16. The Full Bust Adjustment

How To Adjust A Pattern For a Fuller Bust

Many times we see people wondering why swimwear gapes at the armhole, doesn't cover the bust properly or the girls fall through the centre of a one piece or out the bottom of a bikini top. 


How many times have you seen swimwear with no bust darting, gaping at the armhole for example? Many PDF swimwear designers without formal training will design swimwear with no bust darting and expect the fabric to stretch perfectly over the bust ... frequently they design and test on themselves which mean's they can probably get away with it when they're a B cup ... but if you come along with a fuller cup then things start to go wrong. And if you use a fabric with higher rebound or worse still, make it reversible, then your problem compounds very quickly.

I constantly hear them saying just cut the pattern for the bigger bust size ... but that doesn't work. Why not? Well a person with a 82B cup on average has a waist of 76cm ... but if you're a 82D, they're telling you to go up one to two sizes so the garment better matches your bust measurement ... which is fine except that the rest of your body didn't also go up one to two sizes at the same time ... only your cup size ... meaning now you have all this excess fabric gaping or at the very least a distinct lack of tension.

If you're over a B to C cup then you need bust darting. Sure you can pull that triangle bikini top as tight as a corset to force the fabric to conform and it may very well do so ... but it'll also have extremely bad tension ripples where the straps join the triangle, the print will distort and the bust will flatten ... I don't call that a good fit, but many will find it acceptable until their shoulders and neck start hurting from the overly tight straps.

If you're a full bosomed woman and you're making a swimsuit from the average ready to wear PDF or commercial pattern then you're going to need a full bust adjustment. The bigger the cup size, the more shaping it requires to curve properly over the breast. Seems obvious right? Like I said before, anything over a B to C needs shaping ... and shaping is done with darts or darts hidden in seam lines. As the cup size increases, more darting angle is required. And this is where the problems start.

To perform a full bust adjustment and change the dart angle, first and foremost you need to know where the bust point is ... this is essential. But let's be honest, it's not often a pattern has a bust line marked let alone the bust point ... furthermore it's not likely you're even going to know where the centre front line is unless is a one piece cut on the fold, so trying to work backwards to locate the bust point yourself isn't going to be that straight forward either. 

Some swimsuits do have darts included already which makes it slightly easier, but don't assume the point of the dart is the actual bust point ... usually a dart point is pulled back from the actual bust point by 1 to 3cm ... yeah that varies too.

What else could mess things up? Well the obvious one is the amount of negative ease ... the more negative ease present, the less darting angle, proportionally speaking is required to achieve the same cup shape. The quick ones out there will  also realise that this same theory applies to fabrics of differing rebound tensions as well.

You could try asking your PDF designer to make sure a bust point is marked on their patterns but it's unlikely they're going to tell you if they haven't already got them marked. There is a belief among some that if you don't include a bust point or bust line marking then the customer is less likely to be able to mash up different patterns to create a new style and then sell it. If the customer is already at that level then not including landmarks isn't going to stop them.

How do we go about solving all these problems beyond guessing where the bust point is? Guessing isn't going to be all that bad anyway because of one wonderful trait of Lycra under tension ... after you've added a dart focusing on where YOU believe the bust point is, you'll have a cup shape that will move the garment to where it fits the bust under the least tension ... it'll locate itself to your new position! And if you're relatively close then the rest of the garment will adjust to that! 

You could also make up the undarted swimsuit and literally pin out the dart to where it fits best and offers the most support ... but who wants to make up a test garment unnecessarily just to get the dart right when with a little bit of understanding you might be able to do it right the first time? 

So how do you find the bust point or at least approximate it if it's not marked on your pattern. Let's take a look at two types of patterns first: one without any dart and one with a small dart.

Dartless Halter Bikini

If you take a look at the left image it's fairly straightforward to guess where the bust point on the darted pattern should be ... it'll be somewhere just above the dart tip (if the designer has pulled back the dart a little) ... how far above should be enough that the bottom of the pattern touches just below or on the under bust line given that this is a bikini top, so it's a matter of measuring how high the BP is above the underbust line for a B cup of the size you're working with (presuming no vertical negative ease) ... you won't know exactly for far below the BP the pattern maker placed the lower line but you could make a reasonable estimate. Given that you simply extend the dart higher that small amount and you have your approximated BP.  You can do a similar thing with the dartless block and at the very least determine where the bust line height should be ... except that many dartless patterns these days aren't for sprouting teens like they once used to be, but aimed at C to possibly D cups. So in that case, check what cup size the pattern claims to fit ... if it says from B to DD for exmple then assume it its designed for a C/Full C average and find the underbust to BP height for that ... this again, will give you a bust line at the very least.

