Well, this is interesting - bra sizing

I have always worn a size 34B in a bra. Maybe many years ago I got measured or calculated my size from measurements, I don't remember. Typically if I was shopping for a bra, I'd go out and try some on. It is a painful process. Making a bra is a different kind of painful process. You have to ascertain your size without having the opportunity to try it on until you are almost finished making it.

My TNT bra pattern was cobbled from different bits (cup design from an ancient Burda World of Fashion swimsuit pattern, band and bridge from a Pinup Girls pattern (the Sharon front closing bra). Every element was then tweaked as needed to improve the fit. I literally have no idea what "size" this pattern is. I know the band is too long and when I make it, I cut off the amount needed to get the right fit in the band. This is easy and painless because the band is straight as you can see at right. It's acceptable because I make one at a time and I use different fabric and elastic. A too-long band piece gives me insurance and allows me to fine-tune the fit as I go.

So my planned bra making experiment involves some prior planning, i.e. choosing which size to cut. The two patterns (Maya and Harriet) take completely different approaches to figuring out the right size, and neither uses the standard North American approach of adding an apparently arbitrary number to your underbust measurement to get to your size "number".

The Maya instructions start as usual with an underbust measurement. However, instead of basing size on the difference between the underbust and full bust measurements, you determine a completely different value - the wire size. Which curved shape best corresponds to your "breast root"? Based on experience I thought that a 34 wire would be right for me and I confirmed this by holding a size 34 wire against my anatomy. Then you use the supplied chart to identify the size. Under this system, my size is 34B or 75B (EU sizing). That seemed reassuring.

The Harriet instructions are to take your underbust measurement. That number is your band size (logical, no?). Then measure the full bust. The difference in inches (Harriet is from the US) determines your cup size. Based on this process, my size is 30D! Well, that is a very different number from my usual 34B. It instantly makes me nervous.

So I printed off the 34B Maya and the "sister sizes" for the 30D Harriet. Those sizes are 30D, 32C and my usual 34B, interestingly enough. Then I got all my patterns out and did a flat pattern comparison.

What I learned (you knowledgeable people probably already knew it):

• I had read that sister sizes all have the same cup volume, but always thought that the volume would be distributed differently, i.e. the A cup size would represent a flatter breast that was more spread out on a bigger body (bigger band, wider breast root) than the sister C cup size which would project more. This seems to be wrong. All the sister sizes use the same wire so they have the same shape and size breast root.
• The cup pattern in the Harriet bra is exactly the same for all the sister sizes. Only the frame (bridge, outer cradle and band pieces combined) changes. 
• Since all the bras call for a 34 wire, it is not surprising but nevertheless reassuring that the shape of the cradle (the round opening in the frame where it goes around the cups) is very similar in all three bra patterns.  
• My TNT bra pattern and the 34B Maya are the same length, more or less, from CF to CB. But my TNT bra is actually too long for me (I cut the bra band to fit as I'm making it). So I'd have to do the same with the Maya. I could do this as the back band is straight, although it's a bit more tapered than my TNT bra. 
• The bridge on the Maya is far wider than my TNT bridge. It would be over 3cm wide at the top. I can tell that is too wide so I have taken 6mm off the bridge even before testing.
• The Harriet does not have a straight band. It's wider at the side for more support and scooped between the straps. Therefore it's more important to get the band size right from the get-go. 
• The bridge on the Harriet varies in width with the sister sizes - the 34B bridge is far wider than my TNT bridge. The 30D bridge is about the same. 
• The 30D band is far shorter than my TNT pattern. The 34B band is a bit longer. The 32C band seems pretty good, especially if I combine it with the narrower 30D bridge. 

Based on all this, I *think* I'm actually a 32C. (How typical, most women apparently wear the wrong size bra.) Back in a jiffy after I sew up some rough and ready testers in both patterns.