Monday, 29 October 2012

"How to Calculate Your Bust Dart Size: The Formula that Takes the Guess-work out of FBAs and SBAs"

When you look in Pattern-making books they usually have a table of standard measurements including either the bust dart width, or it's angle in degrees. These "standard measurements" are for a B-cup, which means that if you are not a B-cup, you have to draft the pattern and then do either a Small Bust Adjustment or a Full Bust Adjustment.

But how do they get these numbers? They seem to have nothing to do with the rest of the measurements. But there is always a formula. It just takes a bit of working out.

What is a bust dart's function? As I see it, a bust dart is there to make the longer front pattern piece match the shorter back pattern piece at the side seam. The front pattern piece is longer than the back one because it has your bust to go over and the back doesn't.


So to find out the bust dart's width at the side seam,

  1. you measure your front from the neck point the red dot over the apex (the green dot on the drawing), and thence straight down to your waistline = F; (this measurement is the orange line in the picture)
  2. then measure from the red dot down your back to your waistline = B; (this is the red line)
  3. subtract B from F and you have your bust dart width, which is the difference between your front bodice length, and your back bodice length.

Now for a little trigonometry. Scientific calculators at the ready!

We'll split the bust dart into two equal parts so that we have two right angle triangles:

The difference between my front measurement and my back measurement is about 2 cm (halved to 1 cm for the split triangle), and the side dart length is about 13 cm on me.

How to Find Your Bust Dart Angle for Patternmaking in Fashion

If you are using the German method of pattern making, which is briefly taught in Patternmaking in Fashion by Lucia Mors de Castro (mine cost about £40 but it now costs £100 at Amazon due to high demand - I think it is over-priced now by very greedy sellers), you will need the angle of the bust dart, and as we have the length of the dart (the hypotenuse) and the width of the dart (the length "opposite" the angle) we can find the Sine of the angle (sin = O/h) and then use the sin-1 function on our scientific calculators to find the angle. Thus:-

How to Find Your Bust Dart Width for Metric Pattern Cutting

If you are using Metric Pattern Cutting for Women's Wear by Winifred Aldrich, or the instructions on www.BurdaStyle.com, you will want to find the bust dart width for a shoulder dart.
  1. Take the sine you found earlier (here 0.077) and multiply it by your neck to apex measurement (on me about 24 cm) to get half your total bust dart width. (Here, about 1.84 cm). 
  2. Get your total bust dart width by doubling this number = 3.68 cm, roughly 3.7 cm.

So that is my bust dart: 3.7 cm wide at the neck point and 8.8 degrees (which can be rounded to 9 degrees for practical reasons).

So that is how you work out your personal bust dart measurement. Isn't it so much better than using a "standard" measurement?

I hope it helps. : )

Sabrina