Estimating cast on stitches

I saw a pattern for a hat that want to adapt for a 3 yr old. How many stitches do I need to CO? The adult size is 80 st. And, is there a general rule on this, just to know for future patterns or sizes.:muah:

You need to know how large the head is and how many stitches per inch you will have with the yarn.

If you can measure the child’s head you can use this site to estimate hat size and CO.
http://www.mielkesfarm.com/bsic_hat.htm

Or you try this… find the average measurement
http://www.fibergypsy.com/common/children.shtml
http://www.bevscountrycottage.com/size-chart.html

Figure out your gauge … And then you can use this