MN2002 Karri / Pattern Alterations & Adjustments / Sewalongs / Sewing Techniques / Tips & Tutorials

How to do a full bust adjustment (FBA) on princess seams // A Karri dress tutorial

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice

I’ve heard from a lot of people that they find the idea of doing a bust adjustment on princess seams horrifying. Honestly, it’s actually not that different than doing a bust adjustment on a darted bodice – but personally, i feel like you get a smoother end fit with princess seams. With the release of my new Karri dress, I knew many of you would be wondering how to tackle this adjustment, and also whether in fact you actually need one. But never fear! Today we are going to run through everything you need to know.

How to do a full bust adjustment FBA on princess seams // shown on Megan Nielsen's Karri dress sewing pattern


Most sewing patterns, including our 0-20 (or in Karri’s case, the XS-XL range) are drafted on a B cup block and most Curve patterns like our Curve 14-30 range are drafted for a D cup. If your bust is significantly larger than the cup size a pattern is drafted for you may notice some fit issues when wearing and as a result need to adjust your pattern to better fit your bust.

When you need to add room to the bust of a pattern, that’s called a Full Bust Adjustment or FBA.


So how do you figure out if you need a bust adjustment? There are a couple of ways to find out!


You may need an adjustment if you’ve already made a muslin using your full bust measurement to choose your size, but you notice fit issues like:

  • strange drag lines, gaping or pulling around the armscye
  • the waistline being higher at the front than the back of your bodice
  • an inability to close the bodice of your dress around your bust, even though you can over the rest of your body
  • the bust apex hitting you too high


If looking at the size chart you notice that your full bust measurement is in a significantly different size bracket than your waist measurement. For example if your Bust measurements puts you at a 20 but your waist puts you at a 16 you might need an adjustment.


Sewing cup size is determined by comparing the full bust measurement to the high bust measurement in inches. This may or may not correlate with your bra cup size which is calculated using your underbust measurement, so it’s worth measuring yourself rather than working off your expectations of your cup size.

Full bust – High bust = cup size

The number of inches between your full bust and high bust measurement will determine your cup size.

A cup = 1” difference
B cup = 2” difference
C cup = 3” difference

So in order to determine if you need an adjustment you can measure your high bust and compare it to your full bust. If the difference is significantly larger than the pattern is drafted for you’ll likely need to do an adjustment.


So now that you know whether you need to do a bust adjustment, we need to figure out how much of an adjustment to make and what size to cut out of the pattern!

There are two easy ways to calculate the size of your bust adjustment.

The first is using the high bust measurement. After measuring your high bust add on 2” (if using a pattern drafted for a B cup like Karri). We’ll call this your equivalent bust for purpose of explanation. Choose your pattern size based on this “equivalent bust” measurement. Your full bust measurement less the calculated equivalent bust measurement is the amount of your FBA.

For example if your full bust measurement is 42” and your high bust measurement is 38”. Then your equivalent bust measurement would be 38” + 2” = 40”. This means for the Karri pattern, you would cut out a size L (or a size 14 in our 0-20 size range patterns). The difference between your full bust and the equivalent bust is 2” so the amount of your bust adjustment is 2” total. We would then halve this number to determine the adjustment to the half pattern, and would adjust the front pattern piece by 1” (2.5cm).

The second method means comparing the different size bands of your body to the sizing chart. If your waist measures as a size L but your full bust measures at a size XL then you would cut a size L and make your adjustments on that. The size of the adjustment would be the difference between the bust measurements of these two sizes. So in the example we have been using the difference between a size L and size M bust is 2”, so your total full bust adjustment would be 2”. We would then halve this number to determine the adjustment to the half pattern, and would adjust the front pattern piece by 1” (2.5cm).

So now we know if we need to adjust, as well as the amount we need to adjust by, we can get started on how to do it!

