PCB Copper Thickness

When specifying the thickness of copper on printed circuit boards (PCBs) the usual metric is "ounces."  One ounce copper means the thickness you get if you flatten one ounce of copper out to one square foot.  The chart below shows the thicknesses of various ounce weights of copper in both mils (thousandths of an inch) and microns (millionths of a meter).

Copper Thickness (in Ounces)	Copper Thickness (in Mils)	Copper Thickness (in Microns)	Resistance per Square (in mOhms)
1/4 Ounce	0.34 mils	8.5 microns	2.0 mOhms
1/3 Ounce	0.44 mils	12 microns	1.5 mOhms
1/2 Ounce	0.67 mils	17 microns	1.0 mOhms
1 Ounce	1.34 mils	34 microns	0.5 mOhms
2 Ounces	2.68 mils	68 microns	0.25 mOhms
3 Ounces	4.02 mils	102 microns	0.167 mOhms
4 Ounces	5.36 mils	136 microns	0.125 mOhms

The table also shows the approximate "resistance per square," which is the edge to edge sheet resistance of any size square area of that weight of copper.  This is very useful for quickly calculating the DC resistance of a PCB trace by decomposing it into a series of squares.  For example, a trace 10,000 mils (10 inches) long and 10 mils wide is made up of 1000 squares, each 10mils x 10mils, in series.  If that trace is 1/2 ounce copper, the DC resistance would be 1000 squares x 1.0 mOhms / square = 1000 mOhms, or 1 Ohm.  If you are running high current through that trace, 1 Ohm can cause a substantial voltage drop.