I’ve been looking at adult hat patterns on round needles and notice that instructions vary from casting on 42 stitches to casting on 82 stitches. Why is there such a big difference for adult hats? How do I know which to choose for a 19 year old average size girl?
It sounds like the patterns are in different gauges. That is the only thing I can think of that would account for such a large difference in number of stitches.
I bet that the pattern with the small # of cast on stitches uses a very bulky yarn and the one with the larger # of cast on stitches uses a thinner yarn. I even knit an adult hat once that had over 100 sts cast on, but it was knit with fingering weight yarn on size US2 needles.
Choose an adult hat pattern that you like, and trust that pattern - making sure that you do a gauge swatch and use an appropriate yarn and needle size for the pattern, and it will be just fine!
If you’re using worsted weight and No. 7 or 8 needles, 88 or so stitches is usually fine for a young lady. 22" is about average for head circumference for women. You can actually measure her head if you like. You might do a gauge swatch once you choose your yarn just to make sure. Multiply the number of stitches per inch times the number of inches around her head, and there you go.