The siemens (symbol: S) is the SI derived unit of electric conductance. It is named after the German inventor and industrialist Ernst Werner von Siemens [1], and is equivalent to the now-obsolete mho. In English, it is siemens in both singular and plural.


For a device with electrical resistance R, the conductance G is defined as


The unit siemens for the conductance G is defined by 1 S = 1 A/V = 1 A2/W = 1 kg−1•m−2•s3•A2 =1 Ω−1.

Example: The conductance of a resistor with resistance 6 ohms is G = 1/(6 Ω) = 0.16... S.

SI multiples[]

Multiple Name Symbol Multiple Name Symbol
100 siemens S
10¹ decasiemens daS 10−1 decisiemens dS
10² hectosiemens hS 10−2 centisiemens cS
103 kilosiemens kS 10−3 millisiemens mS
106 megasiemens MS 10−6 microsiemens µS
109 gigasiemens GS 10−9 nanosiemens nS
1012 terasiemens TS 10 −12 picosiemens pS
1015 petasiemens PS 10−15 femtosiemens fS
1018 exasiemens ES 10−18 attosiemens aS
1021 zettasiemens ZS 10−21 zeptosiemens zS
1024 yottasiemens YS 1024 yoctosiemens yS


The siemens is equivalent to the now obsolete mho unit, which was derived from spelling ohm backwards and written with an upside-down capital Greek alphabet [2] letter Omega: , Unicode symbol U+2127 (℧). The term siemens, as it is an SI unit, is used universally in science, while mho is still used primarily in electronic applications. The upside down ohm symbol, while not an official SI unit, has the advantage of being less likely to be confused with a variable than the letter S when doing algebraic calculations by hand, where the usual typographical distinctions (such as italic for variables and roman for unit names) are difficult to maintain. Furthermore, in some industries (like electronics) it is common to incorrectly write the symbol for second [3] as S instead of s, causing potential confusion.

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