Sister Sizes, Explained Without the Maths Headache

Your size is sold out. Or it almost fits, but the band is a touch loose and the cup is fine. Or you have moved brands and suddenly nothing lines up.

Sister sizing is the small bit of knowledge that gets you out of all three, and almost nobody explains it properly.

The one thing to understand

A cup letter is not a fixed volume. It is a difference.

A D cup means your bust measures four inches more than your band. That is all it means. So a D cup on a 34 band holds noticeably less than a D cup on a 44 band, because the band underneath it is bigger.

Once that clicks, everything else follows.

How to find your sister size

Move one step in each direction, in opposite directions.

Band down, cup up. 36D becomes 34DD. Same volume, firmer band.

Band up, cup down. 36D becomes 38C. Same volume, looser band.

The cup letter changes but the actual cup size barely does. You are shifting where the volume sits, not how much there is.

When to go band down, cup up

This is the useful direction, and it is the fix for most fit complaints:

• Your band rides up your back
• Your straps dig in
• You can pull the band more than a couple of inches from your body
• You are already wearing it on the tightest hook

When to go band up, cup down

Less common, but real:

• The band genuinely hurts, not just feels firm
• You cannot fasten it on the loosest hook
• You are getting red marks around your ribcage that stay for hours
• You are pregnant, or your ribcage has changed for any reason

A quick reference

Starting at 38DD, the sister sizes going down are 36DDD and 34G. Going up they are 40D and 42C. Every one of those holds a similar volume. They just anchor differently.

The limits, honestly

Sister sizing is not magic. Go more than one or two steps away and the cup shape starts to fight you, because cups are drawn for a particular band width. A 34G and a 42C might be mathematically related but they are cut quite differently.

Use it as a nudge, not a leap.

Why it matters when you shop

Sizing is not standardised. A 36DD in one label can fit like a 34G in another. If you know your sister sizes, an inconsistent size chart stops being a wall and becomes a small adjustment.

It also means being out of stock in one size is not the end of the conversation.

If you tell us your current size and what it is doing, we can usually tell you which direction to move in.