Sister sizes – the magic trick of cup size

5 Apr 2019 10:13 AM

Annica Svensson

So how exactly do bra sizes really work? Even if you know your own exact bra size, you may still fit into another one or two sizes. You’ll find that these bras fit exactly or almost as well. But how can that be? Today, we are looking more closely at what all women should know – sister sizes.

You have probably heard that four out of five women wear the wrong size bra. When we say ‘wrong’ size, we mean a size that doesn’t fit at all, for example when the bust spills over the edge of the cup or the cup is half full. Sister sizes, however, are something completely different. That is when the wrong size is still the right size. Sounds complicated? It doesn’t need to be.

The magic relationship between cup and circumference

To sort out what sister sizes are, we first need to go through how the bra sizing system works. A bra has double sizes – the band size (measured directly under the bust and given as a number, for example 85) and the cup size (given as a letter, for example B-cup).

The cup is always proportional to the band, i.e. it changes according to the increase in circumference. A B-cup with a band size 95 does not have the same volume as a B-cup with a band size of 75. The fact is that if you fill the cup of a bra of size 90B with water, it will hold the same amount as a bra of size 75F!

C-cup is not the same in all sizes

Sister sizes mean that the volume of the cup is the same in several band sizes. For example, 85C has the same size cup as 80D and 75E.

This means that if you try a 75F-cup and think that the cup fits well but that it is too tight around the back, then you can try a bra with an 80 in band size. But instead of trying a 80 with an F-cup, choose a 80E instead. 75F and 80E have the same cup volume. It is only the band size that has changed. If you try a 80F the cup will be too big but the bra will fit you well around the back.

The table below shows the relationship – the orange fields have the same cup volume. The rule of thumb is simple – up one band size, down one cup size. Down one band size, up one cup size.

Sister sizes – when more than one bra size fits

This is what is called a sister size – 85C is the sister of 80D and 90B. Let’s say that you have size 85C. If the bra has an elastic back, you can probably also wear an 80D as it will stretch over time and adapt to your body. If you think that 80 D is too tight around the back you can simply increase the circumference by a few centimetres using a bra extender in the hook and eye fastening. You can thereby wear a 40B on the innermost fastening so that is a little tighter.

The rule of thumb is simple:
down one size in circumference,
up one cup size

When should you not wear sister sizes

So when should you not wear a sister size? If you need help to lift a heavy bust, it is not recommended to wear the ‘big sister’, i.e. the sister size with a bigger band. There is a risk that the band size will be too loose and not provide the support it should. It is more common to go down a sister size (85C to 80D) than to go up, for that exact reason.

The advantage of sister sizes

If, after a long time of searching, you finally find a bra that you feel you just have to have but it is not available in your size, then it is perfect to check if a sister size is available. Up one cup size, down one band size. Or the other way round. Try for yourself. It almost always works.