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | Mark Seam Allowances, Bust Point & Slash Lines

Now the fun part – lets draw some crazy lines all over our patterns. Kidding. They’re not crazy, they have very specific purposes, so let’s run through them now.

  1. First thing’s first, measure and mark your armscye and CF bust curve seam allowances – 5/8″ (1.5cm) from the edges of the Side Bodice pattern piece.
  2. Next it’s time to pay attention to the notch on the CF bust curve of the pattern piece. On Karri this conveniently corresponds to the bust apex. Mark a point in line with this notch along the seam allowances we just marked, 5/8″ (1.5cm) from the pattern edge. Working from this point, rather than the edge of the pattern means our adjustment will be made at the sewn edge of the bodice panel, not in it’s seam allowances.
  3. Now it’s time to draw your slash lines! Draw Line 1 from the bust point you marked, across the pattern piece to the side seam edge. The line should be perpendicular to the grainline.
  4. Line 2 will be a line from the bust point to your armsyce, about 1/3 of the way in. Conveniently, the notch on the armsyce are pretty much 1/3 of the way up. Mark a point 5/8″ (1.5cm) in from the armsyce edge, where Line 2 intersects the seam allowances you’ve drawn.
  5. Draw Line 3 down from your bust point, straight down to the waist edge. This line should be parallel to the grainline marking on the pattern. Unfortunately this line has ended up in the seam.
  6. Finally, draw Line 4, a short line perpendicular to Line 3. This line should hit the Line 3 and edge of the pattern.

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | Cutting The Slash Lines

The next step for our FBA will be to cut our slash lines and spread them by the amount we need to adjust by.

Cut Line 3 from the bottom to the centre bust point, then along Line 2 till you reach the point you marked near the armsyce along the seam allowances, making sure not to cut completely through the pattern paper at this point. Next cut in the opposite direction through the seam allowances, along Line 2 from the pattern edge towards the seamline point you just stopped at. Be careful again not to cut all the way through – this tiny little section that we have left will act as a hinge so that the armscye length will stay the same and will still fit perfectly with our sleeve.

Next cut Line 1 from the side seam towards the centre bust point. Do not cut completely through the pattern paper at the center bust. Once again, we want to leave a bit of paper to use as a hinge.

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | Spread & Secure Slash Lines

Spread the centre slash line (Line 3) by the size of your bust adjustment. Make sure you keep the slash lines parallel (ie keep the distance the same all the way down). For this example I’m going to use an adjustment of 1″ (2.5cm) as calculated above.

Now we need to fix the waistline as the centre front is now slightly shorter. Cut along Line 4, cutting completely through – then move it down till it is in line with the new waistline. I like to use my ruler to keep it straight.

Put some pattern paper underneath, and tape everything together!!

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | Pivoting Out The Dart

Now we have made our adjustment, but we’ve added a dart at the side seam, which we don’t want. So we need to eliminate it, but keep the extra ease.

Extend Line 1 all the way to the centre front of the pattern to create Line 5.

Cut from the centre front bust edge of the pattern along the newly drawn Line 5, and then from the other side, if it was secured to your paper underneath, cut open the dart along Line 1. Be careful to leave a small amount of paper between these two lines to act as a hinge.

Now you can close the side seam dart by bringing the two edges of Line 1 back together. Tape in place to secure. You’ve now moved the dart excess to the centre front!

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | Lengthening The Front Panel - Measuring & Cutting Slash Lines

Having a look at our adjusted pattern piece you can see that we have made the side front pattern piece longer. This means we will also need to lengthen the centre front pattern pieces.

On a standard princess seam dress, the adjustment would look like this. Measure the additional length you added to the side front panel, and transfer these to the centre front pattern piece. First mark your cut lines, then spread them by the appropriate amounts, tape extra paper in the gaps, and cut out your new pattern.

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | Lengthening Front Panel - Measuring & Spreading Slash Lines


So far we’ve been working on an FBA method that will be suitable for most princess seam bodices. But each pattern will have it’s own design features which may affect the way you make your adjustments, so let’s talk about Karri’s!

