Choosing a size and increasing your cup size // Sophie Swimsuit pattern // Closet Case Filesx
Closet Case Patterns, Fitting


Thank you so very much for the truly awesome response to our new pattern, The Sophie Swimsuit! The studio is currently covered in swim lycra and piles of underwires everywhere and and we’ll be shipping out the first batch of swimsuit fabric kits later this week. In the meantime, I thought I would talk about how to choose a size and underwire for Sophie since this is a question we’re getting a lot. I’d also like to explain how you can make the pattern fit you even if you fall outside our cup range (which goes up to approximately DD).


Unlike all of our other patterns, you choose your main size for Sophie based on your under bust measurement. This dictates the size you are going to print, even if your waist and hip are smaller or larger.

Size-Chart copy

Your cup size is determined by the difference in inches between full bust and under bust. For example, if your full bust is 36″ and your under bust is 32″,  the difference of 4″ means you require a #4 cup.


(The reason I choose not to letter the cups is simple; based on the way our sizing model works, many people may find themselves needing a cup size larger than what they are used to when buying bras. I didn’t want you to freak out if you’re a DD in my pattern, so we just want with non-value laden numbers).

Adding additional back support // Sophie Swimsuit pattern // Closet Case Filesx


Since the one piece View A body has 5 cup sizes, we had to split up the files by individual size or else it would have been a complete nested mess! However, we’ve nested the waist/hip lines three sizes above and below in light grey so you have some guidelines for grading between sizes. If you need to grade more than 3 sizes for the one piece, I would suggest using the high-waisted bikini bottom as a tracing guide since those pattern pieces have all 11 sizes nested normally. Here is how you would grade between sizes if you were a size 6 underbust/#5 cup and a size 12 hip. As you can see, it’s not too complicated to connect from the size 6 at the bust to the larger size at the waist. Your cup pieces aren’t effected, just the body pieces. You’ll have to grade both center and side panel pieces.

Grading Between sizes // Sophie Swimsuit Pattern // Closet Case Files


Although this pattern can be made without foam or an underwire, if you’re a #3/C cup or over, you’ll simply get a better fit with the added structure. We are including an underwire in the swimsuit fabric kits and I have a guide to finding your size on the shop, but I thought I’d explain it here too, especially if you’re planning on buying your underwires from another source. Sophie was drafted around the regular length wires from Bra-maker’s Supply so I suggest getting underwires from there if you don’t want to trim them down (I’ve noticed that the wires they sell are slightly shorter than in their sizing chart which is what we used to draft the pattern – you may want to get a pair of long wires too, and see what fits better in your channel).

Cup sizing can be a little confusing the first time you dip your toe in. As I mentioned above, you find your cup size by subtracting your full bust from your under bust, with every inch difference representing a full cup size. However, not all cup sizes are created equal. For example, the actual volume of a B cup on a woman with a 30″ under bust is much smaller than a B cup on a woman with a 38″ under bust – in both cases the difference between full and under bust is 2″, but the woman with a 40″ full bust naturally has more breast tissue than one with a 32″ full bust.

Underwires measure the quantity of that breast tissue, or the diameter of the breast itself, and the most common kind is numbered from 30-60. For every jump in cup size, the number of the underwire goes up 2 in size (ie. 30, 32, 34, 36, etc.) Let’s take a look at my underwire chart to see what I mean.


You can see that as the cup size increases with the chart moving right, the underwire number goes up. You may also notice that size 0 & 2, 4 & 6, and 8 & 10 share the same underwire sizes. Don’t worry too much about this; it’s just how we had to make a cup sizing model work with our existing sizing system, so some of our underbust sizes share cup sizes as well.


To find your underwire size, find your underbust size in the left column and then the difference in inches with your full bust along the top. Where those two lines intersect is your underwire size. For example, a woman with a 31″ under bust and a 3″ full bust difference would take a #36 underwire. If she had a 4″ difference, she would take a #38 underwire, a 5″ difference a #40 underwire, and so on.


If all of this is too confusing and you feel pretty confidant that you’re wearing bras that fit you well in the cups, you can  also choose a cup size based on your existing bra size. To do that, print out this underwire chart from Bra-maker’s Supply at home with no scaling. Lay your underwire on top of the chart until you find one that matches in size, and then use that underwire/cup size for your swimsuit!



