A sister size is a different band-and-cup combination that holds the same cup volume. Go down a band and up a cup, or up a band and down a cup — so a 34C's sister sizes are 32D and 36B. It is the most useful trick in bra fitting, and it is how you find something that fits when your exact size isn't available.
This guide is part of our complete guide to bras: fit, sizing and styles.
What is a sister size?
Two sizes are sisters when the cup holds the same amount, even though the letter is different. This works because the cup letter is measured relative to the band, not in absolute terms.
If you shrink the band by one size, the cup has to go up a letter to hold the same volume. If you widen the band, the cup goes down. For why this is, see bra cup sizes explained.
How do you work out your sister size?
Move the band one step and the cup one letter in the opposite direction. That is the whole rule.
- Band down, cup up: 34C becomes 32D
- Band up, cup down: 34C becomes 36B
More examples: a 32B's sisters are 30C and 34A. A 34D's sisters are 32DD and 36C. A 36B's sisters are 34C and 38A.
Sister sizes in Pakistani numeric labels
The rule is the same on numeric labels — you just convert the band first. The numeric band moves in steps of five where the UK band moves in twos:
- 32 = 70, 34 = 75, 36 = 80, 38 = 85, 40 = 90, 42 = 95
So a 75C (a 34C) has sisters at 70D (32D) and 80B (36B). An 80C (36C) has sisters at 75D and 85B.
This matters more in Pakistan than elsewhere, because local ranges are often narrower — so the size you measure as may simply not be stocked, while its sister is. Full conversion chart: bra size chart for Pakistan.
When should you use a sister size?
Use it when the cup fits but the band doesn't, or when your exact size isn't stocked. Three common situations:
- The band rides up or feels loose: go down a band and up a cup
- The band digs in and feels tight: go up a band and down a cup
- Your size is sold out: a sister size is a genuine fit, not a compromise
If the band is riding up at the back, see how to stop your bra riding up.
Is a sister size a real fit or a workaround?
It is a real fit — the cup volume is identical. But it is not infinitely flexible, and that is worth being honest about.
Moving one step either way usually works well. Moving two or more steps changes the shape of the cup and the position of the straps noticeably, so the bra may fit in volume while sitting oddly. Stick to one step where you can.
The band also has a job of its own: it does most of the supporting. A sister size that leaves the band too loose will not support you properly, however well the cup fits.
How does this help with Erblis sizes?
We stock a focused size range, so sister sizing is genuinely useful here. Being straightforward about what that range is:
- Blossom Beauty (wireless): 32B, 32C, 32D, 34B, 34C, 34D — that is 70B–70D and 75B–75D in numeric sizing
- Radiant Allure (convertible push-up): 32A, 32B, 32C, 34B, 34C
- Opulent Allure (front-clasp): S, M, L, XL
So if you measure as a 30D, your sister size 32C is stocked. If you measure as a 36B, your sister size 34C is stocked. And if you are outside the range altogether, we would rather tell you than sell you something that won't fit — our brand comparison says which Pakistani brands carry wider ranges.
Frequently asked questions
What are the sister sizes of a 34B?
32C and 36A. Going down a band means going up a cup; going up a band means going down a cup.
What are the sister sizes of 75B?
75B is a 34B, so its sisters are 70C (32C) and 80A (36A). Convert the band first, then apply the same rule.
Can I just buy a sister size instead of my own?
Yes, if the band still fits firmly. The cup volume is the same, so it is a genuine fit — but the band must stay snug, because it does most of the support.
How many sister sizes can I go?
One step in either direction is reliable. Two or more changes the cup shape and strap position enough that the fit usually suffers.
Does Erblis deliver bras in Karachi?
Yes, with free delivery across Karachi, Cash on Delivery, and a free tote bag with every order.
The short version
Band down, cup up. Band up, cup down. One step either way keeps the same cup volume, which makes sister sizing the simplest way to find a fit when your exact size isn't on the shelf — especially in Pakistan, where ranges are narrow. Use code WELCOME10 for 10% off your first order.