Pattern/yarn weight changes

I’d like to knit a slouchy type hat pattern but downsize the yarn weight from bulky to worsted. Is there any formula or rule of thumb to calculate the needle size and number of initial cast-on pattern stitches to decrease.

You can do a swatch and figure out your stitches per inch and calculate it with your head size. If it’s other than a stockinette pattern you’d have to adjust based on that as well so the patter would come out correctly.

There are a lot of patterns with different weight yarns though. I’d probably just get one that suits my yarn.

Yes, translate the pattern stitches into inches, then take your sts/inch times the measurement and that would be the number you need to cast on. Though there are hundreds of patterns for worsted weight, you may be able to find something similar or even the same.