Numerical inputs are not error trapped.
This
JavaScript skin depth calculator takes as inputs the frequency, resistivity (ρ),
and relative permeability (μ_{r}). Calculated values do not change until
the "Calculate" button is clicked.
It uses the standard textbook equation to compute the skin depth. An explanation
can be found here.
Since data
sources for materials properties typically present permeability in term of relative
permeability (μ_{r}), that is what is used for input in the calculator.
Most typical materials used for signal and power carrying cables  e.g., copper,
aluminum  have a relative permeability of 1. Materials with magnetic properties
such as steel and iron have relative permeabilities ranging from hundreds to hundreds
of thousands.
Here is a table of materials
with values.
