So, we've already established that I need to make an XS sweater with some more room in the bust.  How much room?  Let's see.

Measure yourself around the fullest part of your bust, wearing whatever clothes you'll probably wear under the sweater.  This is your bust measurement (duh), and we're going to call it YB.

Next, take the finished bust of the sweater size that fits you in the waist and hips.  We're going to call it SB.

The last thing you need is the amount of positive or negative ease (E) you're making the sweater with.  If you're making it with 2" ease, E is 2.  If you're making it with 1" negative ease, E is -1.

Now, what you do is subtract SB from YB, and add or subtract your ease.  In other words:

Here's how my math works out.  My YB is 34, SB is 30, and E is 0.  So 34-30+0= 4.  That means I have to add four inches to the bust.  

Because I have two boobs (and assuming you do too), divide your number by two.  I get 2" per boob.

Now we have to play with gauge to figure out how many stitches that makes out to be.  My gauge is 23 sts = 4".  That means to get 2 extra inches, I have to increase 11.5 stitches.  Since I haven't figured out how to knit half a stitch yet, I'll just fudge it to get 12.

 First off, you want to measure from the hem of the sweater to right where your boob starts. 

Measure from your shoulder (the top of it, where a sweater seam would fall) to the top of your breast.  Mine is about level with my armpit seam, yours may be higher or lower.

 The number you just got is how far you'll knit before starting your shaping.

Measure from your underbust (where the bottom of your bra is) straight up to the fullest part of your bust.  It's probably at the nipple, but all boobs are different, so don't worry if yours isn't. This is your bust height (H).

Now, just a teeny bit more math.  You need to translate H into rows by multiplying it by your row gauge (RG).

Again, here's my math as an example.  My H is 3" and my RG is 7 rows/inch.   So I get an answer of 21 rows.

All the numbers are great, but what do you do with them?  Here's where we combine our answers from Parts A and B.

To make your bust dart you need to incease A stitches over B rows.  We're going to do our increases in pairs, so we divide A by two to get A2.  Then, to figure out how many rows between increases, we divide B by A2 to get C.

If C isn't an even number, just fudge it a bit.  If it's 3.5, just increase every 3 rows sometimes and every 4 rows sometimes.  Don't sweat it, you're making a sweater, not a nuclear reactor.

My math:

A is 12, so A2 is 6.

B is 21, and B/A2 is 3.5.  So I'll increase every 3 or 4 rows.

And this is the very last step, I promise!  

I decided I want my dart to be about 1" wide, so it will be 7 stitches.  You can make yours wider or narrower as you please, though those with larger breasts are advised to make wider darts and vice versa.

That means I will have 7 stitches between my paired increases.  You can use whatever type of increase you like, I'm a big fan of M1 because it blends in to my knitting and doesn't leave gaps.  Your results may vary.

When you start your bust increases, how many stitches do you have across (S)?  Subtract the number of dart stitches (D) from that to get S2, and then divide by two to get how many stitches are before and after the dart.

I've got 37 stitches across.  My first increase row is going to look like this:  knit 15, increase, knit 7, increase, knit 15.

Then knit the in-between rows.  If you're increasing every 3 rows, that means you increase, knit 2 rows, increase, knit 2 rows, etc.

Remember to add in the stitches you increased when you write the second row!  So my second increase row is:  Knit 16, increase, knit 7, increase, knit 16.

Once you've increased the desired number of times, knit a few rows plain to avoid pointy boob syndrome.  Then just do the increases in reverse to decrease back down to size, and continue on with the pattern.

Hope you learned something new!  There are a LOT of different ways to make bust shaping, this is merely one that I pulled out of the air and happened to work for me.  If this doesn't fit you correctly or you don't like the way it looks, feel free to make changes or toss it out and try something different.  We are all shaped differently, and one of the most fulfilling parts of knitting is being able to make things that fit and flatter your individual shape.

R1: K 15, inc, K7, inc, K15

R2-3: Stockinette stitch

R4: P 16, inc, P7, inc, P16

R5-7: Stockinette stitch

R8: P 17, inc, P7, inc, P17

R9-11: St. st

R12: P 18, inc, P7, inc, P18

R13-15: St. st

R16: P 19, inc, P7, inc, P19

R17-19: St. st

Here my pattern says to start the side increases, so I do that (italicised here).  Notice that it affects my numbers on the decreases!

R1: K20, K2tog, K5, SSP, K19

R2-4: St. st

R5: K3, M1, K16, K2tog, K5, SSK, K18

R6-8: ST. st

R9: K19, K2tog, K5, SSK, K17

R10-12: St. st

R13: K3, M1, K15, K2tog, K5, SSK, K16

R14-16: St. st

R17: K18, K2tog, K5, SSK, K15

R18-20 St. st