It is very easy to have wires laying around and not know their size. I originally put this tutorial together with this in mind, but I want to talk about manufacturers for a minute. I've now been through two drafting courses and both sets of teachers (and well known bra making suppliers) have discussed how you can send a schematic to a manufacturer all you want, but they never come back quite the way you expect.  I have a strong opinion on the subject. Wires should be actually put up to the body in a wire fitting and if you know the name of the wire that's great for knowing how to order more. 

It is a little difficult to determine what size will be right by just doing a wire trace and comparing to a chart and hoping that the wire that comes in the mail actually matches the schematics.  For this reason, I highly recommend that if you are making bras for clients, you get a set of wires for doing wire fittings and then do this process of measuring below to get the diameter necessary for doing custom work. If you are doing this process for making bras for yourself and you did a root trace, rather than testing physical wires, I would order the size your root trace and 2 or 3 sizes larger.  Slightly bigger is going to be better than too small and pinching breast tissue. 

The most accurate rule of thumb is to actually have wire in hand for a wire fitting and for drafters to actually have wire in hand while drafting so you can trace it right into your draft.

Without further ado, here is the process of measuring wire diameter:

Lay your wire on a piece of paper so you can trace it.