one size fits all hat...cast on question?

[melinda9723]

I have been trying to find out if there is such a thing in knitting. I would like to know what is an average number to cast on for a hat for a child,women, and men? Something like a one size fits all in each category is what I am trying to accomplish.I am more of a crocheter than knitter and thank you for your help.
I will be using either my circular needle or dpns,have to check thier sizes again.I will be using baby yarn for baby hats and acrylic yarn for kids and adults hats.