The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the degenerate case, electrical resistance.


An ohm is a resistance that produces a potential difference of one volt when a current of one ampere is flowing through it.

1 Ω = 1 V/A = 1 m2•kg•s–3•A–2


The ohm is named after Georg Ohm, a German physicist, who discovered the relation between voltage and current, expressed in Ohm's Law.


R is 1 ohm if V = one volt and I = 1 ampere

By definition from Ohm's Law, a device has a resistance of one ohm if a voltage of one volt causes a current of one ampere to flow (R = V/I). Alternatively and equivalently, a device that dissipates one watt of power with one ampere of current flowing through it has a resistance of one ohm (R = P / I 2).

Since 1990, the ohm has been maintained internationally using the quantum Hall effect, where a conventional value is used for the 'wikipedia:Klaus von Klitzing constant', fixed by the 18th General Conference on Weights and Measures as R{K-90} = 25812.807 Ω.

The complex number quantity impedance is a generalisation of resistance. Its real part is resistance and its imaginary part is reactance. Impedance, resistance and reactance all have units of ohms.

The symbol for the ohm is the Greek alphabet capital letter omega (letter) (Ω). If the Greek letter cannot be used, the word ohm is used instead. The various guides for the use of the International System of Units do not explicitly forbid the elision of the final "o" of some SI prefixes, although there is nothing in them to suggest that it is allowable, either. As a result, one is just about as likely to see "kilohm", "kiloohm" and even "kilo-ohm", and the same holds true for hecto-, micro-, nano-, pico-, femto-, atto-, zepto-, and yocto-. The only other SI unit to suffer from this kind of orthographic uncertainty is the ampere. In the particular case of the ohm, one even sees the "a" prefixes lose that vowel: hence megohm and gigohm. Higher prefixes are rarely used with ohm. In the other direction, milliohms (or millohms) are seen where the resistance of cables, etc., are measured.

Units of ohms, kilohms (103 Ω) and megohms (106 Ω) are used in electronic design documentation. On schematic diagrams kilohms are abbreviated "K" and megohms are abbreviated "M". Thus, 33 kilohms would be rendered as 33K, and 5.1 megohms would be 5.1M. Values less than 1K are rendered without any symbol following the number, so 680 ohms would simply be shown as 680. This does not cause confusion, because the numeric value is placed next to a schematic symbol for a resistor, and the resistor is usually identified by a reference designator, R, plus a numeric part, e.g., R12.


A measurement in ohms is the reciprocal of a measurement in siemens, the SI unit of electrical conductance. Note that 'siemens' is both singular and plural. The reciprocal of the ohm is also called the mho, from ohm written backwards.

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