Hello!
I a new here, my name is Andrea 
I am trying to make a child’s hat on 16 inch, size 11 circular needles. The child’s head is 20 inches around. Is there a simple equation to know how many stitches to cast on? I have made several hats, all the wrong size! I am frustrated!
Thank you so much in advance!
Welcome to Knitting Help, Andrea!
Here’s a basic hat calculator. The cast on will vary based on the yarn weight you’re using so make a swatch and count stitches. Since you’re using a large needle I’m assuming you’re using bulky or chunky type yarn?
Doh! Forgot the link!
http://www.earthguild.com/products/knitcroc/marypat/hatcalc.htm
Uh, Jan, think you forgot to post the link to the calculator. 
Thanks guys! Jan, I will be back to look for the calculator. Thank you again!
And yes, bulky yarn. Softee brand. It has been my go to this winter.
GG, those are some cute hats.
If the child’s head is 20inches, you’d like the hat to be 1-2inches smaller. The stretch will keep the hat on comfortably. If you’re using Bernat Softee chunky (11sts/4inches) then cast on about 49-52sts. You might first check to see what gauge you’re getting but if you’re close to 11sts/4inches, you’ll be fine.
(11sts/4inches = xsts/18-19inches)