I have been talking a bit about actual Japanese clothing like haori here on the blog recently, but the "kimono" style sleeve in western fashion is really nothing like a Japanese kimono sleeve! Hence why I am putting quotes around the word kimono above, because the word kimono gets rather over used these days for things that bear little resemblance to actual kimono (those flimsy chiffon shawl/cardigan things in every shop these days for example are being labeled as "kimono"...). Still, this style of sleeve, which was very common in the mid 20th century, is one of my favorites and I use it in my designs/sewing all the time!
You don't have to buy a new sewing pattern to use a kimono style sleeve for your next project-- they are super easy to add to an existing bodice pattern! So pull out your favorite dress bodice, button down shirt, or even knit top pattern and give this pattern hack a try. (Though as always I suggest you try any new pattern in muslin or a mock up first to test it out and make any fitting adjustments!)
Some more notes before I continue: the patterns I use below are self drafted and I use them over and over so these traced versions have seam allowance already included, and you will notice I don't add any additional seam allowance as I know it is already there (does that make any sense?). If the pattern you start with doesn't have seam allowance do make sure to add some. Also, this isn't some official way to add a kimono sleeve, this is just a method that has always worked for me.
Start by tracing the front of your preferred bodice pattern onto a fresh sheet of paper (this gridded paper is called alphanumeric paper and comes on a roll, I highly recommend it for pattern drafting.)
First add 1/4" to the top of the arm side edge of the shoulder line and redraw the shoulder. See what I mean in red on the illustration below.
Next measure 2 inches down from the underarm at the side seam and put a mark, then put another mark 1/2" further. Draw a line 1 1/2" out from the first (the 2") mark like above.
Draw a curve between the two marks from the last step. As you can see my sleeve ends up starting directly above the side dart on this particular pattern, but this has never caused me any trouble.
Next measure 5" out from the first point you altered at the shoulder line. You can make the sleeves longer than this too of course. I am also factoring in that at least 1/2" inch (or more depending on if you line the bodice or simply hem the sleeves) will come off the sleeve hem in seam allowance eventually when this pattern is sewn up.
Then connect the two points between B and C on our little diagrams. Like magic you have added a kimono sleeve to your bodice front!
Now for the back pattern piece start by measuring just how long that last line you drew for the front (from B to C) ended up being in the end. For my pattern this measurement was 9 3/4".
Next add the same 1/4" to the outer shoulder to the back too and redraw the shoulder line like you did for the front. Then add the same 5 inch extension to the shoulder line too.
The easiest way to progress from here is to have the front pattern piece cut out and to lay it on top of the front matching the shoulders and sides up and then tracing the sleeve you drew for the front. Or you can measure down 9 3/4" (well that was my measurement, but use whatever your measurement was) from the 5" out point on the extended shoulder and connect it to the side seam using the same curved 1 1/2" line like you drew on the front. I hope some of that made at least a bit of sense...sorry!
Again I'll repeat that you should test your lovely new sleeve by making a muslin before you cut into nice project fabric ;) With a muslin/mock-up you can check both the fit and adjust the length of the sleeve if you need to.
I hope this will help someone take the plunge and have a little more fun editing patterns, go ahead and dip your toes into pattern drafting, it's super rewarding! As I use this sleeve style in my own sewing all the time it was well time I explained how I manage it :)