Bust Dart Math!

I guarantee that explaining this will take far longer than it will take you to actually DO the math. Plus, if I were sitting right next to you, I could show you in a nanosecond. But, we have our friend the written word to help us, so here we go: Bust Dart Math!

For a top-down in-the-round sweater, vertical bust darts are lines of decreases that take the full-bust measurement of fabric at your bust and decrease it in size until it matches the measurement under your bust. Thus, we start off with:

1. How much do you need to vertically decrease?

You'll need two measurements of your own beautiful self:
Full Bust (FB): Around your bust at its fullest part.
Under Bust (UB): Around your ribcage, just under your bust.

Now, subtract:
FB minus UB equals Total Decrease Amount (TDA).

However, we need to convert that TDA measurement to rows/rounds and stitches so we know what to knit, right?

2. How much are we decreasing in each individual dart/decrease round? And then, how many decrease rounds do we work overall?

  • Figure out the stitches-per-inch gauge.
  • Divide that gauge into four (4 divided by gauge) because we are working four decreases for each round (two bust darts each side; each dart is a line of vertical decrease stitches). The result is the Decrease Amount Per Round (DAR).
  • To find the number of decrease rounds: TDA divided by DAR = Total Decrease Rounds (TDR). (Hold onto that TDR number for a bit.)

3. Where do you put the dart (decrease) stitches in each round?

Remember that we are assuming a top-down sweater worked in the round. Here's how we figure out precisely where to put the dart stitches in each round:

  • Make sure you have marked the midpoint of each armhole on your partial sweater. This marker is at the "side seam," so to speak. We'll do all the counting/measuring in relationship to that marker, so it's pretty important.
  • Find this "side seam" location on yourself, too. (Go ahead, cheat: Put on a thin shirt that actually has side seams.) Measure forward from the side seam to your nipple. This measurement, side seam to nipple, we'll call N.
  • Try on your partially-knitted sweater, and place a safety pin (carefully…) next to the stitch closest to your nipple (by "next to," I mean on the "armhole" side of that stitch). Count the stitches from this marker to the side seam marker and you have what we'll call Total Side Stitches (TSS). (If all the math works out perfectly, then your stitches per inch gauge times your TSS should equal N, but don't stress too much about this one, because your bust may be stretching the gauge out a bit!) NOTE: Use a safety pin or a marker that looks very different from your other markers to mark the nipple stitch, because you will want to remove this one before knitting and you won't want to get confused which marker is which.

Location of the Center Dart: (On Bertha in the photo above, this dart is represented by the BLACK clip.) This dart is easy. You want it to be about .25" away from your nipple, towards the side seam. Figure out how many stitches that is (use your stitch gauge or just measure), and place another marker at that spot. Whoo! Center Dart Alert! (Now do the same thing on your other side for the other Center Dart.)

Location of the Side Dart: (On Bertha, this dart is represented by the PINK clip.) This one is a leetle more tricky, but we can handle it. There are two ways to figure out this dart: using measurements, or using actual stitch counts.

Measurement Way: N minus .25" was where we put the Center Dart, right? Well, the Side Dart is placed at the point one-third of the remaining distance from Center Dart to side seam: [N-.25] divided by three equals the distance from Center Dart to Side Dart, measuring from nipple towards the side seam. Place a marker there for the Side Dart; do this step again for your other side. (See how on Bertha the pink clip is about a third of the way between her non-existent nipple and her "side-seam"? That's what I'm talking about here.)

Stitch Count Way: For this, you have to figure out how many stitches are in the .25" you used for the Center Dart above. Got that? OK, then: [TSS minus that number] divided by three equals the number of stitches between the Center Dart and the Side Dart. Place marker and repeat on other side.

(Note: On Bertha, her pink clip is a little bit further forward than the "one-third" rule. That's because Bertha's curves are about a B or C cup; my "one-third" rule/suggestion/guideline/loosely-discussed-between-friends-number is what I used for my D/DD cups. If you did the exercise with the hair clips and the loose tee shirt, you may have your own customized measurements for the placement of the Center Darts and Side Darts. Use those. They're all about you, baby!)

4. Now, the knitting instructions:

  1. Remove the safety pin that marked the location of your nipple. (If you leave it in, you might get confused which marker is a dart and which is your nipple. Amusing, but not very helpful.)
  2. Starting at the first "side seam" marker, work to three stitches before the first Side Dart marker, ssk, k1, slip m.
  3. Work to three stitches before the first Center Dart marker, ssk, k1, slip m.
  4. Work across the center front of your sweater until you get to the other Center Dart marker, slip m, k1, k2tog.
  5. Work to the other Side Dart marker, slip m, k1, k2tog.
  6. Finish this round, and then work one round even.
  7. Repeat those two rounds (one dartly decrease round and one work even round) a total of TDR times (the number you held onto in Step 1).


Wow. Know what? YOU HAVE DARTAGE!! Notice that the decreases will form vertical, diagonal lines that are actually quite attractive.

An example is always better, but we've run out of space to do that today. We can do that on Friday, plus I can start answering questions then, too.


P.S. Have something to add to the discussion? Need to ask a question? Leave a comment! I'm not quite actually and fully back yet due to Unforseen Circumstances, but remote access is a beautiful thing.

Previous Bust Dart Posts:

The Return of The Bust Darts

Introduction to Darts: Vertical and Short-Row

Bust Dart Placement

Getting Started With Your Bust Darts: Shoulders To Bust

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