OK! I’ll admit it. I’m one of those odd ducks who likes math. (I’m know I’m not alone in the knitting world; there are lots of knitters who have math, IT or science jobs / backgrounds!)
Patterns are written for a specific stitch and row gauge. For shawls and many other accessories, it doesn’t really matter that much other than the fact that you might run out of yarn. For many sweaters, gauge matters a great deal. For the ‘boxy style’ sweaters I love to knit and wear, not so much but I still do some math to make sure I’m getting an amount of ease that’s consistent with my desired fit.
So I swatch. And I to measure my swatch before (if I remember) and after I wash and block it. *In this case, blocking is simply laying it out to dry naturally. It’s not a lace shawl!
If I’m lucky enough to match gauge exactly, I pick my sweater size and cast on. Usually, I’m not so lucky. If I like my fabric, it’s time for sweater math. (Amy Herzog has a great YouTube video on evaluating your swatch to determine if you’ve got a good fabric for a sweater.)
Now for the math. It’s not hard. And… it’s worth learning how to do it. I’m going to use Pistachio Saffron, a free Knitty pattern designed by Carol Feller for my example.
Calculating gauge:
My blocked swatch measurements: 22 stitches over 4 3/16 inches; 33 rows over 4 1/4 inches. Calculate the stitch gauge over 4 inches by dividing the stitch count by the distance and multiplying by 4. 22 / 4.1875 x 4 = 21.01 stitches over 4 inches.
Do the same to calculate row gauge. 33 / 4.25 x 4 = 31.06 rows over 4 inches.
My gauge vs. the pattern:
Stitch count: 21.01 vs. 22. This is 1 stitch every four inches. Doesn’t seem like very much but I need to so some simple math to see if I’m right!
Row count: 31.06 vs 31. I’m fine here. But… I’m short so I’ll have to do some math for body length, rate of increase for a-line shaping, arm length and rate of decrease for the sleeves. (I’m not going to cover that in this post.)
The 4th size has a 48″ bust circumference which is about right for me for a very relaxed sweater. But… I’m not getting gauge. What size do I need to knit to create something close to that?
And… now for the sweater math!!!!
Because I have fewer stitches per 4 inches, I’ll get a bigger sweater. This is good to know because it will help me make sure that my math is right. 3rd size (43.75 at bust on schematic) has 240 stitches at the bust.
- Pattern gauge: 240 stitches at bust at 22 stitches / 4 inches = 43.63 inches
- My gauge: 240 stitches at 21 stitches / 4 inches = 45.71 inches
4th size (48″ at bust on schematic; circled in red on the schematic) has 264 stitches at the bust
- Pattern gauge: 264 stitches at 22 / 4 inches = 48 inches
- My gauge: 264 stitches at 21 stitches / 4 inches = 50.26″ circumference.
The pattern states that the model is wearing a sweater with 5″ positive ease. If I had just assumed that my gauge was close enough, I’d end up with a sweater with too much ease. Thanks to some simple knitting math, I figured out I should knit the 3rd size instead!
Useful Knitbits 
thank you for your math help and gauge talk :). it’s crazy to imagine just 1st off makes for a 2.25 inch difference. i used to do the math for things like that but it lied to me as did my swatches so i stopped lol.
Thanks! It’s does seem like so little but sure adds up over a sweater circumstance.
Thanks Robbie! I think this will help me go in the right direction for my next sweater, especially since the designer’s gauge is making the fabric much looser than I like for a sweater.
Thanks. If the designer deliberately made a loose fabric, the drape may be very different. That may or may not be a good thing. Be sure to check out Amy Herzog’s video. Don’t forget that you’ll need more yarn! 😉
I just cannot figure out how you’re getting your answers here. What am I missing? 3rd size: 43.75×21.01/4=229.797. Where are you finding that equates to 45.8” circumference??
4th size: 48×21.01/4=252.12. How is that 50.25”.
I know there’s an easy explanation but I’m not seeing it. Help. I do do want to learn this!
I’m so sorry. I just saw your question. I need to pay more attention to my blog!
Clearly some of the stuff in my example wasn’t clear enough. I rewrote the math portion to include the number of stitches for each size. I hope that it’s clearer now.
Thanks so much for letting me know that it wasn’t clear.