38D Sister Sizes

The sizes in bold are the ones to use. The outer two share the cup volume in theory, but are proportioned for a noticeably different frame.

When to choose 36DD

Choose 36DD if the 38 band rides up or needs the tightest hook. This is the usual direction, as larger bands tend to be worn too loose.

When to choose 40C

Choose 40C if the 38 band is too tight and the cups are right.

What a 38D measures

All three sizes are designed around a bust of about 42 inches. The difference is how much of that total is ribcage and how much is breast, which is why the band changes and the cup letter moves the other way.

At this band, a firm fit makes a big difference to comfort, because the band is carrying more weight. Wider bands with three hooks hold their position better.

If neither sister size works

Sister sizes only help when the cup volume is already right. If you spill out of a 38D or the cups gape, keep the 38 band and change the cup letter. If you are unsure which problem you have, when to sister size up or down will help you decide, and the calculator will give you a fresh starting size from your measurements.

Frequently asked questions

What is the sister size of a 38D?

36DD on a firmer band and 40C on a looser band. Both have the same cup volume as a 38D.

Is a 36DD the same as a 38D?

The cups hold the same volume. The 36DD has a band one size smaller, so it fits more firmly around the ribcage.

Can I wear a 42B if I am a 38D?

That is two steps away. The cup volume is the same on paper, but the wires, straps and proportions are made for a larger frame, and the band will be four inches longer. It is unlikely to support you well.