Bra Sister Sizes: What They Are and When Switching Helps

The bra you want is sold out in your size, but the rail has one in a size you have never worn. Or the cups on your favorite bra are perfect while the band has stretched and rides up. Both situations have the same answer, and it is one of the most useful ideas in bra fitting: the sister size. In this sister sizes bra guide I will explain why a 34C and a 32D are closer relatives than a 34C and a 34D, how to work out your own sister sizes in a few seconds, and, just as important, when switching to one will not fix anything.

What a sister size is

A sister size is a different bra size with roughly the same cup volume on a different band. You get there with one simple rule:

• Go down one band size, go up one cup letter. 34C becomes 32D.
• Go up one band size, go down one cup letter. 34C becomes 36B.

The reason this works is that cup letters are relative, not absolute. A cup letter describes the difference between the bust and the band, so the same letter holds more on a bigger band. When you make the band smaller, the cup that holds the same volume needs a higher letter. If you have read my guide to the bra size chart, this is the same idea seen from another angle.

The practical upshot: a 34C, a 32D and a 36B have cups of about the same volume. What changes between them is the band, and with it how firmly the bra is anchored.

A chart of common sister size families

Each row below is one family. Read across a row and the cup volume stays roughly the same while the band changes. Sizes here use US letters; in UK sizing the letters after DD continue as E, F, FF and so on, but the rule is identical.

To find your own family without a chart, start from your size and step diagonally: one band down and one letter up, or one band up and one letter down. Remember to count the double letters as full steps. After D in US sizing comes DD, then DDD; in UK sizing after D comes DD, then E.

When switching to a sister size helps

The band is too loose but the cups fit

This is the classic case. The band rides up at the back or you have to fasten it on the tightest hook, yet the cups are smooth and comfortable. Going down one band size firms up the band. If you went down the band and kept the same letter, the cups would shrink, so you go up a letter to keep them the same. A 36C with a loose band becomes a 34D.

The band is too tight but the cups fit

The reverse. The band digs in or leaves deep marks even on the loosest hook, while the cups are fine. Go up one band and down one letter: a 32D becomes a 34C. Before you do, though, check that the band is genuinely too small and not just new. Bands relax with wear, and a band that feels firm on day one may be right by week three.

Your usual size is out of stock

If the cut and style suit you, a sister size is a reasonable thing to try on. Expect the band to feel firmer or looser than you are used to, and decide whether you can live with that. One step usually is tolerable; more than one usually is not.

You fall between two band sizes

Bands come in even numbers, so a measurement in between leaves you with a choice. Trying the two sister sizes on either side, such as 32D and 34C, tells you which band suits you, since the cups should feel similar in both.

Different makers and styles run differently

Some patterns run tight in the band, some loose. If you know a particular style runs small, a sister size up in the band can be the right size in that bra even if it is not your usual label.

When a sister size will not fix the problem

Sister sizing solves band problems. It is much less useful for anything else, and it is easy to reach for it when the real issue lies elsewhere.

• The cups are the wrong shape, not the wrong size. If the top of the cup gaps while the bottom is full, or the cup is too shallow or too tall, a sister size has the same cup shape and the same problem. A different style is the fix; the bra styles section covers how cup shapes differ.
• Both the band and the cups are wrong. Then you need a different size, not a sister size. Re-measuring is the quickest way to see where you are.
• The wires are the issue. Wire width and length change with the size. Moving to a sister size can make wires sit wider or narrower, which may help or make things worse.
• The straps are digging. Usually that is the band failing to carry its share of the weight. A sister size with a firmer band can help, but loosening the straps and checking the band fit comes first.

An example, step by step

Here is a very common situation. Someone has worn 36B for years. The bra feels fine when she puts it on, but by lunchtime the band has crept up her back and she keeps pulling the straps back onto her shoulders. The cups, on the other hand, never wrinkle or overflow.

Everything points to the band. It is too loose to anchor the bra, so the straps take over and slide. The sister size down is 34C: a firmer band with the same cup volume. She tries it on, fastens it on the loosest hook, and the band stays level through the day. If the 34C cups had felt too small, the next try would be 34D, a change of cup on the new band rather than another sister size.

Notice what she did not do: tighten the straps further or go straight to the tightest hook on a stretched band. Both just move the problem around.

The limits of the rule

Sister sizing is a rule of thumb, and it gets less accurate the further you travel along a row. A few things to keep in mind:

• Stay within one step. 34C to 32D or 36B is a useful move. 34C to 30DD is technically in the same family, but the band is now two sizes tighter, and the cup and wire proportions have shifted too.
• The cup is not literally identical. Sister sizes hold roughly the same volume, but the cup sits on a different frame. The wires get wider or narrower, the straps may be placed differently, and the center panel may change width. Many people find a sister size feels close but not quite the same.
• Makers grade sizes differently. Each pattern is scaled from size to size by its own maker, so sister sizes in one line may be closer than in another.
• The band is still the foundation. Sister sizing does not change the basic rule that a firm, level band does most of the supporting. Do not use a sister size up to make a loose band “more comfortable” if it means the bra no longer stays put.

A quick way to use sister sizes in a fitting room

1. Start with your measured size.
2. Judge the band first: level, firm, comfortable on the loosest hook.
3. If only the band is wrong, try the sister size in the direction you need.
4. If the cups are wrong too, change the cup letter on your preferred band instead.
5. If the size seems right but the fit still is not, change the style before changing the size again.

Working this way means you change one variable at a time, so you can tell what each change actually did. It is slower than grabbing a handful of sizes, but you come out knowing why a bra fits rather than just that it does.

If you have never measured yourself, that is still the best first step. My guide on how to measure bra size walks through it.