What bra sister sizes are and how to use them
A sister size trades band size for cup size while keeping the cup volume close, so you can loosen or tighten the band without losing your fit.
What is a sister size in bras?
Sister sizes are bra sizes that hold close to the same cup volume with a different band. Move down one band and up one cup letter to tighten it, or up one band and down one cup letter to loosen it.
A 38D and a 36DD hold a similar cup with a tighter band. A 38D and a 40C hold a similar cup with a looser one. It isn't a guarantee of an identical fit; cup shape and construction still vary by style. It's the fastest way to try a second option when your first size is close but not quite right.
When should you use a sister size?
Reach for a sister size in two situations: the cup fits but the band digs in or rolls, or your measured size isn't offered in the style you want.
- Cup right, band wrong. If the cup covers you well but the band is too tight and rolls, or too loose and rides up, try the next band size with the opposite cup letter before you give up on the style.
- Your size isn't stocked in that style. Band and cup ranges are set per style, not brand-wide, so a sister size can put you in a similar cup when your exact size is out of range. Check the full lineup of Woman Within bras before ruling out a style.
How to find your bra sister size
Start from your current or measured size, then move one band size and one cup letter in opposite directions: band down and cup up to tighten, or band up and cup down to loosen.
Bra bands run snug more often than loose in shopper feedback, so the tighter-band option tends to work more often than the looser one. Measure your band and bust before you buy: sister sizing is a starting point, not a replacement for your own numbers.
| Direction | Band Change | Cup Change | Example |
|---|---|---|---|
| Tighter band | Down one band | Up one cup letter | 38D to 36DD |
| Looser band | Up one band | Down one cup letter | 38D to 40C |
Why band and cup ranges differ by style
Every bra has its own band and cup range, so a sister size only works within what that specific style offers. Always check the Size Chart on the product page first.
The Front-Close Wireless Posture Bra runs band sizes 38 to 54 in cup sizes B to G, while the Playtex 18 Hour Ultimate Shoulder Comfort Wireless Support Bra runs bands 36 to 54 in cups B to H, two different ranges on two different styles. If your sister size falls outside a style's listed range, it isn't available in that bra, even if it's available in another.
That's true across categories too: full coverage bras and front closure bras each set their own ranges, so confirm the chart on the specific style before you swap sizes.
Frequently asked questions
What is the sister size of a 38D?
A 38D's closest sister sizes are a 36DD (tighter band, same cup volume) and a 40C (looser band, same cup volume). Which one fits depends on how your current band feels: too tight or too loose.
Should I go up a band or down a cup?
Go up a band and down a cup if your current band is too tight. Go down a band and up a cup if it's too loose. The cup volume stays close either way; only the band changes.
Do sister sizes fit the same?
Not exactly. Cup volume stays close, but cup shape, wire placement, and fabric still vary by style. Treat a sister size as your next size to try, not a guaranteed match.
Is a 34-inch band really 34 inches around?
Not usually. A measured band size and a labeled band size aren't identical; sizing runs from a body measurement through a fit formula, not a direct inch-for-inch match. Use the size chart on each style rather than a tape measure alone.