister steps can I go?
One step is always safe. Two steps usually works. Beyond that, the changes in underwire width, band tension distribution, and strap placement are significant enough that fit will be noticeably compromised. If your size is frequently unavailable, it's worth investing in a few bras in your true size from a reliable brand rather than relying on sister sizing as a permanent solution.

Don't know your true size yet?

Sister sizing only works if you start from the right place. Use our free calculator to find your exact size first.

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