Bra Sister Sizes Explained: The Chart, and the Two Times It Won't Save You

The first time someone told me my sister size, I assumed it was a sales trick. I had walked into a shop wearing a 36B that rode halfway up my back, and I walked out in a 34C. Different letter, different number, and somehow the same cups.

It is not a trick. It is the single most useful idea in bra sizing, and it explains why the number on your label tells you so little on its own.

Why a cup letter means nothing by itself

A cup letter is not a volume. It is a difference: how much bigger your bust measures than your band. Each inch of difference adds a letter. So a C cup on a 30 band is a much smaller cup than a C cup on a 40 band, because the same three inches is wrapped around a smaller ribcage.

That has a consequence nobody mentions while you are shopping. Go down one band size and the cups on that band shrink too. To keep the same cup volume, you have to go up a letter. That pair, one band down and one cup up, is a sister size. It works the other way as well: one band up, one cup down.

The sister size chart

Read along a row. Every size in it holds roughly the same cup volume.

US and UK sizes agree up to DD, so this chart works in both. Above DD, count letters in your own system. The US runs DD, DDD, G and the UK runs DD, E, F, so the UK sister of a 34E is a 32F, not a 32DDD.

When to reach for a sister size

The rule is short. If the cups fit and the band does not, change the band and move the cup the other way.

If the band rides up your back and the cups are fine, the band is too loose: go down a band and up a cup. This is by far the most common case. A loose band hands its job to the straps, which is why a riding band and digging straps tend to arrive together.

If the band digs in or the centre gore rubs, and the cups are fine, the band is too tight: go up a band and down a cup.

And if your size is sold out, a sister size is the closest substitute. Online, it is often the difference between getting the style you wanted and not.

The two times a sister size won't save you

This is the part the charts leave out, and I think it is why sister sizing has a reputation for not working.

The first is when the cup is a problem as well as the band. If your cups gape or spill, a sister size keeps the same volume, which means it keeps the same cup problem. Fix the cup first, on your current band, and only then think about sisters.

The second is distance. Sister sizes are close to the same volume one step away. Two or three steps away, the cups change shape: the wires get wider and the cups shallower as the band grows, and the reverse as it shrinks. A 30F and a 36C sit on the same line of the chart, and they are not remotely the same bra. One step either side is a reliable swap. Past that, you are guessing.

There is a smaller third one, which is shape. A plunge and a full cup in the same size fit differently, because the style changes where the volume goes. A sister size fixes a band. It does not fix a style that is wrong for your shape.

Finding your starting size

All of this assumes you have a size to start from. If you are not sure yours is right, measure. Two numbers, a snug underbust and a bust at the fullest point, and our bra size calculator turns them into a starting size with both sister sizes listed beside it. Try the size and one sister, and let the band tell you which one wins.