Grading this pattern was a bit of a feat, so although I wanted to offer a bigger range of cup sizes, it just wasn’t possible to grade nicely. However, it isn’t hard to go up cup sizes. What you need to do is figure out what your underwire size is. Remember that every inch difference equals a cup size, and that each underwire size goes up by a factor of two. So, if your underbust is 31″ and your full bust is 37″, that 6″ difference means you would require a #42 underwire (one size above the #40 required for the 5″ difference). Looking at the chart you can see the #42 underwire is used for the size 12/cup 5. You would therefore need to use the cup pieces from the size 12/5 cup and then trace off the cup opening from that size onto the size 10 body.

To do that, line up the size 12 body over the size 10 body – you can use the grey “grading lines” as a guide. The line in blue below shows the size 12 on top of the size 10.

Grading cups up 2

You would then simply trace off the size 12/cup 5 bust line onto your size 10 body. That is shown in red below.


The principle is more or less the same for the bikini top, but if you’re only going up one size around the underbust you can probably just get away with making the size up and then adjusting the length at the center back if necessary.

Support wise, I will say that the bikini top is definitely more supportive than the swimsuit. The band really anchors everything into place around the ribcage. If you have a very large bust and would like to make the one piece, you may have to sew in some sort of concealed band support along the back of the suit to help provide enough support to the front. One of my lovely testers constructed this powernet & elastic back band and she said it worked really well:

Adding back band support to Sophie Swimsuit pattern

Hopefully that clarifies sizing for this pattern. Feel free to hit me up with any questions in the comment section!

  • Jacey Grusnick

    This may be a dumb question, but when you are measuring for your sizing should you be wearing a bra or no bra?

    • I generally think its better to wear a bra since that replicates your dimensions in the garment. It should be a naturallly fitting bra though, ie. nothing like a crazy padded push-up bra.

  • Heather- above you say “Your cup size is determined by the difference in inches between high
    bust and under bust. For example, if your full bust is 36″ and your
    under bust is 32″, the difference of 4″ means you require a #4 cup.” Which is correct to use as the larger measurement, high bust or full bust? Thanks!

  • One more question (just trying to decide if I want to buy as I am slightly out of the size range). Is the cup drafted for the stretch of swimwear material or is it lined with something less stretchy? I would like to line it with foam, but that is significantly less stretchy than swimwear material. I love the idea of a swimsuit with underwires and have been meaning to hack one but your design is much cuter than anything I would come up with, I’m sure! I just can’t imagine wearing a bathing suit without some foam in the cups. Thanks!

    • Hi Megan, sorry that was a typo! Its full bust all the way 😉 The cup is drafted for foam, but can be unfoamed as well, although obviously the support won’t be the same.

      • awesome, really good to know!! I’m excited to try this pattern out, its exactly what I had in mind, so I’m glad I was dragging my feet on trying to hack it since your will be much better 😉

  • Cadi

    My underbust is 37 inches, would I go up to a size 18 or down to the 16 on the bikini version? I think I know the answer, but figure if I have this question someone else might too so is it’s worth asking. Thanks for another amazing pattern!!

    • Hmmm. I’d go up… Always easier to take in than let out!

  • Jane

    This is so well explained, thank you! It’s dead of winter here in Australia but come summer I think I may have to give this a go. Looks fab!

    • I’m so glad – I spent a few hours editing it so it made sense 😉

  • julie d.

    this is an amazing pattern, and what a great post because i always have to grade from the bust to waist to hips. i have zero boobs, but for me an underwire and slightly padded cups makes me FEEL like i’ve got the boobies, so bravo! excited to make it!

  • Thank you for this post, I’m looking forward to using it. I thought about perhaps hacking the bikini top back onto the one piece back so that the back strap supports would be there, but I really like the idea of the power mesh back band as well.

    • That’s an awesome idea! One of my testers did something similar and it worked a treat!

  • CM

    I am SO THRILLED by this pattern- exactly what I have been searching for over the last several YEARS! Thank you.

    I want to make the bikini top. I will need to grade up the cups.

    My under bust is 31″ // CCF Size 8
    My full bust is 41″
    =10″ Difference

    According to the wire charts, I would be an underwire #50; which corresponds with CCF Size 20 Cup 5.

    So would I be using the cup pieces of 20C5 on the band and straps of a Size 8? Am I understanding this correctly?

    I feel a bit like a student asking you to check my homework… but I’m also a beginner at making this kind of adjustment!! 🙂

    • You got it! I also suggest using a wider strap or the halter – those little elastic straps will probably not be supportive enough for you… Basically you just want to get the cradle and band fitting right – you can adjust the wing as needed. Based on your bust size I would also narrow the bridge by about 1/4″ – just fold it out between the cups!

      • CM

        Awesome!! I am so fired up about this!

    • Jennifer Jae Boyd

      I have the same measurements! How is this working out for you?

