<math>R=\frac{L\rho}{A}</math>
where L is the length of the object, and A is its cross sectional area. <math>\rho </math> is the resistivity of the material the object, and is a measure of how much the free electrons in a material are scattered by the ion lattice.
Resistance in a circuit made of purely resistive elements (i.e no [[inductors]] or [[capacitors]]) can be calculated by the [[voltage]] divided by the [[current]] (R = V / I).