Bra Cup Block

Time To Get Technical

Bra Block Science

Note: this section describes how to make a bra block for non-stretch fabrics and is based on wire size, where there is no added ease beyond that dictated by the wire. We are not trying to produce a perfect fitting pattern for each individual here ... just a good approximate block that can then be fitted at the pattern level. Furthermore, this website is specifically about teaching pattern making to fashion students and it is not aimed at teaching home sewists bra construction. I realise many home sewists want to know how to sew a bra, but that is outside the scope of my tutorial and better left to the hundreds of dedicated sewing books and courses already on the market.

I also realise there is a relatively big group of custom fit bra makers out there looking for the Holy Grail of the absolute perfect fit bra ... this page is probably too RTW for you and you've also probably gone straight from breast to customised pattern anyway.

I strongly urge people read the two preceding pages to this one if you haven't already. The first is an explanation of standard bra sizing and the second is about how the fashion industry interprets breast volume into a bra shape and gives an introduction of the implications this has on pattern making. This page takes the first two and explains how to turn measurements into a block that you can manipulate into a bra pattern. Further modifying the bra block for stretch fabric and hence swimwear will be handled after all the bra basics have been explained.

Bra Cup Block

In order to make a block or pattern you will need to choose your wire first. Commercial patterns need to assume all wires are the same size and shape, or they could never sell you a pattern. Besides they have no idea where you buy your supplies so while it's a bad assumption it is reasonable. The truth is all wires are not the same. For example, an Australian 12C (US 32-34C) can vary significantly in profile from 11 to 13cm in diameter across the horizontal depending on who manufactures it, however there is a consistency. As horizontal diameter increases, vertical radius decreases. Essentially you have a circle of 12.3cm diameter that is squished into an oval in one direction or another and changes the profile and diameter of the wire, while changing the breast shape but not significantly changing the volume. All 12C wires approximate to a circle of 12.3cm, so this means we have a start point for making a 12C cup. Remarkably each cup varies in a even increment of about 9.8mm diameter as a circle.

Based on this it's possible to create one cup pattern for each wire (ignoring AA and AAA for a moment) and just place it on different bands to make any combination of standard bra/cup sizes. Here's where the system of separating cup block from band block is so good. Creating a block for the band doesn't need to consider the cup. It assumes a completely flat chest with no cup. You make the cup block and the band block separately then put them together and you have a bra block to suit you. Sounds harder than it is but all this theory is important to understanding.


The picture above shows how underwires increase with equal increment. The wires I illustrated on the bra sizing page fit reasonably well on this grid. Most wires end half way up the hemisphere at centre front and have an extended side tail that heads off toward the armpit. Some wires extend vertical of the hemisphere at centre front and some join left wire to right, but in most it's the hemisphere (bust line in reality) or fractionally above. The side tail is important because this is where the wire strays from the circle. Again, how much and how high depends on the manufacturer, but if you centre the wires like in the picture above and draw a line along the side tails it should end at the centre of the circle. That angle is usually how the tail length is specified by the designer. The example above is a 24° tail, demi cup wires might be only a 12° tail, while some really full cup wires can be as much as 36° ... any higher than this and you end up with wires that dig in.

Note: At this point I want to remind people that this technique is about making a bra block. The technique assumes you have chosen the correct/closest circular wire. Of course, if you are able to obtain a specially made ovoid based wire you will need more specialist pattern making assistance. As these are not generally commercially available I have placed them outside the scope of this text.

The table below lists the diameters of wires in relation to band and cup size. If the wire you wear regularly doesn’t match these numbers it’s most likely there’s a size translation error between sizing systems (i.e.; you might have a Taiwanese wire interpreted to American sizing being sold in Australia – Spotlight stores!). Simply measure your wire as a circle and find the closest spot on table where it corresponds to your underbust measurement.

Ok so you now know what your wire diameter is, how long its tail is, and what your underbust is. This is all you need to make your cup block. Let's start with the lower hemisphere. Using the Australian 12C (US 34C) as an example, the diameter of the half circle is 12.3cm which makes the half circumference 19.3cm (the length of the wire without the tail) ... this will also be the length over the breast from centre front wire to nipple to tail wire. As most bras have a two panel lower cup let's split the panel in half as illustrated in red below. Essentially you end up with two equilateral triangles each with curved sides 9.65cm in length. You could do this process with one, three or four panels as well, although two is the most common.

