What Is Sister Sizing? How to Find Your Bra When Your Size Is Sold Out

Found a bra you love, gone to buy it, and — your size is sold out. Here's the trick the shops rarely tell you: sister sizing lets you wear a different band-and-cup combination that holds the exact same cup volume. Your 34C has twins — a 32D and a 36B — that fit almost the same. When your size is out of stock, one of them usually isn't.

There are just two moves: go up a band and down a cup, or down a band and up a cup. Below is the chart, the two directions explained, and how to tell if the sister size actually fits.

What is sister sizing? (The simple version)

Think of a recipe that serves the same number of people in a wider, shallower dish or a narrower, deeper one — same food, just a different vessel. Sister sizing is that, for bras: the same cup volume carried on a different band.

Here's why it works. A cup letter isn't a fixed amount — it's relative to its band. So as the band number goes up, the same letter holds more, and as the band goes down, it holds less. To keep the volume identical, you trade one for the other: band up, cup down — or band down, cup up. The 32D, 34C, and 36B all cradle the same amount of tissue; only the band length (and the wire width) changes. (Bra sizing is built on this band-and-cup ratio.)

Sister size chart: your size's "sisters"

Find your size, then step sideways. One step in either direction is your sister size:

Smaller band ←	Your size	→ Larger band
30D	32C	34B
32D	34C	36B
34D	36C	38B
30DD	32D	34C
32DD	34D	36C
34F	36E	38D

The pattern never changes: move one band size down and one cup letter up to go left (firmer band), or one band size up and one cup letter down to go right (roomier band). The cup volume stays put the whole way across the row.

A quick worked example. Say you're a 34C and you've found the perfect bra — but only 32 and 36 bands are left. You don't go home empty-handed. The 34C's sisters are 32D (firmer band) and 36B (roomier band). If your band tends to ride up, grab the 32D; if 34 bands usually feel a touch tight, grab the 36B. Either way the cups hold the same as your 34C. One size just became three options on the shelf.

When to go up a band and down a cup

Use this when the cups fit but the band feels too tight — it digs in, you can't take a full breath, or you can't fasten it on a comfortable hook.

The move: go up a band, down a cup. A 34C becomes a 36B. You get a roomier band, and the B-on-a-36 holds the same as the C-on-a-34. Same cups, more breathing room.

This is also the one to reach for when your size is sold out and only the larger band is on the shelf. One caution: as the band gets roomier, the straps carry a little more of the load, so check that they aren't digging in — if they are, you've gone a step too far in this direction.

When to go down a band and up a cup

Use this when the cups fit but the band is too loose — it rides up your back, or it only feels supportive on the tightest hook. (A loose band is the most common fit mistake there is, so this direction gets a lot of use. If that's your problem, our guide on the band riding up your back goes deeper.)

The move: go down a band, up a cup. A 34C becomes a 32D. The band grips your ribs properly, and the D-on-a-32 holds the same as the C-on-a-34. Same cups, firmer anchor.

The same trick rescues a cup that's too small: if you spill over the top, sister sizing up the cup while dropping the band keeps things snug. (More on that in our guide to breast spillage over the cup.)

How to tell if the sister size actually fits

Same volume on paper doesn't always mean perfect on your body — the wire width and band feel shift slightly. So check it the normal way:

• Band: snug and level all the way around, two fingers underneath with light tension, no riding up.
• Cups: smooth — no spillage over the top or sides, no gaping or wrinkling.
• Centre gore: the bit between the cups lies flat against your breastbone.
• Wires: sit in the crease under your breast, not on tissue or out toward your arm.

If all four pass, the sister size is a keeper. If the cups now gape or spill, the volume was never quite right to begin with — and that's a measuring job, not a sister-sizing one.

One thing that genuinely changes is the wire width. A smaller band comes with a narrower wire, a larger band with a wider one — so even though the volume matches, the cup can feel a touch deeper-and-narrower or wider-and-shallower than you're used to. Give a new bra a full day before you judge it; the band will also relax slightly with wear. If after that the wires still sit on your breast tissue instead of in the crease beside it, the width is wrong for your frame, and you've stepped too far from your true size. That single feeling — wires off the ribcage — is the clearest signal to step back the other way.

Where sister sizing stops working

Be honest about its limits — this is a small adjustment, not a magic wand:

• Only go one or two steps. Each step shifts the wire width and cup proportions a little; go too far and the bra simply won't sit right.
• It won't fix a cup you already dislike. If the cup gapes or spills at your true size, a sister size carries the same problem along.
• It's not for big body changes. After weight change, pregnancy, or nursing, re-measure for your real size instead of sister-sizing around it.

In plain terms: sister sizing solves availability, not a genuine fit problem.

Where to find your size

This is exactly where sister sizing earns its keep, because local shelves are thin. A few honest pointers:

• Don't accept "we only have up to C" as your size. That's the shop's stock, not your body. Extended cups (D and up) are genuinely hard to find on local shelves.
• Shop online for range. Brand websites and online marketplaces carry far more sizes than most physical shops, and you can read the size chart before buying. Sister sizing gives you two or three extra sizes to search for, which dramatically improves your odds of finding something that fits.
• Search all your sisters at once. When you're scrolling listings, type each sister size into the search, not just your "main" one. A bra that's sold out in 34C may be sitting in stock in 32D — same cups, ready to ship. Keep a little note of your two sister sizes in your phone so you never have to do the maths at the counter.
• Lean on each brand's own chart. Different brands cut differently from each other and from UK/US labels, so your sister size is a starting point — confirm with the brand's chart and a try-on where you can. A free online bra size calculator is fine to double-check the maths, never the final word.

The bottom line

Sister sizing is the quiet superpower of bra shopping: the same cup volume in a different band, so a sold-out size or a slightly-off band is no longer a dead end. Cups fit but band too tight? Up a band, down a cup. Band too loose? Down a band, up a cup. Stay within a step or two, run the four fit checks, and you've turned one size into three.

NovellaFit has no shop and nothing to sell you, so the only thing we're pushing is the fit that works. New here? Meet the site on the about page, or work through the rest of our fit guides one problem at a time.