November 17, 2020

How to Do an FBA on a Princess Seamed Bodice

The full bust adjustment is an essential technique to know if you have a large bust, but it doesn’t need to be hard! Here’s how to do an FBA on a princess seamed bodice – there are quite a few steps, but take them one by one and you’ll get a great result.

how to do an FBA on a princess seamed bodice

Here at Cashmerette, we include three cup sizes (C/D, E/F, G/H) with all of our patterns so that you can get a great fit right out of the packet, and most people don’t have to do an FBA. But if you’re using a pattern that is drafted with a smaller cup size than what you need, or you are significantly bigger than Cashmerette cup sizes, you’ll likely need to do an FBA.

In this tutorial, we’re showing how to do a full bust adjustment on a pattern that has princess seams – it works for armhole or shoulder princess seams. If you’re new to FBAs, check out this guide to figure out whether you need to make this adjustment, and how much circumference you’ll need to add.

Let’s get started!

Preparing for an FBA

Here’s what you need on hand for doing your FBA: your front bodice pattern, some tracing paper (we like Swedish tracing paper, but anything that you can see through will do), a pen or pencil, a ruler, scissors, and some tape. It helps to be working on a cutting mat or a surface that you can tape things down onto temporarily—you’ll see why pretty soon.

We always recommend tracing your pattern piece before starting to cut into it. This is so that you can refer to the original piece later on if needed—once you cut into the real thing, it’s hard to go back.

How to do a princess seamed FBA

  1. We’re going to start with the side princess bodice panel. First, draw on the seam allowance around the armscye and front sides as shown. Then you need to draw three lines: the first one is line A, and it goes horizontally across the piece to the bust apex (that’s where the curve sticks out the most). Line B goes from the bust apex down to the seam allowance at the bottom. Line C goes from the apex up to the seam allowance at the top.

2. Cut up line B and C, stopping at the seam allowance. Then make a little hinge but cutting into the allowance on the other side. Swing the left section up.

3. Now cut line A, from the side seam to the end of the line, leaving a little hinge. You can now swing the bottom piece down, and then adjust all the pieces so that the vertical opening has parallel sides, and that the opening is the width you need to add for your FBA.

4. Now we need to make the piece level at the bottom again. To do that, cut across the right hand side piece at any point below the notches, and slide the bottom piece down to align the bottom. You also need to measure the gap that opened up – that’s gap A.

5. Trace your pattern piece, including all the notches, gap A, and draw in the dart that’s been created.


6. On your new piece, draw a line that goes along the bottom leg of the dart, through the top of the dart, to the edge of the piece. Now cut through that line from the side seam, all the way to the end of the dart. Snip into the line from the other side, creating a little hinge.

7. Swing the bottom piece up to close the dart (the dart legs will be on top of each other). Tape closed. When you do that, a small gap will open up on the princess seam – measure how long that is, at the seam allowance. That’s gap B.

8. Re-trace the piece, making sure you mark gap B. Your side princess piece is now done!

9. Now we have to adjust our central princess seam to match. To do that, measure the distance from the bottom of the side piece to gap A, and the distance from A to B. Measure the same distances from the bottom of the central princess piece, and then draw horizontal lines across at the height of gaps A and B.


10. Cut across at both horizontal lines, and spread the pieces apart by the distances of gaps A and B.

11. Trace your new piece!

And you’re done! Make another muslin and admire your beautifully fitting princess seams.


Want to learn more about fitting clothes to your body?

Check out our Fitting for Curves: Pattern Adjustments for the Upper Body online workshop.

17 thoughts on “How to Do an FBA on a Princess Seamed Bodice

  1. Thank you for these very clear directions. I just finished a FBA on my princess seamed jacket. I cannot wait to make a new muslin and see how much better the fit is.

  2. Helen says:

    Hello – I want to do a FBA on a princess seam pattern with multiple panels – 4 front panel, 4 back. Which panels do i need to increase? I am thinking still start with the side piece, than the other three adjusted to match the seam lines?
    I am excited to try this technique – I have used the pivot and slide method, but then end up needing to add underarm darts.

    1. Martha Banks says:

      Ref. Jacqui Holloway question on how to do a FBA on the Harrison Blouse, there is no response. Will you provide instruction for the adjustment to the 2 panel princess seams. Thanks much.

      1. On the Harrison Shirt, the majority of the shaping is in the central princess seam (closest to the center front), so I recommend performing the FBA on that seam, with the middle panel playing the role of the side panel in this tutorial.

  3. Jo says:

    Hi, the size calculater tells me I need a 4inch FBA, I’ve tried following the method above but am really struggling with closing the massive dart and figuring out gap B….the pattern piece ends up bending. Any advice??? Is there a better way of doing a 4″ FBA?

    1. Send us an email to with your measurements and which pattern you’re making and we’ll be happy to help!

  4. Joanna says:

    Hi! Thank you for the clear tutorial, but now that I have used it to make a large enough adjustment that both a and b added lengths are below the notches and tried sewing my muslin, I see one big problem which you didn’t cover: by adding breast space I have also added waist space and my waistband won’t fit the new bodice. How do I remove the extra cm from the waist?

    1. You can simply trim the extra off the waist at the side seam – draw a line from the bust point on the side seam (where the notch is) to the waist where you need it to be, and draw a curved line

  5. One more from me 😀
    The Harrison Shirt features double princess seams. How does one go about adding an FBA there? I have checked the list of videos in the course, but it looks like that’s only about sewing the shirt, not adjusting what needs to be adjusted

    1. On the Harrison Shirt, the majority of the shaping is in the central princess seam (closest to the center front), so I recommend performing the FBA on that seam, with the middle panel playing the role of the side panel in this tutorial.

  6. Jacqui Holloway says:

    I would also like to know how to do FBA on the Harrison shirt as it has the two princess seams and I can not find any instructions anywhere. Thanks

    1. On the Harrison Shirt, the majority of the shaping is in the central princess seam (closest to the center front), so I recommend performing the FBA on that seam, with the middle panel playing the role of the side panel in this tutorial.

  7. Kaz Collins says:

    I just wanted to thank you, Jenny! This was my first time doing an FBA in a princess seam and it worked! I am so grateful for your clear instructions and illustrations

  8. Helen Spencer says:

    Thanks so much for these instructions.
    I’m making a bodice which is lined and the lining pattern is just a single piece. Could you suggest how I adjust the lining pattern piece to match my adjusted main front?

    1. Ayelet says:

      Hi Helen, it’s hard to say without knowing the details of that specific pattern, but you generally need to make the same adjustment to the lining.

      If the bodice has princess seams but the lining doesn’t, you could try making your own lining using the main pieces (after you’ve done the FBA).

      -Ayelet at Cashmerette

  9. Jennifer says:

    Thanks for the awesome, clear directions! I’m a beginner when it comes to fit/sizing and appreciate the help learning new skills. I wonder if you could help me clarify the basic math behind the FBA as I’m running into difficulties. I’m a curvy person with a small frame, so I can almost never wear a buttoned shirt (bra cup size H). My band measurement (under bust) is 30″ while my upper bust is 34″. Based on the formula: full bust (36.5) – upper bust (34) = 2.5″ (B/C cup), most tutorials seem to suggest I don’t even need FBA, which has me confused. Any tips?

    1. Hi Jennifer! Check out our sizing calculator at which also takes your typical bra size into account .We’ve found on smaller framed busty people there is often not such a big difference between high and full bust and the calculator takes that into account. Also, make sure you’re taking high bust (only) WITHOUT a bra.

Let me know what you think!