UK Bra Sister Sizes Chart & Guide | Find Your Perfect Fit

Have you ever tried on a bra where the cups fit perfectly, but the band was just a little too tight or too loose? It’s a frustratingly common problem, and it often leads to women giving up on what could have been the perfect bra.

But what if there was a simple trick to solve this? There is. Welcome to the world of sister sizing.

This guide will demystify one of the most useful concepts in bra fitting. Understanding how bra sister sizes work is a true game-changer, giving you the power and flexibility to fine-tune your fit and find comfort you never thought possible.

What Are Sister Sizes?

In the simplest terms: Sister sizes are different bra sizes that have the exact same cup volume.

Visually prove that three sister sizes have the exact same cup volume

The cup letter (like D, E, F) does not represent a fixed size. The actual volume of a “D” cup on a 32 band is much smaller than the volume of a “D” cup on a 38 band. Sister sizing is the system that explains this relationship.

The core rule is simple:

  • When you go UP one band size, you must go DOWN one cup letter to keep the same cup volume.
  • When you go DOWN one band size, you must go UP one cup letter to keep the same cup volume.

For example, the sizes 32DD, 34D, and 36C are all sister sizes. They all have the same cup volume, but are designed for different ribcage measurements.

The UK Sister Size Bra Chart

Use this chart to easily find your sister sizes. First, find your recommended size from our bra size calculator, then look left and right to find your alternatives.

Go Down a Band (Snugger Fit)

Your Starting Size

Go Up a Band (Looser Fit)

30B

32A

34AA

30C

32B

34A

30D

32C

34B

30DD

32D

34C

30E

32DD

34D

30F

32E

34DD

32C

34B

36A

32D

34C

36B

32DD

34D

36C

32E

34DD

36D

32F

34E

36DD

32FF

34F

36E

34D

36C

38B

34DD

36D

38C

34E

36DD

38D

34F

36E

38DD

34FF

36F

38E

34G

36FF

38F

36DD

38D

40C

36E

38DD

40D

36F

38E

40DD

36FF

38F

40E

36G

38FF

40F

36GG

38G

40FF

When Should You Use a Sister Size?

Sister sizing is a problem-solving tool. Here are the two most common scenarios where it becomes essential.

A simple, directional graphic that explains the rule of thumb of sister sizing.

Scenario 1: The cups fit perfectly, but the band is too loose.

This is a classic sign you need to sister size down. You’re likely experiencing problems like your band riding up your back or your straps digging in because the band isn’t providing enough support.

  • The Solution: Go down one band size and up one cup letter.
  • Example: If you are trying a 36D and the cups are perfect but the band is loose, your next size to try is a 34DD.

Scenario 2: The cups fit perfectly, but the band is too tight.

If the band feels uncomfortably snug, digging into your ribs even on the loosest hook, you should sister size up.

  • The Solution: Go up one band size and down one cup letter.
  • Example: If you are trying a 32E and the cups feel great but the band is far too tight, your next size to try is a 34DD.

A Quick Word of Caution

While sister sizes have the same cup volume, the shape of the cup and the width of the underwire can change slightly. A 32DD will have narrower-set wires than a 36C. Always check how the new size feels and make sure it solves your original fit problem without creating a new one. For more information, check our Ultimate Guide to Bra Fit Problems.

Conclusion

Understanding sister sizing is like having a secret superpower in the fitting room. It gives you the flexibility to prioritise a perfectly snug and supportive band—which provides 80% of a bra’s support—while keeping the cup volume you know works for you.

If you’re unsure of your starting size, the first step is always to get a reliable measurement. Use our accurate UK Bra Size Calculator to find the perfect starting point for your fitting journey.

Frequently Asked Questions

You don’t need a calculator! Just follow this simple rule: to keep the same cup volume, if you go down one band size, you must go up one cup letter. If you go up one band size, you go down one cup letter. For example, the sister sizes for a 34D are 32DD (down) and 36C (up).

Yes, they are. A 36C and a 34D have the exact same cup volume. The 34D is designed for a person with a smaller ribcage (a 34-inch band) but the same breast volume as someone who wears a 36C.

Possibly, but the overall fit will be different. While the cup volume is the same, the 36C will have a looser band. If you wear a 34D and the cups fit well but the band feels too tight, a 36C is the perfect size to try next. However, if your 34D band fits perfectly, a 36C will be too loose and won’t provide enough support.

In terms of cup volume, a 34D is equivalent to its sister sizes: 32DD and 36C. It is also the same as a US 34D and an EU 75D. You can see a full list in our International Bra Size Conversion Chart.

This is a very common question. While “big” is subjective, 34D is actually a very common and average bra size in the UK. Due to misleading portrayals in media, many people think of a “D” cup as being very large, but it often represents a much more average bust size than people expect. The most important thing is finding a size that is comfortable and supportive for you.