Bra Making: What is Sister Sizing?

Have you heard the term “sister sizing”? It’s a great term for explaining how bra sizes shift in band and cups. The basic idea is this: Sister sizing refers to the same cup but on a different size band. As a bra band size gets smaller, the same cup volume will have a bigger letter, and vice versa.

In other words, a B is not a B is not a B. A 36B is a full cup size larger than a 34B. So I’m not just “basically a C cup”. Nor is someone else “an F”.

The use of cup size in non-bra sewing patterns like blouses or dresses can make this a bit more confusing for sewists. Your cup size in dress patterns (like Simplicity’s Amazing Fit series, for instance) might be completely different from what you need in a bra. Dress patterns use the difference between the upper chest and full bust measurements to determine cup size, but bras are more dynamic in their sizing.

So how can sister sizing help you?

If you can’t find your size in a bra pattern, there’s a good chance the pattern has a “sister size”, and you can find the equivalent cup on another band, and simply alter the band to your preferred fit (a very easy adjustment!). For example a 28F on a 32 band would be a 32DD or 32E.

(Note: in the U.S. and UK “DD” is usually the cup size above “D”. There are other double letters, too, but most bra patterns use the normal alphabet. I personally find the normal alphabet less confusing!)

Obviously I didn’t fill out the chart with all the possible bra sizes and cup letters, but hopefully you get the idea!

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There are some exceptions to the sister sizing “rules”, both in lingerie brands and in home sewing patterns. These exceptions do what I call “jump grading”, where the cups on the same band size will jump by two cup sizes after D cups. I won’t go into details of how that works because it can get confusing unless you’re really into the nerdy ins and outs of bra grading. Suffice it to say, there are a few upscale brands that use this kind of size grading (Empreinte, Prima Donna) and Pin-up Girls patterns use this method as well.

Whatever size you start out with, there will always be a good chance that a bra pattern doesn’t fit the same way as your size in your favorite brand. That’s just the nature of the beast. There are many measurements that go into bra drafting, not just the bust and ribcage, and no measuring method is going to capture them perfectly. Before you try online bra calculators remember that you are making a bra, which is a bit different than buying one. Find a starting size by using the pattern’s measuring suggestions, and then go from there to make a “you size”!

Some tips for fitting a bra pattern:

1. Fit a wire before you do anything else. A good-fitting wire solves other fitting problems further down the road.

2. Try to stick with the same band pattern, tweaking it until it is firm enough for you, in the fabric you want to use.

3. You may end up needing to try different cup sizes. Sewing has a way of tuning our eye to fit issues we never noticed before and bras are no different. Nearly everyone I’ve fit has had to go up at least 1 cup sizes, usually two, from their “usual” size. And my top tip: it is better to start large in the cup and take in adjustments, than starting too small and trying to figure out where the room needs to be.

Have you had trouble fitting a bra pattern? I’ve got more pattern fitting posts in the pipeline so I’d love to hear what ails you.

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