Cart 0

A Better Way to Adjust the Bicep

Posted by Betsy C on



Guys, I’m not gonna lie, this was one project where I bit off more than I could chew. Every fit revisions has so much nuance that it’s going to be a lot to cover. Way back in January I wrote about how to determine the right bicep for your arm. Since it’s now many months later, I figured it would be a good thing to get back on the discussion and talk about how to adjust your bicep.

I'm a little nervous about posting this because it is an advanced/abstract process, so bear with me. Deep we go!

Let’s get the main thing out of the way:
The way you have been increasing your bicep is wrong. Sorry!
Please don’t cut your pattern to bits to do some kind of movement that resembles paper art. This is a ton of work and the results are not so pretty. If you don't know what I'm talking about this is it:

If you need to add more than 1/2” to the bicep width it’s not ok to do this. Why? Because you are trying to fit at least a size bigger bicep in the same armhole. This combination doesn’t work well together. Consider this: the armhole is pretty closely related to the bicep, so why would only the bicep get bigger? Essentially, by only adjusting the bicep you are creating a pouch for the bicep, but not addressing armhole comfort.


Also, if your bicep is bigger then it not only means that you need more fabric in width, but also in length to help cover the extra girth. It’s not a lot, but if your sleeve cap is too short you are going to get a twisted appearance to your sleeve because there is too much fabric at the underarm being pulled upwards.

This type of revision above reduces the sleeve cap height too much. Here's an example of what happens to the sleeve cap when you slash and spread for a larger bicep. Maybe this looks ok to you. But I say nay!

It's all about the armhole

Maybe you see where I am starting to go with this. The fact is that you can’t easily or correctly adjust the bicep without taking the armhole into consideration. The armhole circumference needs to increase to create a bigger hole for a bigger bicep to fit into.

Let me show you how I approach this problem:

Start out by marking the seam allowances on your pattern. It's important to establish the sew lines because these are the measurements we will need. 

Take your sleeve pattern and measure out the total cap length along the sew line. Mark this number down. 

Prep your sleeve piece by taping on extra paper for the width additions.

Take your front and back body pieces and measure each armhole curve along the sew line. Add together and mark this number down. 

So it will make sense, I think a little bit of actual measurements may show how this works, so here's our scenario: Let's say that the total armhole circumference is 20". Your sleeve cap length is 20". The bicep on the pattern is 14", but you need to get to 15 1/2", adding 1 1/2" to the total sleeve cap length. Take the 1 1/2" and divide by 2. This is 3/4". Go back to your sleeve and add the 3/4" to each side of the sleeve at the bicep. 
Now here's the tricky part, because I usually eyeball this, I would say for each half inch you increase total, drop the inseam 1/8". As I mentioned above, this is because the cap needs to get slightly higher with each size up to cover extra girth.
Measure  the new cap length along the sew line. Mark this number down. Let's say that your cap length is now 21 1/2". That means that your armhole needs to be 21 1/2" as well. 
It's time to adjust the armhole!
The first step of this is to drop the armhole level. If you only need to increase your circumference by 1/2", then just drop the armhole level 1/2" and re-measure. If you got it right on the first try, congrats! If not, then no worries because sometimes this is trial and error and you have to do try it again until the circumference is right.
How low is too low? I would not exceed 1/2" drop to the total armhole level. Why? Because in pattern grading terms, this means you need to go a size bigger on the chest width. 
To do so, start by adding 1/4" at each side seam, blending to zero at the waist. Re-check the total armhole circumference. One you hit that matching number then connect the armhole and the side seam.
Honestly, if you need to add more than 1/4" each side seam, you may have chosen the wrong size to sew because you have to increase the bicep, the sleeve cap height, the armhole level drops, and the chest--basically you are grading up at least one full size! My tip is to do the math before you start cutting to make sure you are choosing correctly. Is this part fun? no, not really unless you are into crunching numbers, but we all want a project that's going to fit and look beautiful.
Here is my general disclaimer about this. This is an advanced practice, but it's the right thing to do for your fit. It's advanced because there is no single definite answer for you. It's not even a drafting or pattern brand "thing". Each style is different and depends on the ease in the cap. This is the closest to what I can put together to explain what I do on a regular basis when I am negotiating between armhole and bicep for pattern drafting.  There is a lot of trial and error involved in figuring it out. Sometimes you can get away with making only two of the above adjustments, other times you need to do four. It completely depends on the numbers you are working with. 
If this is something you struggle with? Do you think this way will help you out?

Share this post

← Older Post Newer Post →


  • Great article!! Another suggestion that has worked so well for me……Make the sleeve 2 piece- works like a dream, especially for shirts/blouses.

    Aless on
  • So. Clear. Now. I have muslined out about four different tops re-watching the full bicep tutorial videos over and over each time and they all still got the twisted arm. I never understood why until now and it makes perfect sense. I will try your technique on the next one!

    GreenDoor on
  • Thanks!!! I purchased some shirts that were designed by someone who thinks size 24 women have stick arms. Obviously stuff from china. too much of a hassle to return so I want to adjust the sleeves and arm holes to allow for my arms. This looks just about right.

    laura on
  • This is a really useful post. I’ve made adjustments using the ‘traditional’ method and they have worked but as you point out I think that is related to the scale of the change needed. It’s always a good thing to have alternatives to try and also to question traditional thinking. Thanks for taking the time to share.

    Jacqui on
  • This is exactly what I need. I’m a UK size 14/16, but my flabby arms are more like a size 22. Even when I was younger and slimmer, I had trouble finding RTW to fit my biceps. Thank you so much.

    Gillian BC on

Leave a comment