Bra Sister Sizes: A Comprehensive Guide to Finding Your Perfect Fit (2026)

What Is a Sister Size?

A sister size is a bra that holds the same cup volume on a different band. The math: go up one band, down one cup (or vice versa). The volume of breast tissue the cup holds stays the same — only the band and cup number change.

Real-world example:

• You usually wear 34C.
• Your sister sizes are 32D (one band down, one cup up) and 36B (one band up, one cup down).
• All three bras hold approximately the same cup volume — but the band fit and overall feel differ.

That's it. Sister sizes aren't a "compromise" — they're a tool for finding better fit when your usual size is unavailable, when your weight has shifted, or when a specific bra runs tight or loose in the band.

How to Find Your Sister Size

1. Start from your measured size. Use the bra measuring guide to find your band and cup.
2. Try your measured size first. That's your baseline.
3. Walk up a band (and down a cup) if: the band feels too tight, the band leaves marks, or the cups gape. Try one band up, one cup down.
4. Walk down a band (and up a cup) if: the band rides up in back, the straps dig, or the cups feel small. Try one band down, one cup up.

You can repeat this for 1–2 sizes in either direction, but the further you go, the less the proportions match your body. Most people find their best fit within one band of their measured size.

Sister Size Chart (US, AA–DDD)

Find your usual size in the center, then read up/down/left/right to find the sister sizes that hold the same cup volume.

Reading the chart: a 32D holds the same cup volume as 30DD (one row up, one column right) and 34C (one row down, one column left). All three sizes are "sisters."

For the full 28A–50DDD chart with bust measurements, see the bra cup size chart hub.

When to Sister-Size Up vs. Down

Go up a band (and down a cup) when:

• Your usual band feels too tight or leaves red marks
• You want a more relaxed, all-day fit
• Your usual size is sold out and you need an alternative
• You're between bands (e.g., measured 33) — round up to 34

Go down a band (and up a cup) when:

• Your band rides up in back
• You want more support from the band (snugger = stronger lift)
• You feel like the cups are too small but the band is fine
• You're between bands (e.g., measured 33) — round down to 32 for a tighter fit

Why Volume Stays the Same

Here's the secret: when you go up one band, the cup also gets slightly wider (because the cup is attached to a longer band). Going up one band and down one cup net-net cancels out — the cup is now a little wider but a little shallower, and the total volume of breast tissue it can hold is the same.

This is also why the same bra in 32D and 36B fits so differently even though they hold the same volume: the cup geometry is different (the 36B cup is wider and shallower, the 32D cup is narrower and deeper). For bodies that are more wide-rooted or shallow, 36B will fit better. For bodies that are more narrow-rooted or projected, 32D will.

5 Common Sister-Size Pitfalls

1. Sister-sizing across brands. A 34C at Brand X is not the same as a 34C at Brand Y. Sister-size across one brand only.
2. Walking too far. 32D → 36B is one step. 32D → 38A is two — at that distance, the proportions stop matching your body.
3. Ignoring cup shape. Two sister sizes hold the same volume but with different cup geometry. The shape change can mean a bra that was perfect in 34C doesn't work in 36B.
4. Forgetting the band can stretch. A snug band will relax 5–10% over the first few wears. Don't sister-size up just because the new bra feels tight out of the package.
5. Sister-sizing to fix cup problems. If the cup is wrong, the cup is wrong. Sister-sizing the band won't fix a cup that's too small or too large.

FAQ

What is my sister size if I wear a 34C?

Your sister sizes are 32D (one band down, one cup up) and 36B (one band up, one cup down). All three hold the same cup volume. For the full chart, see our bra cup size hub.

Are sister sizes the same in every brand?

No. Each brand uses its own fit model, so a 34C at Brand X may grade differently than a 34C at Brand Y. Sister-size within a single brand for the most reliable fit.

Can I sister-size to fix a tight band?

Yes — if your usual band feels too tight, try one band up and one cup down. The cup volume stays the same, but the band is roomier.

Can I sister-size down for more support?

Yes — one band down, one cup up gives a snugger band (more support) and a slightly deeper cup. Good for when your usual band feels loose.

What's the difference between sister sizing and trying a different brand?

Sister sizing moves within one brand's size system. Switching brands is a different exercise — you'll need to re-measure and use each brand's own size chart.

Does sister sizing work for sports bras?

Yes, but with a caveat: sports bras use a different grading system (often XS–XL or S/M/L). For sports-bra-specific sizing, see our sports bra sizing guide.