So how do you make the curved triangles? Let's first consider a little simple geometry. An equilateral triangle has three equal internal angles of 60°, but clearly the lower hemisphere pattern pieces need corners of 90°. Take a look at the illustration below. Rather than get into some really complex geometric equations for calculating arc lengths and the like, I find it easier to break up each side into 4 pieces as shown ... for the 12C example that means 4 lines each 2.4cm (9.65/4 = 2.4125). To create the 90° corners you need to angle each section 10° from the last (or 170° inside angle). Sounds complicated but really it's just a faster way to make accurate hemisphere quarter patterns ... and the same technique will work for multiple panels or unequal sided triangles if you've located an ovoid wire or are making a direct customised block.

Lastly smooth off the sides into a curve and you have the perfect 2 panel lower hemisphere. Personally I tend to leave the curving process until the finished pattern but many don't like the look of faceted block panels. I think it's easier to manipulate the blocks this way but it's simply preference.

So what about the top half? Here's where things get a little complicated. Take a look at the diagram below. The lines a, b and c represent cross-sections through the breast and rows 1, 2 and 3 represent different breast volumes. What the diagram is designed to show is how the shape of the top hemisphere changes as volume increases up or down.

Row one illustrates the fuller height breast (vertical ovoid, or more typical of larger cup sizes on smaller bands), row two represents an average volume (circular) and row three represents the low volume breast (still technically a circular wire but horizontally ovoid in shape). Of course it's possible for any breast to have features of more than one of these shapes. For example, some breasts are fuller toward the armpit, standard at the bust point and hollow at centre front. Add to this that more often than not the breast can meet the chest at the same point irrelevant of volume and you can't even use a bendy wire to get an idea of the breast outline without a bit of guess work. The answer however is not immediately obvious, but lies in the fact that you really don't need to cover much of the top half of the breast at all (non-underwire bras will be covered separately) and thus another hemisphere is a reasonable start point.

Take a look at the above diagram again. In row two you will notice that the breast leaves the hemisphere guide about halfway up. This is also similar for row one! Row three, because of the lack of ovoid wires is likely to be wearing a smaller wire resulting in much of the volume being pushed into the hollow (not to mention this shape woman tends to wear padding in smaller cup sizes). The resulting profile is illustrated below in red. This might also help to answer why the majority of commercial wires are 24° wires! It's the point on the wire where length is no longer required for supporting the top portion of the breast so why waste money making it longer!

And if you rotate the whole thing everything starts to become clear (the green line should give you one idea of where this could go).
If, however we're talking about much larger cups generally, you will find that more coverage is required in the upper potion ... the same technique is used to create the block however you do not continue to close darts beyond the red line unless required by the pattern.

Make a copy of the bottom hemisphere quarters you made before and invert them. If you measure around the circumference of the circle you'll find that the profile is one third of the hemisphere quarter. If you draw a line down the centre of each triangle and mark it's one third point you can directly draw the above profile onto the hemisphere without any more calculations.

Now put the four pieces together and you have the foundation of a basic bra cup block. At this point what you have is really no more than a hemisphere with a bit of the top chopped off that sits inside your chosen wire. Clearly this does not yet take into account the area between the circle and the tail. Simply take this from your flat, front-on style drawing and cut and paste it around the block by slashing and rotating the pieces as shown below. Now the question becomes, is this a block or a styled pattern? If I intend to cut several different styles from this single overall shape, then it's a block. If I intend to cut just this one style, then I need to smooth everything off and it becomes a pattern. The problem with bras having so many potential strap positions means it's really open to interpretation. Compare this method to the close fit lingerie block method and once you've taken out the extra darting you'll see they're very similar. I tend to call this a block, because obviously a breast is not a hemisphere and still needs some style and shaping, but the perfect hemisphere is the current shape so who am I to decide!!

IMPORTANT: This block is only the bra cup section of the bra block. To complete the bra block you need to add it to the bra band block. Final tweaking of the bra cup block is left until the pattern stage.


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© 2020 Pattern School Online by Stuart Anderson