The Guide to Sister sizes

How does the bra sizing system actually work? And why does it confuse almost everyone

Eight out of ten women wear a bra that doesn't fit properly. That's a well-documented fact, and it almost never comes down to the body being difficult to fit. It comes down to a sizing system that isn't intuitive, and that nobody has really explained.

This guide hopes to changes that. You'll learn why 80D and 75E are exactly the same volume, how to read your size correctly, and how to find a bra that fits, whatever the label says.

What is a sister size?

A bra size has two parts: a number (band size, the measurement around your ribcage) and a letter (cup size, the volume). What most people don't know is that these two parts are connected in a way the label doesn't show.

The cup letter isn't a fixed volume. It's a ratio — the difference in centimetres between your ribcage measurement and your bust measurement. That means a D cup on a 75 band is a smaller volume than a D cup on an 85 band, even though they share the same letter.

This is the most important thing to understand: when the band size changes, the cup letter has to compensate to keep the volume the same.

Sister sizes are sizes that share the same bust volume but have a different band and cup letter. They're interchangeable in volume, but differ in how the band fits around your body.

How the sizes connect

Every time you go up one band size (for example from 75 to 80), you go down one cup letter to keep the same volume. And the other way around: go down in band size and you go up one cup letter.

Sister sizes with the same bust volume:

70E

75D

80C

85B

All four sizes above hold exactly the same bust volume. The only difference is how snugly the band sits around your body.

Why does this matter to you?

Your "usual size" may not be your right size

The most common mistake when measuring for a bra is measuring too loosely under the bust. This gives you a band size that's too large, and to compensate, you end up in a cup that's too small. The result: your breasts spill out at the sides or upward, the band rides up at the back, and the bra feels uncomfortable without you knowing why.

Many women have worn a 80D their whole lives when they actually fit a 75E. The volume is the same. The feel is not.

Your size can shift without your body changing

Bra manufacturers differ in how they cut their patterns. An 80C from one brand can fit like a 75D from another. That's normal, and it's one reason why knowing your sister sizes is useful: when you shop a new brand, you know which sizes to try without starting from scratch.

The right volume in the wrong band still gives a poor result

Sister sizes don't solve everything. The volume is the same, but if the band is too loose you lose the firmness and the bra won't do what it should. The starting point is always to find the tightest band you're comfortable in. Adjust the cup letter from there.

Where do you start?

The simplest way is to measure yourself. You need a measuring tape and about two minutes.

Measure under the bust: Hold the tape snugly and horizontally, directly under your breasts. It should feel firm. This gives you your band size.

Measure over the bust: Measure over the fullest part of your breasts, without compressing the tissue. This gives you your bust measurement.

Calculate the difference: Each centimetre of difference between the two measurements corresponds to one cup letter (roughly: 10 cm = A, 12 cm = B, 14 cm = C, 16 cm = D, 18 cm = E, and so on).

You can also use our size calculator.

This gives you a starting point. Try your size, and if something doesn't feel right, try a sister size.

Ready to find your size?

Use our size calculator and we'll help you find the right starting point — including any sister sizes worth trying.

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