and and
Where:
s_{%}_{IACS} 
= 
Electrical Conductivity (% IACS) 
s_{S/m} 

Electrical Conductivity (Siemen/meter) 
s_{mS/cm} 
= 
Electrical Conductivity (mSiemen/cm) 
r 
= 
Electrical Resistivity (mohmscm) 

The SI derived unit for conductivity is the Siemen per meter, but conductivity values are often reported as % IACS. IACS is an acronym for International Annealed Copper Standard, which is the material used to make traditional copper electrical wire. The conductivity of the annealed copper is 5.8 x 10^{7}S/m and copper is defined to be 100% IACS at 20°C. All other conductivity values are related back to this conductivity of annealed copper. Therefore, iron with a conductivity value of 1.044 x 10^{7} S/m, has a conductivity of approximately 18% of that of annealed copper and this is reported as 18% IACS. Conductivity values in Siemen/meter can be converted to % IACS by dividing the conductivity value by 5.8 x 10^{7}S/m (the conductivity of annealed copper) and multiplying by 100.
Often reference materials contain the "172.41" term as shown in the second equation. This term is simply arrived at by taking the inverse of the 5.8 x 10^{7}S/m copper conductivity value. This make it very easy to convert to % IACS when conductivity values are reported in mSiemens/cm because the value is simply multiplied by 172.41.
Also, conductivity and resistivity are inversely proportional. A conductor with a conductivity of 58 MegaSiemens per meter (MS/m) will have a resistivity of 1/58 ohmmeter. To convert resistivity values to % IACS conductivity values, simply divide 172.41 by the resistivity value in mohmscm.
An interesting side note is that commercially pure copper products now often have IACS conductivity values greater than 100% because processing techniques have improved since the adoption of the standard in 1913 and more impurities can now be removed from the metal.