Q:

Copper weight/thickness current capacity

i have been reading about how Copper weight/thickness is important in several areas of PCB design. The thickness and width of a trace determines the amount of current (amps) the trace can carry. The thickness is also used in the calculation of trace impedance (ohms) in RF and high speed digital circuits however i have yet to find numbers or at least a simple chart that explains the amount of current per copper weight ( kind of like the 40mils per amp rule)
For example how much more current can my 40mils track carry at 1 oz vs 6 oz copper weight?
also, i have yet to se a track burn itself down by the excess current, i have been using the 40mils rule always, how can i be sure o make my pcb safe enough to carry up to 8 amps without using 320mils tracks? (lets assume im limited to 200mils)

A:

I always use this website to calculate current carrying capacity. 
http://skottanselektronik.com/

