Resistivity is to resistance as density is to mass.

A long, thin copper wire and a short, thick wire made of the same grade of copper will have the same density but may have different masses. Similarly, the two wires will have different resistances but the same resistivity.

The resistance of a sample of material (e.g. a metal wire) depends on its resistivity but also its physical dimensions (length and cross-sectional area). Resistivity is a fundamental property of the material which determines how readily it resists electrical current.

Resistivity typically has the unit ohm-meters (Ω.m) and is represented by the Greek letter rho ρ (often mistaken for an italicised p by students!)

Resistance is related to resistivity by the following formula (Pouillet's law):

Resistance (Ω) = resistivity (Ω.m) x length (m) / cross-sectional area (m²)

R = ρl/A

Resistivity is defined as the resistance of a sample of 1m length and 1m² cross-sectional area (as such, resistivities of metals are tiny numbers!) and is often given by the re-arranged formula:

Resistivity (Ω.m) = resistance (Ω) x cross-sectional area (m²) / length (m)

ρ = RA/l