Resistivity 

We have, 

Rearranging,

If the resistance of a conductor with length 1 m and area of cross-section 1 sq. m be R Ω. Then, Resistivity, ρ = R (1 sq. m) / ( 1 m ) 

ie, ρ = R The resistivity of a substance is the resistance of a conductor of unit length and unit area of cross-section. The resistivity of a substance is a constant at fixed temperature. But, it will be different for different materials. 

The SI unit of resistivity is Ωm.