Folks often ask me how tall they should make their hats. And it always sort of strikes me as a funny question, sort of like saying ‘how long should I make the leg of my sock’ or ‘how deep should I make this cowl.’ I can tell you how long mine is, I can offer a suggestion or two, but the real answer is ‘however tall/long/deep you like it!’
The first one I ever came across was in Knitting Rules by none other than the Yarn Harlot herself. She says “the distance from the base of a person’s palm to the tip of the longest finger is equal to the height of a hat (for that particular person’s head) from cast-on edge (when placed just above the eyes) to the point where you begin decreasing (to shape the crown).”
I’ve found that works awfully well as a guideline if you want a fairly fitted hat (with no folded up brim) and you’re doing something like 8 decrease points with decreases every other row. If you’re doing something a bit more dramatic (I love to do more like 16 decrease points and decrease so fast the top of the hat goes sort of flat), you may want to do something more like ‘knit the hat until it’s a good inch taller than the person’s head from crown to base of ear, then start your decreases.’ But that means you need to know how tall someone’s head is. Luckily the lovely Woolly Wormhead had a handy chart that will help.
And of course, if you like your hats slouchy, you’re going to want even more height than that. So the ultimate truth is that you’re probably going to need to experiment a bit to find the perfect height for you. Luckily, I happen to know that 7 to 7.5 inches from the cast on gives me the perfect slightly slouchy hat for me (big head, lots of hair). But it’s totally worth experimenting to see what works for you!