I’m knitting a sweater whose pattern calls for short sleeves but I want 3/4 length sleeves. I’ve knit the back and part of one front, so I have a good idea of my actual row gauge. It was time for some calculations.
First step was to sketch out the sleeve according to the pattern.
I filled in the stitch counts and general instructions. This helped me visualize the shape of the sleeve. I’m not changing the sleeve cap, just the part below the armhole bind off. The pattern has me cast on 81 stitches and increase to 93 stitches at the widest point. I know that the width of the sleeve at my upper arm will be far too wide for just above my wrist so the next step is to figure out how many stitches I’ll want to cast on instead.
Using a very scientific method, I took the front panel and wrapped it’s bottom edge around my arm where I want my sleeve to end. I overlapped the edges to get the fit I wanted (slight positive ease) and pinched the overlap point. Then I counted how many stitches overlapped, subtracted that from the number I’d cast on and got an approximate stitch count for the cuff. The cuff edging wants an odd number of stitches so I adjusted my estimate accordingly.
Now I know my cast on, 43 stitches, and my widest part count, 93 stitches. Next I measured the sleeve of a favorite sweater to get my desired sleeve length.
Now to draw out my new sleeve. I sketched the general shape and filled in the cast on stitch count, pre-armhole shaping stitch count and the length between those. Next step is to determine my increase rate.
I need to get from 43 stitches to 93 stitches in 14″. That’s an increase of 50 stitches overall. I’l increase one stitch at each edge so that means I need 25 increase rows in 14″. My row gauge is 9.5 rows/inch. 9.5 rows x 14″ = 133 rows which I’ll round to 134 so I’m always increasing on the same side of my work. If 134 were evenly divisible by 25 I’d be set but that comes out to 5.2 rows between decrease rows.
Rounding to an even number of rows greater and less than 5.2 I find that I’ll increase ever 4th row and every 6th row some number of times each. But how many of each? I dug out some notes from a class on knitting math* I took years ago, plugged my numbers into that formula and found that I’ll increase 1 stitch each side every 4th row 8 times, then every 6th row 17 times.
The final step was to sketch in the placement of the lace motifs to make sure that they looked right on my new sleeve. If I’d ended up with a partial motif then I would have had to adjust the placement. Here’s the final sketch of the new sleeve.
Now all I have to do is knit!
* The class I took was Math for Knitters with Edie Eckman. You can find increase/decrease calculations as well as lots more knitting math in most knitting design books.