When I first planned the Ngaio Blouse as a pattern I intended to offer it in one standard size, and do a tutorial on how to do a full bust adjustment (FBA) on it, since the pattern pieces aren’t standard shapes that most sewers are used to adjusting. Then I thought: why not just do it for you?
I thought you might appreciate a bit of insight into the method I used, the calculations behind it, and what affect that has on the final fit.
The simplest way to measure cup size is to measure the difference between your full bust measure, and your high bust measure:
According to this system, an A cup has a 1″ difference, a B cup has a 2″ difference, a C cup a 3″, etc, etc.
This is the measuring system I use for the Ngaio blouse, because it works reasonably well for most bodies, is the same system most other pattern companies use (so I’m not throwing a bunch of weird measures you’re not expecting at you), and is simple to measure and calculate. However, it isn’t a perfect system by any means, because it doesn’t take into account:
Ribcage shape: If you have a very square/rectangular ribcage, or a very triangular ribcage, the same size pattern will fit you differently, For example, Ms. T and Ms R have the exact same high and full bust measure, but Ms T has a very triangular ribcage, and Ms R has a square/rectangular ribcage, so their underbust measures are 4″ different. For her bra to fit properly, Ms T is almost certainly going to wear a bra that is significantly smaller in the band width, and significantly bigger in the cup size, than Ms R.
Breast shape: The overbust vs full bust measure system assumes that breasts stick out from the body proportionately to increases in cup size, but that isn’t how many busts work. Breasts can be small, but very pointy, or large, but flatter and more spreading.
Because of these imperfections in the measuring system, the cup size you get when calculating your bust size for the Ngaio blouse may not match the cup size you’re used to wearing in a bra. That’s OK! Just follow the pattern, but make a toile (as is recommended with every pattern) to check the fit and make adjustments.
If you’re between measures according to the system you should think about your back and shoulders. If you have a narrow back and small shoulders, you’re probably going to fit better in the larger cup size that you’re between, which will put you in an overall smaller size. If you have a wide back and wide shoulders, you’ll probably fit better in an overall larger size, with a smaller cup size.
To show the difference in cup sizing, the gorgeous Jenni (who sits best in the Large cup sizing) modelled both Ngaio in both the Medium and the Large cup sizing.
Here is Jenni in her size, but with a Medium cup:
Note how the bust line sits on her bust, instead of under it, and how the top wants to crease and pull from back to front, and all the extra fullness below the bust.
Now, in the correct bust size according to her measures:
The bust line sits under her bust, and the blouse sits smoothly below the bust, skimming the stomach, with no pulling and wrinkles from back to front.
Of course, different cup sizing still doesn’t take into account how high or low your bust sits on your body, which takes its own adjustment.