Can anyone explain how to correctly gauge a pattern? I was pretty far into a gorgeous alpaca hat when I noticed that it was big enough to be a small sweater. I thought I did everything right but obviously not. I knit a gauge swatch and everything!
My head is 23 inches (i know, big head). The gauge swatch was 20 stitches in stockingnette, and was 3.5 inches. I did 23/3.5=6.57. so then i did 6.57*20 stitches = `131. The pattern is a repeat of 12 so I cast on 132 stitches. I tried it on and it was huge so I pinned it and now it looks like I only needed 96 stitches. Help!!
When you did your gauge swatch, did you do it in the same pattern stitch as the hat? Did you knit the gauge swatch in the round?
Did you measure the gauge swatch over the middle 2 inches? Ideally, if the gauge is, say 20sts/4 inches you would cast on 24 or even 36sts and measure over the middle 4 inches.
The reason for this is that the edge sts are smaller than the rest of the sts and they will throw off the measurement.
Another possibility is that your gauge in the round is different from your gauge knitting flat. Under the Free Videos tab, there’s a section called Getting Started and you’ll find a video there for swatches in the round that makes this easier.
It’s too bad about your hat. I know how disappointing that can be.
What salmonmac said, and: If you go from stockinette to rib on the same size needles your rib can be much looser than you want. I either go down a couple of needle sizes or knit the rib with fewer stitches.
Are you using a pattern that you can link to? That can be helpful to people trying to help. 
You will be able to make a hat, it just sometimes takes persistence.
Thanks for all the help. I didn’t knit it in the pattern and didn’t know about measuring the middle stitches, so this will help a lot. The pattern is here: http://stitcheryprojects.com/2011/10/11/two-lace-baby-hats/
And don’t forget that your head might be 23", but you probably want some negative ease in your hat, so you want it to be 20"-21" in circumference, not 23". In DK weight yarn, 96-112 stitches sounds about right for that size.
(Although, your gauge definitely is off since 96 stitches should only get you a 16" hat according to the pattern.)
You were asking about an alpaca yarn in the general forum, but the yarn you identified there was bulky, not DK. Is that what you’re using?
You will be able to make a hat, it just sometimes takes persistence.