A while back, I described several ways of altering Silver Streak in order to make it into a shorter cardigan. At that point, I simply discussed how one might go about modifying the length, but didn’t explain how to calculate even spaced decreases or increases.
Knowing how to adjust length is you first tool towards custom fitting your knitwear – patterns are written with an average size in mind and while the circumference may fit you with nary an adjustment, you’ll want to adjust some length if you’re shorter or taller than 5’6″.
Before you can begin to calculate your increases or decreases (the formula is the same for both), you need to translate your length into rows. Let’s take a simple sleeve as our example: we intend for it be 19″ long and our row gauge is 7.5sts/1″, therefore we’ll need 142 rows to complete it (it really was 142.5 rows but we knitters prefer even numbers). We need to work even for about an half an inch before binding off, so lets take off 6 rows to bring down the number of rows to 136. This will be our dividend or ‘A’.
This sleeve begins with 60 sts at the wrist and increases to 88 sts at the bicep for a difference of 28 sts. Since the increases will be worked in pairs, however, we need to divide this number in two to obtain the number by which to increase – 14. This will be our divisor or ‘B’.
Easy enough so far. The next part is what I can never remember by myself – others keep notes for working the kitchener stitch, I keep notes on this. You’ll probably find it helpful to add tags such as I do here (A, B, C, etc) if you’re anything like me. Once you understand the concept, the illustration above will suffice.
First, divide A by B: 136/14=9.7 (‘C’)
Had C been a whole number(such as it would have been had you started with 126 instead of 136), you’d have no need for further calculations and could simply increase every 9 rows. I know what you’re thinking: every 9 rows?!? Don’t worry, we’ll talk about how to obtain nice, even numbers in a minute.
Second, multiply B by C: 14*9=126 (‘D’)
Third, subtract D from A: 136126=10 (‘E’)
Fourth, subtract E from B: 1410=4 (‘F’)
Finally, here are our results:
Increase on every 10(C+1)rows 10(E)times
Increase on every 9(C)rows 4(F)times
Again, the result would have us work increases every 9 rows. In order to obtain increases only on right side rows, we need to run the calculation in a somewhat modified manner by halving A and doubling ‘C+1′ and ‘C’ in the results:
136/2=68
68/14=4.8 (‘C’)
14*4=56 (‘D’)
6856=12 (‘E’)
1412=2 (‘F’)
Increase on every 5(C+1)rows 12(E)times
Increase on every 4(C)rows 2(F)times
Double the results from ‘C+1′ and ‘C’, and we end up with ‘increase on every 10th row 12 times and on every foll 8th row 2 times’
That’s it. Next time you’re planning a sweater, take a few minutes to compare its schematic against a sweater that fits you well and make adjustments as needed. It only takes a few minutes and you’ll soon be custom fitting every sweater you knit.
15 Comments

I love technical posts! Filing this one for future reference thanks tons.

OMG – that is so totally over my head right now!! I would need at least a couple of drinks to make those numbers stop scaring me. LOL

Whew. Interesting. I follow the concepts until you get to multiplying b*c, then you lose me conceptually. I’d love to hear more about the why’s of these calculations.

OMG, that was just some crazy math. I was reading very diligently in the beginning and then it got to the part about biceps and dividing by two for decreases and then *flop*! My brain died! I think I just need to be in the right head space. I’m was never a great math student in high school, unless I fully concentrated. So that was my problem there!
I have been neglecting to start Silver Streak and now that it’s almost spring, I’ll have to have it ready for next fall instead!
Thanks for your tutorials, I really do appreciate them.

Hello my dear. My name is Marly and I am the host of the Yarn Thing podcast. I have had a lot of request from my listeners to see if you would be interested in doing an interview with me on the show. So, I thought I would write to you and see if you would be interested.
I do book reviews as well if you want to connect me with your publishing company. Up to you Drop me a line in email and let me know what you think, okay?
xoxox
Marly aka Yarn Thing
http://knitthing.mypodcast.com 
That is a terrifying looking equation, but I can’t wait to try it sometime!

It’s not as bad as it looks – really, it’s one of those things where you just need to follow along and not necessarily understand it. Not being a mathematician, it’s outside of my scope to explain it but found it to work whenever I’ve double checked the results.

Whew I’m sure I’m going to thank you for posting this, even though I’m letting my eyes glaze over at the moment.
p.s. Go for the podcast and put up a link afterwards! 
Just Say No to Math and Stuff. (Thank You).

Aaaw…poor V getting picked on for the math…LOL It too is lost on me BUT I totally get what you say about using the math and not necessarily “understand” it. I spend most of my life like that! ;oP
Hope to see you at pub night! 
I love your blog, your book, your patterns!! Thank you for such great knitting help.

Wonderful!
But I still like it better when you do the math… 
This post brought back terrifying visions of high school. Yet another reason that being 50 something isn’t so bad.

OK, I think I get it. I will have to give this a try the next time I need to modify a pattern. Thanks for your explanation of this porcess.
Take care, Joanne

Thanks for this formula, it is just exactly what I was wanting!