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | Karri: Lengthening The Front Panel

Unlike other princess seam bodices, the front of the bodice of Karri is made of 3 panels. The easiest way to extend the front of the bodice, is to adjust the Front pattern piece (pattern piece 1) only. Since all of our adjustments happened below the bust notch on pattern piece 4, we can essentially ignore pattern piece 2, the front top.

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | Karri: Position Of The Horizontal Bust Seamline

However please keep in mind that though this is the simplest way to adjust the pattern, it will mean that the horizontal bust seamline in the pattern will sit higher than the bust apex. In the original pattern this line goes directly through the bust apex. You may like this look with the style line higher, but if you prefer the seam to hit across the bust apex as in the original pattern, than i would suggest a further adjustment.

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | Karri: Mark the New Bust Point

Choose a new notch location on pattern piece 4, that corresponds with the bust apex. You will need to test this on yourself, but for this example I’m using the top leg of the pivoted dart, the upper edge of Line 5.

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | Karri: Repositioning The Bust Seamline

Cut off the seam allowance at the notch on pattern piece 2. Discard the seam allowance, and tape pattern piece 2 on top of pattern piece 1 so that the notch lines up. Draw the new notch location on pattern piece 1, in this example it will be the top cut line.

Cut along this line. These are your new pattern piece 1 and 2. Don’t forget to add seam allowance to the top of pattern piece 1, and bottom of pattern piece 2.


Now one last note! In this tutorial I didn’t want to complicate things, so I have shown the standard method for an FBA. This standard approach will always result in making the waistline wider in addition to the bust. This means though, that if your bodice needs to fit back together with a lower section of garment (in Karri’s case, the skirt) you will need to remove some width from the waist of your altered side bodice pattern piece to maintain its correct size.

Megan Nielsen Patterns | How To: Full Bust Adjustment on a Princess Seam Bodice | FBA Waist Correction

You can do this by re-cutting Line 3 up to our hinge point again and pivoting the section below Line 5 so that the waist is corrected back to it’s original width along the waist seam line. Note that this is not at the bottom edge of the pattern, but 5/8″ (1.5cm) up from the bottom edge, so there may be a very small amount of overlap of the two sides of Line 3 in the bottom corner of the pattern piece.

And with that final correction, we’ve completed our Princess Seam FBA! How did you go? Was it less scary than you imagined? I hope that you’ve been filled with confidence and are feeling ready to dive into creating the your own perfect fitting princess seam bodice.

Happy sewing! x

Order the Karri dress!

Don’t have the Karri sewing pattern yet? Get it in store here! We absolutely love seeing what you make, so don’t forget to tag your creations with #MNkarri and @megannielsenpatterns if sharing on social media.

About Author

Meg is the Founder and Creative Director of Megan Nielsen Patterns, and is constantly dreaming up ideas for new sewing patterns and ways to make your sewing journey more enjoyable! She gets really excited about design details and is always trying to add way too many variations to our patterns.

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[…] Megan Nielsen demonstrates a full bust adjustment on princess seams. […]

7 years ago

Would I need to do a SBA for an A cup? If so…how??!!


[…] How to do a Full Bust Adjustment on princess seams […]


[…] How to do a Full Bust Adjustment on princess seams […]

Meg White
7 years ago

As you mention, this tutorial leaves you with lots of room added to the waist. As described, it also won’t match up to the waistline of the skirt. I’d love to see the way you actually do your FBA to fix this. I’ve looked at these two tutorials ( and that address the waist measurement by slanting the side-seam in. Does that affect the grain line? Is that a problem? How is your way different? I think you are saying that you slant it in along the princess line. Cool, this would make sense since you can see right there exactly how much to take out of the waist. Is that better for the grain line? I’m very interested in FBA information, having a rather FB myself. Thanks, Megan, for all your expertise! This pattern is super cute.