Tire size math

[magilla]

Hey, guys, I've lost my formula for figuring out section width, overall tire height, & tread width based on size descriptions, i.e., how to figure the tire height from, say.....315/35/17. I realize that each tire manufacture's numbering system might differ slightly from others, but they're all fairly close. My memory's not as sharp as it used to be. Any help? Also, my memory's not as sharp as it used to be.