How UK Bra Sizes Work (Band + Cup)

UK bra sizes are built from two parts: a band size (the number) and a cup size (the letter). Understanding how these two work together makes it much easier to measure correctly, interpret your result, and fix fit problems.

If you want to calculate your size now, go to the UK Bra Size Calculator

The short version

  • The band size comes from your underbust measurement (around your ribcage).
  • The cup size comes from the difference between your bust and your band.
  • Your full UK bra size combines both, for example: 34D, 36C, 38F, etc.
  • Cup letters are relative to the band size (a 34D is not the same volume as a 38D).

What the band size means (the number)

The band size is the number part of your bra size (for example: 30, 32, 34, 36, 38).

It represents:

  • The circumference of your ribcage just under your bust
  • The main source of support in a bra (not the straps)

How it’s determined

You measure snugly around your ribcage (underbust).
That measurement is then rounded to the nearest standard UK band size.

Examples:

  • Around 31–32 inches → usually a 32 band
  • Around 33–34 inches → usually a 34 band
  • Around 35–36 inches → usually a 36 band

A correctly fitted band should:

  • Sit level all the way around
  • Feel firm but comfortable
  • Stay in place without riding up

If you haven’t measured yet, see: How to Measure Your Bra Size (UK)

What the cup size means (the letter)

The cup size is the letter part of your bra size (A, B, C, D, DD, E, F, etc. in the UK system).

It does not mean:

  • “Big” or “small” on its own
  • The same volume in every band size

It does mean:

  • The difference between your bust measurement and your band measurement

How cup size is calculated

In simple terms:

  • Measure your bust at the fullest part
  • Measure your band (underbust)
  • The difference between these two numbers determines the cup letter using UK size steps

Larger difference = larger cup letter.

Very important: Cup size is relative to the band

This is one of the most misunderstood parts of bra sizing.

A 34D and a 38D are not the same cup volume.
When the band changes, the cup volume changes too.

That’s why:

  • A 34D has less cup volume than a 36D
  • A 36C can have a similar cup volume to a 34D

This is also why sister sizes exist.

Learn more here:
Sister Sizes Explained (UK)

How the full UK bra size is formed

Your full size is simply:

Band size + Cup size = Your UK bra size

Examples:

  • 32C = 32 band + C cup
  • 34DD = 34 band + DD cup
  • 38F = 38 band + F cup

The band controls support and stability.
The cup controls volume and coverage.

Both must be right for a bra to fit properly.

Why two people with the same cup letter can look different

Because:

  • The band size changes the actual cup volume
  • Body shape and breast shape vary
  • Bra styles and cuts change how volume is distributed

So:

  • A 30D and a 38D are very different sizes in reality
  • The letter alone never tells the full story

How this relates to the calculator

Our calculator:

  • Uses your underbust to find the correct band size
  • Uses your bust measurement to calculate the cup size
  • Combines both using UK sizing logic
  • Shows sister sizes so you can fine-tune fit

To see the exact logic, read:
How This UK Bra Size Calculator Works

Common misunderstandings

  • “A D cup is always big” → Cup size depends on the band
  • “My band feels tight, so I need a bigger cup” → Often you need a bigger band (or a sister size)
  • “If the cups gape, the size is wrong” → Sometimes it’s a style or shape issue, not just size

For help with this, see:
Bra Fit Problems & How to Fix Them
Why Your Bra Size Feels Wrong

UK sizing vs other systems

The UK uses a different cup progression from some other regions (like the US or EU).
That’s why a size label can change when you shop internationally.

See the full guide here:
UK vs US vs EU Bra Sizes

Quick summary

  • UK bra sizes use band (number) + cup (letter)
  • The band comes from your underbust measurement
  • The cup comes from the difference between bust and band
  • Cup size is relative to band size, not absolute
  • Both parts must be right for good fit
  • Sister sizes exist because cup volume changes with band size

Ready to find yours? Go to the UK Bra Size Calculator