Understanding knitting dimensions

Tereza Okava wrote:if you want to know for sure, you could knit a gauge swatch. Usually the pattern will say something like your yarn does:
"gauge:  12sts x 16 rows = 4” | US Size 10 1/2 (6.5mm)"
so you get out those needles and cast on 12 stitches, knit 16 rows, and then measure. If it were really important, you might block it too, just to be super sure.
Your needles, your way of knitting, and even your stress level may affect the size of the work (I used to knit when I visited people in the hospital, and noted the dimensions varied wildly compared to if I were knitting in front of TV, for example). Swatching will help you pay attention to that.

In my reality, I do not often have time for swatching (much as I like to frog knitting).
I would just cast it on the numbers you mention and keep my measuring tape handy.
Knit a couple rows and see if the work is close to the width you want. If it's too big or small after 5 rows in the pattern (unless it's some fancier stitch) I'd rip it out and start again with a few more or fewer stitches cast on.
If you're not making something with a fancy stitch pattern you could probably just knit til the length you want it. Measure your top border, if that is a different stitch, and stop the body accordingly to add on the bottom border.


(edited to add: your math looks right, but I find that for some reason the length is always more than the calculations would indicate, and so I am always measuring.)

Jan White wrote:I don't knit, but I crochet. I find, especially if I haven't done it in a while or if I'm using a stitch I don't use all that often, the stitch size can change a lot once I get into the groove and start moving more automatically than consciously, so swatching isn't that useful for me. I'd do what Tereza suggested.