Last Updated: September 25, 2026
Table of Contents
• What Is Sister Sizing?
• How to Find Your Sister Size
• When Sister Sizing Works Well
• When It Doesn't Work
• Frequently Asked Questions
Finding your usual bra size sold out is genuinely frustrating - especially once you've already gone through the work of measuring and finding a fit you trust. Sister sizing is the trick many fitters reach for in exactly this situation, and it's worth understanding even if you never need it, since it can quietly widen your options the next time a size is unavailable.
What Is Sister Sizing?
A sister size is a different band-and-cup combination that gives roughly the same cup volume as your usual size. The relationship works like a see-saw: going up a band size while going down a cup size (or the reverse) keeps the overall cup volume close to the same, even though the numbers on the label look completely different.
Key Takeaway - A 34C and a 36B hold a very similar cup volume. The label changes, but the
actual space for your bust barely does.
How to Find Your Sister Size
The rule is simple once you know it: move one band size, and shift the cup letter one step in the opposite direction.
• To go up a band size: move the cup letter down one (e.g. 32D → 34C)
• To go down a band size: move the cup letter up one (e.g. 32D → 30E)
Your Usual Size Sister Size (Band Up) Sister Size (Band Down)
32C 34B 30D
34C 36B 32D
36C 38B 34D
34D 36C 32DD
36D 38C 34DD
Pro Tip - If your usual size is sold out, check the sister size with the band going up first. A
slightly looser band with a slightly smaller cup letter is often the easier of the two adjustments to get used to.
When Sister Sizing Works Well
Sister sizing is most reliable when you're staying within the same style and the same brand. Cup shape, wire length, and how a brand grades its patterns all stay consistent, so a sister size genuinely behaves like a close substitute rather than a guess.
It's also a useful trick when you're torn between two sizes at checkout, or when a specific colourway is only left in a size next to your usual one.
When It Doesn't Work
Sister sizing gets less reliable the further you stretch it. A single sister-size jump (one band size, one cup letter) is usually safe; going two steps in either direction starts to change the actual shape of the cup more than the volume calculation suggests, since real bodies aren't perfectly proportional the way the maths
implies.
It also doesn't carry across brands reliably - a 34C in one label can fit quite differently to a 34C in another, so a sister size calculated from one brand's chart won't necessarily translate to a different brand's stock.
Watch Out - Sister sizing is a starting point for trying something new, not a guaranteed fit. If you can, check reviews or reach out to us before ordering a sister size you haven't tried before, especially in a style with less adjustability (like a moulded or underwired cup).
Frequently Asked Questions
Is a sister size exactly the same fit as my usual size?
Not exactly - the cup volume is close, but band tightness and cup shape can feel slightly different. It's a good substitute, not an identical twin.
Can I use sister sizing across different brands?
It's less reliable across brands, since sizing charts and pattern grading vary. Sister sizing works best within the same brand and even the same style.
How far can I size up or down using sister sizing?
One step (one band size, one cup letter) tends to be the safest adjustment. Going further than that increases the chance the fit changes more than just the numbers suggest.
Struggling to find your size in stock, or unsure which sister size to try? Get in touch with our team - we're happy to help you find the right fit before you order.
Shop lingerie at Dressed 2 Digress - dressed2digress.co.uk/collections/lingerie-sets
