To figure out how many stitches to cast on for a hat:
1. Measure around head of which hat is to fit. (If a surprise, measure around the wrist and wrap the measuring tape around twice.)
2. Measure how many stitches per inch you get with the needles you will be using to make the hat. Or, in other words, knit up a swatch with the needles you plan to use.
3. Multiply the # of inches around the head by the number of stitches per inch. For example, if I got a person with a head circumference of 20 inches and I got 3 stitches per inch, it would look like this:

20
x 3
60

Hope the tip helps!
PS - For children, I usually use a 16" size 11 circular needle and worsted weight yarn, and I CO 48 stitches.
Hope this helps!

Good tips! I also usually subtract 1 to 1.5 inches from the head size for negative ease.

candice

(If a surprise, measure around the wrist and wrap the measuring tape around twice.)

Are you saying that twice around your wrist equals the distance around your head? I come up 10" short when I tried that out on my dd and on myself as well. Three times would leave it 3-4 inches short.

Did I misunderstand something in the formula?

i actually believe it is 3 wraps. let a little slack! dont make it too tight!