The resistance of a given material increases with length, but decreases with increasing cross-sectional area. From the above equations, resistivity has SI units of ohm⋅metre. Other units like ohm⋅cm or ohm⋅inch are also sometimes used.
The formula R =
ho ell / A can be used to intuitively understand the meaning of a resistivity value. For example, if A=1 ext{m}^2 and ell=1 ext{m} (forming a cube with perfectly conductive contacts on opposite faces), then the resistance of this element in ohms is numerically equal to the resistivity of the material it is made of in ohm-meters. Likewise, a 1 ohm⋅cm material would have a resistance of 1 ohm if contacted on opposite faces of a 1 cm×1 cm×1 cm cube.
Conductivity σ (Greek: sigma) is defined as the inverse of resistivity: