If we start by imagining a woman with absolutely no bust, if she chose to wear a bra it would essentially just be a band with two perfectly flat cups which really aren’t cups at all. Sound silly? Well it isn’t as silly as you might think and is the basis for the bra band. A woman’s band size doesnt change with cup size. In fact it’s very likely that most women will wear the same band 95% of their lifetime, with only the cup changing. The band is based on the size of the ribcage. If you make the band fit correctly then you can apply any cup to it and have a perfect bra. Meaning, that while the band is the easy part, it’s also the most usefull as it can be used over and over again.
There really is only one question when deciding on how to proceed. How much ease will the band have? If you were to make the band from completely non-stretch fabrics, like in a corset for example, then a certain amount of ease would be added to achieve the final diameter (think carefully about that before emailing me about corsets!). If you were making it from stretchy materials, like swimwear fabrics, then some amount of negative ease would be required.
But what happens when some part of the garment is stretchy but others not? Bras are not usually stretchy in the front … with stretch only used from the wire back in order to create tension. Well quite simply there is little to no ease in the front and negative ease in the back. How much depends on the width of the back strap. The narrower the strap and the weaker the fabrics rebound tension, the more negative ease that needs to be applied.
I do not add ease to my bra band blocks at all … anywhere. The reason is the amount of ease depends on the design or style of the bra. First I make the bra and then I add in any positive or negative ease at the end. My band blocks are exactly the size I measure at. Think about that for a moment and before you say that you won’t be able to breathe, go back to my second last sentence … I do the ease last … and all my bra patterns usually have stretchy back bands! You wouldn’t think I’d need to say this but in EVERY class I’ve ever done this is always asked.
OK, so now we can assume the underwire line is the main measurement we need. If rib cages were perfectly parallel then we’d be finished already with a rectangular pattern and our bras would all reside in our armpits. But women are not parallel and bras only work because of this fact. As we go higher from the underwire line, the diameter of the ribcage increases … but best of all it increases reasonably uniformly across the population. Now if you go and measure yourselves you’ll all find that the diameter is not uniform because you are including, in most cases, the breasts in your measurements. If you consider the section of ribcage covered by a bra, you are looking at a slice of an inverted oval cone, which as a general rule of thumb seems to apex almost precisely at the floor (see image below).
I have no idea who came up with this theory originally. I learned it at college and have seen it only a few times since, but it has always worked quite well. What it tells us is how to determine the shape of the band. The formula is quite simple, for example, for a size 10 it would be …
angle = (underwire line x 180) / (3.14 x bust to floor)
= (70 x 180) / (3.14 x 118.8)
= 33.8 degrees
So if you were to draw a rectangle the length of the underwire line and however high you wanted the bra band to be, all you need to do is open up that rectangle by 34 degrees. OK, that’s a fairly rough rule of thumb, but I’m yet to find a more accurate one that covers the majority of the population. For people with wider shoulders they may want to open another few degrees and vice verca for people with narrow shoulders, but considering there is still likely to be some amount of stretch in the back band then this will work.
The next question is where do we add that 34 degrees? We do not add it uniformly as suggested by many people who make bras based on a close fit bodice block. We add it proportionally to where there is most curve in the ribcage. The following images show the process step by step.
Draft a rectangle the length of your underwire line (underbust measurement) and the height of the underwire you’ve selected.
Draft in the center front in the middle of the rectangle and then divide into quarters for the side seams. The outside edge of the rectangle is the center back.
Draft in the bust line (or center front wire height if higher).
Mark in where you would like the wires to sit. Now there’s much speculation about exactly where they should sit, but I recommend starting where they sit naturally to begin with. You can always bring them a little closer later for that extra bit of lift. Check the spacing on a comfy bra you already own. Typically that’s 20cm between bust points on a 10B-C. Mark the vertical center line of each wire.
Draft in another vertical line two thirds of the way from center back to each side seam.
Open up each dart as shown. If the total angle you need to add is more or less than 34 degrees then add/subtract this from the side seam dart.
Smooth off the block as shown. Above is essentially what the band block ends up looking like before you add/subtract ease to allow for stretchy back bands at the pattern stage. Remember, the narrower you make the back band the more you need to reduce the pattern as the rebound tension diminishes. Of course you can move pattern lines all over the place, but more on that later … this is just the start of the band block to which you can now add your cup block at the underwire positions to complete the bra block. Below gives you an idea of how the block might progress to the pattern stage.