http://www.cvel.clemson.edu/emc/calcula ... index.html
This calculator can be used to determine the AC or DC resistance of common power/signal conductor configurations. DC resistances are calculated using the formula R= length / (conductivity x cross sectional area). AC resistances are calculated using the formula R = length / (conductivity x skin depth x cross sectional perimeter) when the skin depth is small relative to the dimensions of the cross section. Note that AC resistance calculations assume the current is uniformly distributed around the perimeter of the conductor. In situations where this assumption is not valid (e.g. a wide trace very close to a ground plane), the AC resistance will be higher than the calculated value.