So a dartless block might mean two things ....

  • Designed for a small bust: hence we can perform a regular full bust adjustment on it.

  • Designed incorrectly for a larger bust: we will need to perform a reversed full bust adjustment to correct the design fault.

I refer to a dartless pattern aimed at larger busts as a design fault because the pattern relies not on shaping created by the pattern, but instead it relies on the performance of the fabric (which may vary considerably). From a technical standpoint that's a design fault even though it may work in many cases. Pattern making is taught to create shaping with darts ... some designers don't like darts because that creates extra seams which they consider unsightly (reversible swimwear, for example) ... many will soon find out that causes gaping and the other problems above ... hence I call it a design fault ... if you can create consistently successful cup shaping without darts and without all the problems then by all means let me know because I've never seen it done, even by the best pattern makers!

Screenshot 2022-05-15 153928.jpg

But let me show you the best trick ever for estimating bust point. If your pattern is made with 12% negative ease then put it over a 12% block ... or whatever matching percentage you might have ... you'll need to check the negative ease on the pattern or ask the designer ... if they won't tell you that much then don't use that designer again!! Having blocks is not only good for making your own patterns, but also for reverse engineering others' patterns when they won't help you do things just like full bust adjustments! I make no apologies for suggesting that.

In the case of the B cup darted pattern (rightmost image above) when you place the pattern you're looking for the best visual fit that lines up the existing dart with the BP on the block ... that's the easy one. 

For the dartless pattern, if it's designed for a B cup or smaller then use a B cup block (left most image), if it's designed for C to D then use the C cup block (center image). What you're trying to do when you place the dartless pattern on a block is to get the best visual fit ... think about where you want the design lines of the pattern to sit on the bust ... once you create a cup in that position then that's where it will move to on the breast and you can then use that to help move the breast to where you want in garments like one pieces!

Try to visualise where on the block the breast or underwire line sit, keeping in mind that the front of the block wraps around the side too, so don't move it too far to the side. In the case of this pattern, the strap also points towards the BP (because it's my pattern!) ... photos by your pattern designer will hint at if that's the case for your pattern. You're looking for hints like this that will help you line it up on the block (or even without a block).


If you don't have swimwear blocks with varying cup sizes then I have them in my shop ... but before you go running off to buy them, this exercise is about a quick way to determine bust point ... you can still estimate without the block, the block just gives you full perspective.

OK before we go any further I'm going to give away a little secret about darting angles .... well two secrets really.

Firstly, if you have a block with no vertical negative ease then it doesn't matter whether your block/pattern is anywhere between 5% to 15% negative ease ... the dart angle is almost identical in every case for each cup size. Think about why ... the dart table below was calculated for a side dart which can still be rotated to any position ... the difference between 7 and 15% negative is less than a couple of degrees of actual dart angle ... and almost negligible between 8 and 12% (which I base all my work on). Outside the range of 7 to 15% the angles need adjusting in the larger cup sizes. Please note that this is a rule of thumb that works fairly effectively ... different negative ease and fabrics will affect it but not greatly within the safe range.

OK ... Secondly, the dart angle obviously increases with increased cup size but it increases in a non linear fashion with smaller increments for each successive cup size. See the table below ...

Angle of the dart for successive cup sizes in non-linear and linear progression

Irrelevant of the overall size, the angle of the dart is the same for any one cup size ... and in any negative ease amount between 7% and 15%!! The angle doesn't change more than one degree over an I cup so effectively that's the limit of this trend. The problem comes however when you're making ready to wear in a range of cup sizes ... you can't really grade using a linear method for the pattern but non linear for the dart ... which is technically what's required. So I approximate the angles on a linear grade, which is ok up to about a G/H cup then it goes bad ... so I don't offer higher than a G in RTW. It is possible to do a non-linear angle after you've graded your pattern by a linear grade using the right hand column ... but you are manually changing the angle for each cup ... extremely tedious and time consuming, but more accurate and then you can extend the cup sizes further.

That's enough for the second instalment ... I worry this may have gotten a little technical so I'll take questions in the group first so we can improve it ... stay tuned for part 3!