Bra Sister Sizes Explained: A Practical Reference Chart
A bra can feel right in the cups yet too tight or loose around your ribs. Bra sister sizes offer a useful next size to try: they change the band size while keeping cup volume approximately similar. The key is adjusting both parts of the label together—and knowing when a different cup size or bra shape would help more.
How bra sister sizes work
A cup letter does not describe a fixed volume. Within the same sizing range, a 36C generally has larger cups than a 34C.
Sister sizing compensates for that difference:
- Go down one band size and up one cup step for a shorter band with similar cup capacity.
- Go up one band size and down one cup step for a longer band with similar cup capacity.
For example, 32D, 34C and 36B are sister sizes. Their cups have approximately comparable capacity, but their bands fit differently.
“Similar volume” does not mean identical construction. Cup width, depth, wire shape and strap placement may change between sister sizes. Treat the relationship as a guide for trying on bras, rather than a guarantee of fit.
Bra sister size chart: UK sizing
Every size in this chart uses UK labels. Read across a single row to find the nearest sister sizes. The middle column is your starting size; the other columns offer a shorter or longer band with approximately similar cup volume.
| Starting size — UK | Shorter band sister — UK | Longer band sister — UK |
|---|---|---|
| 30D | 28DD | 32C |
| 30F | 28FF | 32E |
| 32B | 30C | 34A |
| 32D | 30DD | 34C |
| 32DD | 30E | 34D |
| 34B | 32C | 36A |
| 34C | 32D | 36B |
| 34D | 32DD | 36C |
| 34DD | 32E | 36D |
| 36D | 34DD | 38C |
| 36F | 34FF | 38E |
| 38G | 36GG | 40FF |
The UK cup sequence used here is:
A → B → C → D → DD → E → F → FF → G → GG → H
A cup “step” means the next entry in that sequence. Moving up from D therefore means DD; moving up from F means FF.
Start with the nearest sister size. Although you can continue the calculation across several bands, each move changes the fit around your torso. A size several steps away may have similar nominal cup capacity but an unsuitable band.
Check the sizing system before using the chart
UK, US and European labels are not interchangeable. Differences become particularly noticeable beyond D, as shown in Wikipedia’s comparison of international bra sizing systems.
- UK: Bands commonly run 30, 32, 34, 36 and so on, with cup steps including D, DD, E, F and FF.
- US: Band numbers often resemble UK labels, but cup sequences vary by brand. Some use DD and DDD; others use E and F or additional combinations.
- EU: Bands commonly run 65, 70, 75, 80 and so on. Many ranges use consecutive cup letters, but their sizing should be checked separately.
Within a conventional EU range, 70D, 75C and 80B are commonly treated as sister sizes. This is an example within EU sizing, not a conversion of the UK chart.
When shopping internationally, first identify the system used for the product. Convert your starting size using that brand’s guidance, then find sister sizes within its own cup sequence. A retailer’s location alone does not tell you which labels it uses.
When sister sizing is useful
Sister sizing is most useful when the cups already fit reasonably well and the band is the main problem.
Suppose a UK 34C contains your breast tissue comfortably, but its band shifts during movement. Trying 32D lets you assess a firmer band without deliberately reducing cup capacity. If that same 34C feels restrictive around your ribs while the cups fit, 36B is a reasonable comparison.
It can also help when trying a style with unusually firm or stretchy fabric. Keep your usual size as a reference and compare the actual garments.
If your size is sold out, a sister size may be worth trying, but availability should not decide the fit. Reject an alternative that slips, presses painfully or needs constant adjustment.
Fit problems sister sizing cannot resolve
Cups that need more or less volume
If the band fits but breast tissue spills over the cups, try a larger cup on the same band. In UK sizing, that might mean moving from 34D to 34DD.
Moving from 34D to its sister size 36C keeps cup capacity approximately similar, so it does not provide the intended increase.
Likewise, persistent empty space may call for a smaller cup or a different cup shape. Gaping alone does not prove that the cup is too large.
A mismatch in cup or wire shape
Two cups can have similar capacity while distributing that space differently. One may be wider and shallower; another narrower and deeper.
If wires sit on breast tissue, cup edges cut in despite space elsewhere, or a moulded cup repeatedly gaps, compare another style. Changing band and cup labels cannot reliably correct the underlying shape mismatch.
Different needs on each side
Sister sizing changes both cups together. It cannot independently accommodate breasts of different sizes.
Try fitting the fuller side comfortably, then assess whether stretch fabric, separate strap adjustments or a removable insert helps the other side.
Check the band before choosing a sister size
Cups that are too small or shallow can make a band feel tighter than it really is. To compare, fasten the bra with the cups hanging down your back, clear of your breasts. Assess the band at its normal height. If it feels noticeably more comfortable this way, investigate cup size or shape before increasing the band.
Then try the bra normally, settle your breast tissue into the cups and adjust the straps. Raise your arms, sit down and take a comfortable breath. Check for shifting, spilling, pressure and empty space.
For a new bra, assess the fit on the loosest hooks so there is room to tighten it as the elastic wears. Record the brand, style and size that works: that combination is a more useful shopping reference than a size label alone.