Sister Sizing and Underwires

If you read the previous post you should now know how to measure yourself for your bra and how to calculate your frame and cup size. Remember that your bra size is made up of 2 parts. Your frame size (32, 34, 36 etc) and your cup size (A,B,C,D,DD/E etc). The frame size is also referred to as your band size.

Now you have your size you might be overwhelmed at wondering what size underwire to pick out because if you look at all the size combinations that your pattern offers you might be looking at 60 different sizes and you may be worried about picking the wrong size. Fear not because bras and underwire use what is known as sister sizing.

Screenshot 2015-02-09 22.12.23

Sister sizing means that sizes with the same volume in the cup have the same underwire. Underwires are named after the band size for the B cup. So a 36B will use a size 36 wire but so will a 38A and a 34C. So the easy way to pick your wire is to use the chart to find the size you picked out based on your measurement and then find the wire size that represents your size by following along the row.

Sister sizing also makes it easier to fit your bra. Let’s say you make your size 36B bra but the cup size is too small but the band fits ok. Then you go up a wire size from a 36 wire to a 38 wire BUT you want to keep the band size as a 36 so bra size wise you have to go up a  letter cup size to a 36C. If you kept the cup size as a B but when up to a 38B your cup size would fit but your band would be too big.

Conversely if the cup and the front of the bra fit ok but the band is too tight then you’d keep the same wire size but go up one band size. But when you go up one band size you’d need to go down one letter cup size. So from a 36B to a 38A. But you’d still keep your 32 underwire.

Sister sizing makes it a lot easier to shop for underwires since you can pinpoint what wire size you might be and then just buy 1-2 bigger or smaller. In my next post I’ll talk about how to find your wire size based on the actual pattern pieces themselves and talk about what springing an underwire is.

For your enjoyment here is a very technical underwire and breast volume/mass chart.

Screenshot 2015-02-09 22.10.11


One thought on “Sister Sizing and Underwires

  1. natashaestradalvn says:

    Double A’s use the same size underwire as an A cup even though they don’t have the same volume they have the same approx breast root shape.


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