Although I used worsted weight for the above, calculations are included for gauges between 6 st./in. (fingering) and 3 st./in. (heavy worsted) in 1x1 ribbing. I have done the heavy lifting for you as far as math, though you will have to do a teensy bit.
I hope you get a kick out of this pattern. Something about that double-decreased X on the top of a hat gives me the happies. A clever crown is a joy forever.
When it comes to yarn quantity, you will have to guesstimate. I usually find that 4 oz of wool is enough for the largest adult hat, though if you make it extra long you might need a titch more. The two brown 20" boys' hats pictured took about 100 yards of worsted weight; the green 22" boys' hat took about 150, and the extra-long adult-sized hat (pink, white, brown) took almost all 4 oz of fingering weight yarn, held double. Experiment and let me know how it works out for you in the comments.