Cup Size vs Bust Dart Width

First things first—most patterns are drafted for a B cup. But let’s be real, most of us aren’t a standard B cup! And just to keep us on our toes, a “B cup” in sewing isn’t the same as a bra brand’s B cup. Sewing patterns use ‘dressmaker’s cup sizes’—a whole different kettle of thread! So, let’s break down how to figure out your ideal cup size for making clothes that fit like a dream.

So, how do you know how big (or small) your bust dart should be?

Let’s stitch this step-by-step! (Assuming you’ve already got your bodice pattern, whether self-drafted or shop-bought.)

Step 1: Measure Up

Grab your best-fitting bra and get some help if you can—it’s much easier when the tape measure stays parallel to the floor!

• Underbust measurement: Right where your bra underwire sits
• Full bust measurement:** Around the fullest part of your bust.

Step 2: Calculate the Difference

Subtract the underbust from the full bust measurement.

> E.g., 34” (full bust) – 32” (underbust) = 2” difference

Step 3: Work Out Dart Width

• -Dart width (on the front bodice side seam) = 25%of the difference above
• Using our example: 2” x 0.25 = **0.5” dart width

Step 4: Determine Dart Length

• For wearing, darts should stop about 1–2 inches before the bust point (apex)
• For drafting, draw the dart legs all the way to the apex—this makes it easier if you want to reposition the dart later.

Step 5: Draw and Balance

• Draw two dart legs from your dart width measurement out to the apex
• The front and back side seams need to end up the same length after sewing
• If you adjust the dart, also adjust the side seam length to match the back!

Step 6: Fold and True the Dart

• Fold the dart just as you would before stitching (darts are generally folded down or outward)
• Align the folded front side seam to the back pattern’s side seam
• Mark where to add or trim to even up the seam—add paper if it’s short, trim the back if needed
• Use a pinwheel marker to trace along the side seam while the dart is folded. Unfold, redraw the seam, and voilà -matching dart legs!

Pinwheel marker

I recommend this Clover 3-in-1 tracing wheel. You can use this interchangeably for marking patterns AND carbon marking.

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References

I've done all the scrolling for you, and I LOVE Jin's tutorials, so I'm not going to reinvent the wheel. Here's a link to the video that will provide a visual for you.

A final round of applause

A big round of applause for taking this step! Make your toile, tweak your fit, and sew up your perfectly fitted masterpiece.

Happy sewing!

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