how to caculate current density

Fairly simple.

First, calculate the cross sectional area of the copper conductor. (You will need to calculate the total for each strand in the wire if it's not a solid wire or busbar)

Second, find out (measure) the current flowing through the copper conductor.

Finally divide the current by the cross sectional area and you will have a value in "Amps per square milimeter" (Or per square inch if you are in that other part of the world). That value is the current density in the conductor.

My University professor would insist that some of the terms above be a little more precise like "perpendicular cross section", but I don't think he's going to be looking at this site.

There are some other special conditions that happen at extremely high frequency where the current tends to flow in only the outside skin of the conductor, but I'm guessing from the question that this complication is probably not necessary to consider.