How Many to Cast On? Baby Hat w/ No Pattern

I just finished a sweater, [B]sized 6 months[/B], for a friend who is pregnant and I want to make a matching [B]hat/beanie [/B]and [B]baby socks [/B]to match with the remaining yarn.

[U][B]How do I determine how many stitches to cast on?[/B][/U] I want to knit the beanie on US #3 circulars (using the Magic Loop method) since that is the sized needles used to make the sweater.

I am using Debbie Bliss “baby cashmerino” and Safran “Bomull” yarns (Sport Weight)

I appreciate any input,
Thanks You,

Here are a couple of hat calculators where you can make a hat based on your gauge and head size.