Bra Sister Sizes Chart

Bra sister sizes are nearby band and cup combinations with approximately similar cup volume. To move down one band, move up one cup. To move up one band, move down one cup. Sister sizes can help when cup volume feels close but the band is too firm or too loose.

What Are Bra Sister Sizes?

Sister sizes are bra sizes that use different band numbers and cup letters while preserving approximately similar cup volume.

They are most useful when the cup volume feels close but the band tension needs adjustment.

Why Sister Sizes Work

Cup volume changes with the band. A D cup on a 34 band has more volume than a D cup on a 32 band.

Changing the cup letter in the opposite direction helps compensate for the band change.

General Bra Sister Sizes Chart

Cup labels differ by regional system and brand. Confirm the retailer's progression before applying a chart.

Example: Sister Sizes for 32D

The smaller-band sister size is approximately 30DD. The larger-band sister size is approximately 34C.

Example: Sister Sizes for 34DD

The smaller-band sister is approximately 32F in many US systems or 32E in UK sizing.

The larger-band sister is approximately 36D.

Example: Sister Sizes for 36C

The smaller-band sister is approximately 34D, and the larger-band sister is approximately 38B.

When to Try a Smaller-Band Sister Size

Try a smaller band when the current band rides upward, shifts during movement, or provides little support.

Move up one cup to preserve similar volume.

When to Try a Larger-Band Sister Size

Try a larger band when the current band causes pain, prevents normal breathing, or cannot close without excessive strain.

Move down one cup to preserve similar volume.

When Sister Sizing Can Help

• The cup volume feels close but the band is too firm.
• The cup volume feels close but the band rides upward.
• The preferred style is unavailable in the starting size.
• A brand's bands run unusually firm or stretchy.
• You need a temporary comparison during fitting.

When Sister Sizing Will Not Solve the Problem

• The cup is clearly too small or too large.
• The wire width is wrong.
• The cup is too shallow or too projected.
• The center gore rests on breast tissue.
• The cup height interferes with the arm.
• The bra style does not match breast shape.

Sister Sizes Are Not Identical

A smaller-band sister size generally uses a narrower frame and firmer band. A larger-band sister generally uses a wider frame and looser band.

Cup width, wire shape, strap spacing, and cup height may change.

How Wire Width Can Change

A larger-band sister size may use wider wires even when approximate cup volume remains similar.

This may work better for a wider breast root but worse for a narrow root.

How Strap Placement Can Change

Larger bands can place straps farther apart. Smaller bands can place them closer together.

Shoulder width and cup construction affect whether straps remain secure.

How Cup Depth Can Change

Sister sizes may distribute similar volume differently across width and depth.

A sister size can therefore feel shallower or wider than the starting size.

Do Not Sister Size Several Bands Away

One band step is usually the most useful comparison.

Moving two or three band sizes changes the bra frame, strap placement, wire width, and overall proportions substantially.

Sister Sizing in US and UK Systems

The principle is the same, but cup letters after D differ.

Convert the cup progression correctly before choosing the next cup step.

Sister Sizing in EU Systems

EU bands commonly change in five-centimeter steps, such as 70, 75, and 80.

Moving down one EU band generally requires moving up one cup.

Sister Sizing and Band Extenders

A non-elastic extender can help when the cups fit correctly but a new band is slightly firm.

It does not correct an oversized or undersized cup.

How to Test a Sister Size

1. Fasten the bra on the loosest usable hooks.
2. Check whether the band stays level.
3. Place all breast tissue inside the cups.
4. Check the center gore and underwires.
5. Check cup edges for spilling or gaping.
6. Move the arms, sit, bend, and breathe normally.
7. Compare support with the original size.

Common Sister-Sizing Mistakes

• Changing the band without changing the cup
• Moving the cup in the wrong direction
• Assuming sister sizes are identical
• Moving several band sizes away
• Ignoring cup shape and wire width
• Using sister sizing to solve every fit problem
• Mixing US and UK cup progressions
• Choosing a loose band only because it feels easier to fasten