This also varies with tire manufacturer (BFG seems to be pretty bad about it). Go to the manufacturer's site and they usually list the exact height of their tires somewhere.

and for what it's worth, those numbers do mean something

xxx/yy/zz

xxx is the width of the tire tread in mm, zz is the rim size and the sidewall height is (%yy)*xxx

I.E
265/50/15 is a 15" rim, 265mm wide tread, and sidewall height is .50 * 265 mm.

So if you do the math, the 265/70/17 tire equals a 31.6" tall tire.