Sister Sizes Explained

Last reviewed on 28 April 2026.

Walk into a bra fitting in any country and you will eventually hear the phrase "try a sister size." The fitter pulls a different size off the rack — usually one band number bigger or smaller, with a cup letter moved in the opposite direction — and you put it on, and somehow it fits better than the size you came in wearing. Sister sizes look like a sleight of hand the first time you encounter them, but the logic is straightforward once you can see what each part of a bra is doing.

A bra size is two measurements, not one

A label like "34C" packs two independent things into one tag. The number — 34 — describes the band, the loop of fabric and elastic that wraps around your ribcage. The letter — C — describes the cup, which is everything the band has to hold up.

Cup letters are not absolute. They describe the difference between your full bust and your underbust, with one inch of difference equating to roughly one cup letter (A is about one inch, B is about two, C is about three, and so on). That word "difference" matters more than people think. A C cup on a 32-inch band is not the same physical cup as a C cup on a 38-inch band — the larger band has more body underneath it, so the C cup that sits on it has to be physically larger to give the same proportional shape.

The label tries to communicate proportional fit, not absolute volume. Once you internalise that, sister sizes start to look obvious.

What sister sizes share

A sister size is a size with the same cup volume as another size, expressed against a different band. The mechanical rule is simple: drop one band size and add one cup letter, or add one band size and drop one cup letter. Either move keeps the cup volume the same.

So 32D, 34C, and 36B are all in the same volume family. The cup that sits on each of those bands has to hold roughly the same amount of breast tissue. What changes is the band tension and the position of the wires.

The clearest way to see the relationship is in the sister-size matrix on the home page, which lays out every band-and-cup combination so each row shares a cup volume.

When sister sizes are useful

You almost never reach for a sister size at random. They solve a specific class of fit problem: the cup feels right, but the band feels wrong.

• Band rides up across the back. A band that rides up is too loose. Drop one band size and add one cup. Example: 36C rides up — try 34D.
• Band feels too tight to clip. A band that cannot close comfortably is too small. Add one band size and drop one cup. Example: 34C feels strangling — try 36B.
• Same brand, different style, wrong band. Manufacturers use different elastic tensions in different cuts. The cup is engineered for the size on the label, but the band can run small or large. Sister sizing across one step in either direction is usually enough to compensate.
• Bands stretch with wear. The same bra at six months old has a looser band than it did new. Sizing down the band — and sizing the cup up to keep volume constant — sometimes restores the fit a little, although a stretched-out bra eventually has to be replaced.

When sister sizes are not the answer

Sister sizes assume the cup volume is right and only the band is off. Plenty of fit problems are not band problems:

• Cup gapping at the top. The cup is too large or the wrong shape. Sister-sizing across will not fix it because the cup volume stays the same. Drop a cup size, or change to a demi-cup or balconette shape that does not extend over the top of the breast.
• Centre piece (gore) not lying flat. The cup is too small, almost always. Going up a cup size — not sister-sizing across — is what brings the gore back to the chest.
• Underwire on tissue, not behind it. The cup is too small, the wire shape is wrong, or both. Sister sizes will not move the wire; only a different cup size or wire profile will.

The general principle: if the cup is wrong, change the cup. If the band is wrong but the cup is fine, sister-size in the appropriate direction. The fit test on the home page is a faster way to walk through the visible symptoms and arrive at the right move.

Worked example: 34C in three directions

Take a wearer who measures into 34C with the BraCalc calculator but has problems with their current 34C bra:

• Band creeps up by lunchtime. The band is too loose. Drop to 32, add a cup: 32D. The cup volume is identical to 34C, but the band is firmer and stays put.
• Band leaves red marks and is hard to clip. The band is too tight. Add a band, drop a cup: 36B. Same cup volume, gentler band.
• Cup is half-empty at the top, especially when leaning forward. The cup is too large or the wrong shape, not a band problem. Sister sizes will not help; try 34B or change to a balconette cut in 34C.

Sister sizes solve the first two cases cleanly. The third case is a cup problem, and that is where many people get stuck — they sister-size across forever and never address the actual issue.

A common mistake to avoid

Sister sizing in only one direction. People who have been wearing a too-loose band for years tend to know exactly which way to step, but people whose first bra in adulthood happened to be slightly too tight often spend years in too-loose bands because they assume the band is "supposed" to feel tight. Both directions of sister-sizing are valid; what matters is which direction matches the symptom.

If you are starting from a calculator result, treat the result as the centre size. The BraCalc calculator already shows two sister sizes alongside the main result for exactly this reason — start at the middle, then step left or right depending on how the band feels, not on what feels familiar from before.

Where this all comes from

Sister sizing is a working approximation, not a measured constant. Cup volume in real bras is a function of the cup pattern, the materials used, and the choice of wire shape — not just the label. Two bras in the same nominal sister-size family from different manufacturers will not be identically interchangeable. What sister-sizing reliably gets you is a sensible second size to try when the first one is wrong in a specific way, which is usually enough to find a fit that works.

For the broader picture of how the band and the cup share the load — and why the band is the part that does most of the work — see Bra band vs cup. To see exactly which sizes share which volumes, the home-page sister-size matrix is the single most useful reference.