Sister Size Calculator UK – Find Equivalent Bra Sizes Easily

If your bra cups fit well but the band feels too tight or too loose, sister sizes can help.

A sister size is an alternative bra size that keeps the same cup volume while changing the band size. This allows you to adjust comfort without guessing a completely new size.

Use this sister size calculator to quickly find equivalent sizes based on the bra size you already know.

What Are Sister Sizes?

Sister sizes are bra sizes that share the same cup volume but have different band and cup combinations.

When you:

• Go up one band size, you go down one cup size
• Go down one band size, you go up one cup size

This keeps the cup volume similar while changing how the band fits around your body.

When Should You Use a Sister Size?

This calculator is useful if:

• Your band feels too tight but the cups fit well
• Your band feels loose and rides up your back
• You’re between two sizes
• A bra feels close to fitting, but not quite right

Sister sizes are a fit adjustment, not a replacement for measuring your bra size.

If you don’t know your starting size yet, use the Bra Size Calculator first.

How This Sister Size Calculator Works

1. Enter your current or calculated UK bra size
2. The calculator finds sizes with the same cup volume
3. You’ll see alternative band and cup combinations to try

The results help you fine-tune comfort while keeping proper support.

Example of Sister Sizes

If your bra size is 34F:

• A tighter band option would be 32FF
• A looser band option would be 36E

All three sizes hold a similar cup volume, but feel different around the ribcage.

Important Things to Know

• Sister sizes are a starting point, not a guarantee
• Fit can vary by brand, fabric, and bra style
• The band should feel firm and level
• Straps should support, not carry weight

If multiple sister sizes feel uncomfortable, your original size may need re-measuring.

Sister Sizes vs Measuring Again

Sister sizes help when:

• You’re close to the right size
• One part of the bra feels off

They don’t replace proper measuring.

If your bra feels wrong in multiple ways (band, cups, straps), it’s best to measure again using a full bra size calculator.

Related Fit Help

If you need more guidance:

• Learn why sister sizes matters for support
• Understand common bra fit problems
• Explore bra styles that suit different breast shapes

These guides explain why fit issues happen and how to fix them.

Quick Reminder

A sister size:

• Adjusts band comfort
• Keeps cup volume similar
• Helps avoid trial-and-error shopping

Comfort always matters more than the label.