Bust Cup Sizes for Blocks, Patternmaking & Commerical Patterns are not to be confused with Bra Cup Sizes. They may not be the same, and may be very different; e.g. you may be an F cup in bras, but be only a C or D cup for patternmaking.
How are Bust Cup Sizes Defined?
For the purposes of block and patternmaking, bust-cup sizes are based on the difference between the measurement of your Upper Bust and your Bust. If you subtract your high bust from your full bust, the difference determines your cup size.
- 1″ = A cup
- 2″ = B cup
- 3″ = C cup
- 4″ = D cup
- 5″ = DD cup (or E cup)
- 6″ = DDD cup (or F cup)
In the image, the Upper Bust measurement is 34 inches. The block for a B Bust cup (blue lines) will fit a bust of 36 inches - 2 inches more than the Upper Bust. The block for a C Bust Cup (green lines) will fit a bust of 37 inches - 3 inches more than the Upper Bust. The block for a D Bust Cup (the pink lines) will fit a bust of 38 inches - 4 inches more than the Upper Bust.
Note: The image below was updated on 11 April 2019. When I updated the image, I indicated the A cup line that would be used to create the block for a bust of 35 inches, but the A cup block isn't drawn (i.e. you can see the blue outline for the B cup, the green block outline for the B cup and the pink outline for the D cup, but there is no yellow outline for the A cup block).
The Difference between Patternmaking Cup and Bra Cup
With a bra, cup size is based on the difference between your bust and your under-bust. In patternmaking the under-bust measurement is not relevant for the Bodice Block; any fitting that needs to be done under the bust can be done by darts or design lines; e.g. an Empire Line can reduce the amount of ease in the under-bust to provide more fitting.
If you think if how the fitting of a bodice garments differs to a bra, and their different purposes, it should be clear why the patternmaking cup is different to the bra cup; When drafting a bodice block, you are concerned with the angle that the fabric falls from the upper bust and how this affects how much fabric is needed at the side of the body to accomodate the bust. The bra on the other hand is about supporting the breasts; the actual support is done by the band, which goes around the under-bust. The underbust is therefore an essential measurement in bra construction, while it is irrelevant in drafting a Bodice Block.
This image may help you see how the upper-bust to bust measurement is so different to the under-bust to bust measurement.
Some people with very large busts in bra-cup sizes (F, G, H), may also have a very large fleshy upper-bust and have only a C or D cup for patternmaking purposes.