  • Good for you for ditching cup size letters! I’m a C/ D in RTW and had the hardest time believing I had to keep going up all the way to F in the Watson…kwim? Obvi, you ditched the letters! Genius!

    • It’s really confusing! Once you know a 32D is the same cup size as 36B its not quite as scary but sister sizing can be a real head scratcher when you start making bras….

  • Hi Heather! Would you suggest sizing down if not using foam?

    • Hmm, not necessarily. I’ve made tester suits without foam and didn’t find I needed to test down, as did some of my testers, I really suggest making a tester cup and seeing how it fits…. That said if you’re not using foam you could baste them in and make fit adjustments pretty easily since you won’t have all teh infrastructure to correct too…

  • OK, so after working with the pattern this weekend I’ve discovered this method doesn’t quite work for me. My measurements put me at a size 14 under-bust, and I should blend to 18 at the hip. Because I have 8″ difference between the under and full bust I would need to trace the cradle/bridge and #5 cup that matches the size 20. This would put me into a 50 wire which I know to be too large and would also contradict following the under-bust to chose my base size 14. So, I have decided to trace the size 16 as my base size with the cradle & bridge for the #5 cup (46 wire, same as my best fitting bras) and I am increasing the depth of the cup only through the apex. The 3 piece cup make this quite easy to do. I made a test size 16 #5 cup first to gauge how much I would need to increase and then another after I made the revisions and I **think** it’s going to work. I’m hoping to finish up my first bikini test top this week.

    • Hi Grace! It may not work for everyone, absolutely…. Boobs are the absolute trickiest thing to size/fit since there are so many ways they can differ from person to person. I think that for many people, choosing based on existing bra/underwire size may be the way to go if they feel confidant they’re wearing the right bra size!

  • Lauren

    Hi Heather,

    Just wanted to clarify – in the example of grading up if your full bust is greater than 5″ above your underbust, well, I just happen to be those measurements – or close enough to it (30.5″ underbust, 37.5″ full bust, 40″ hip). I’m just a bit confused if it’s a typo or if I’m missing something – where you say you trace the size 12/5″ pattern onto the size 10 pattern – do you mean trace the size 12/5″ cup onto the size 8 (which is the size you would select if you have a 31″ underbust – right?) Or do you go up a pattern size for another reason? Honestly not trying to nitpick, so I hope it doesn’t sound that way – writing these blog posts is quite a feat and I really appreciate it!! I am just easily confused (sleep deprived mum here). I’m also confused because here in Australia our bra sizes are like 10, 12, 14, etc, so that’s also bogging my brain down. I will also take into account what Grace said in the comments and maybe size up to make the grading less drastic between bust and hip. I love the pattern and am seriously excited to get cracking. Thank you so much!

    • Hi Lauren! Sorry if you’re confused – this is my fault. Yes, I meant size 8…. You want to trace the cup size you need onto the band/body size that corresponds with your underbust measurement!

      • Lauren

        Thank you, Heather! 🙂

  • Kohlrabi

    Hi Heather,
    Loved this tutorial, managed to grade up the cups and made a kind of a foam muslin, just cutting out the three foam pieces to see if I was going in the right direction. Now: the cup fit fine, but I took some volume out around the apex just in the middle without messing with either the seam length of the horizontal cup seam nor the seam where the cup meats the cradle. The issue I have is how to apply these changes to my main fabric cup pieces: I thought the pieces would be basically the same as the foam pieces with the added seam allowance but there seem to be some slight changes. How can I best change the main pieces to fit my changed foam pieces? Appreciate any help, thank you for this awesome pattern! Just looking at the drafting of all the gazillion cup/underbust sizes makes me dizzy!

    • I’m not gonna lie, this pattern *almost* killed me, hahaha!

      So, the foam cup pieces are the fabric pieces minus the 1/4″ seam allowance along all the inner seams (the seam where it connects to the swimsuit is left as is). What I would suggest doing is dotting in the seam allowances on your fabric pattern piece, lay your modified foam piece on top and then draw in your new modified cup line. Does that make sense?

      • Kohlrabi

        yep, it does! Thank you for your quick reply. I actually started doing that but then it looked as if you had worked some extra magic in there (I was speculating that the fabric pieces were slightly smaller so as to stretch over the cup or something else) and figured I’d just come running to you, Queen of Swimsuits and General Awesomeness!

  • Kim Kight

    Hi Heather, how do you grade up the cradle piece on the bikini top? I need to grade up from 12 to 18/5 but can’t figure out how to fit the cup while preserving the width of the 12.

    • Are you making the bikini top? The easiest solution is just to make the 18/5 and then adjust the length of the band at the center back and side seam.