I like knitting hats, especially improvised hats. But then I don’t know how many sts to cast on or when I should begin the crown shaping.
So here you are, a spreadsheet with all kinds of gauges (seriously, from 5 to 50sts/10cm) and a wide range of sizes (from 30 to 100cm circumference).
Find your gauge and size and just cast on!
There are two sheets in this document: the first one for cast-on number calculation, with an editable cell for negative ease and a second one with instructions for a rounded crown shaping.
Note that this is not a step-by-step pattern, but a set of numbers to help you knit a fitting hat for any head using your own favourite stitch patterns.

Update March 2019: added more head circumferences (70-100cm). After all not everybody needs a hat the size of their head, some people have HAIR.