The siemens (S) is the derived unit of electrical conductance (G) in the SI system of units. The siemens is derived from the SI quantities volt and ampere by the relationship G = I/E, where I is current (measured in amps) and E is electromotive force (measured in volts). The siemens and the ohm are reciprocal to each other, i.e. 1S = 1Ω^–1 and 1Ω = 1S^–1.

The siemens was formerly known as the mho or reciprocal ohm. It is named after Ernst von Siemens (1816–1892).