A full bust adjustment (FBA) is one of the MOST common adjustments that a sewist will need to make. Traditionally, patterns have been made for a 'B cup.' This is not a bra cup size, as I'll get to in a minute, but rather that there is a 2" difference between the high bust and full bust measurement.
If you'd rather watch me do the adjustment on paper, you can check out this YouTube Video over on Rivet's channel.
Many of our patterns have either cup sizing or a FBA (full bust adjustment) pattern piece.
Your sewing cup size is NOT your regular bra size. To determine your sewing cup size, you will want to measure your High Bust and your Full Bust wearing the bra that you intend to wear with your final garment.
SUBTRACT YOUR HIGH BUST SIZE FROM YOUR FULL BUST SIZE. The difference is your sewing cup size.
1” difference = A cup
2” difference = B cup
3” difference = C cup
4” difference = D cup
5” difference = E cup
6” difference = F cup
Rivet’s regular bodice includes cup sizes A-C. The FBA bodice includes cup sizes D-F.
Sometimes, the FBA piece just isn't enough. For example, let's say you have a high bust measurement of 42 and a full bust measurement of 48. While you are still within the 6" difference, your full bust measurement is 1" larger than the FBA piece can accommodate. The FBA piece is meant to help a majority of people, but it's not a one size fits all solution.
If you are on the higher end of the cup sizing, have a higher cup size than drafted for, or simply want a more accurate bodice for yourself, it would be highly beneficial to you to learn to do your own Full Bust Adjustment.
Ready to do this? Let's go! First step is locate the bust point. This is the fullest part of the bust, or the 'apex'. It's typically where the nipple is, but not always. You can get a pretty good idea by holding the pattern up to your body and marking it that way. You are also going to draw in your seam allowance through the armscye.
We're now going to use the apex to start drawing adjustment lines. The first line you will draw is a vertical line from the apex to the hem. Next, draw a horizontal line from the apex to the side seam.
Now, you'll draw a third line from the apex to the armscye. You're aiming to hit about 1/3 of the way up from the bottom of the armscye. Lastly, draw a horizontal line from your first line to the center front, about 2" above the waistline.
Before we get to the actual adjusting, we need to make a hinge. This will make the pattern pieces move smoother. Cut to, but not through, the seam allowance line at the armscye to make the hinge.
Cut the vertical line from the hem up to the apex. Once you've reached the apex, pivot, and cut out to the line that intersects the armscye. Cut as close to the seam allowance line as you can without snipping through where you made the hinge cut.
Next, you're going to cut the horizontal line from side seam to apex. When you reach the apex, don't snip through it. You want to leave a small bit attached to make a second hinge. Look at that! We've got moving parts. For a FBA, just keep scrolling. For an SBA, head on down the page awhile.
To begin, you'll need to spread the vertical line apart by the amount you wish to add. Remember, you're only adding half of what you need because this is only half of your bodice. Our FBA piece is a 1" FBA, adding an extra 2" in total. You can see that, the more you add, the more distorted the armscye can get. There are other types of FBAs that may work better for you. You can see where you spread the piece apart at the side seam, you would be creating a dart (the gray area). In this particular tutorial, we won't be adding a dart, but it's good to note.
Cut the horizontal line you made just above the waist.
When we increase the cup size, we also add length to the center front. Move the center bottom piece down to match the side seam piece. Place a piece of paper behind your pattern, and we can start putting this back together!
Tape the center bottom piece to the paper beneath it. This piece will not be moving again. Draw the line from the apex to the waist on to the paper. Make sure that your distance is still correct. This will give you a reference line in the next steps.
Tape the cut back together at the side seam. Now cut the opposite direction, from the apex to the side seam, leaving a little hinge at the side seam in the tape.
Bring the bottom side seam piece up at the apex to line back up with the reference line. Go ahead and tape those pieces in place. Now, trace all around the new piece. At the waist line, just draw a gentle line. We'll be smoothing out lines and bringing in the waist in the next step.
Last couple things before you cut out your new piece. Making a FBA also makes the waist wider, so we need to bring it back in. First, put your original pattern piece back together (or print/project a new one), and overlay it with your new piece, matching up the necklines. Mark the width of the waist (green line). Now we're going to redraw the side seam and waist. Gently curve the bust line to get rid of the point. Under the bust, curve the side seam in to match the top of the green line. Lastly, draw the waistline (or hemline if you are making a top/dress), back to the center front.
And that's it! One thing to note: when you curve the bust, you may add just a smidge of length. It's no big deal for knits! Simple pin and the armscye and waistline, and ease the bust of the front to the back. Perfect!
Happy Making!
Rachelle