Milliohm

Milliohm (symbol mΩ) is a unit of electrical resistance equal to one thousandth of an ohm. As a submultiple of the ohm, 1 mΩ = 0.001 Ω, and 1 Ω = 1000 mΩ. The milli- prefix denotes 10^-3, so milliohm is commonly used for very low resistances. The milliohm is an accepted non-SI unit for use with SI, with the symbol mΩ.

The milliohm is frequently used to specify low resistance components and measurements, such as current-sensing shunt resistors, battery internal resistance, contact resistance in connectors, and trace or wire resistances in high-current or precision contexts. It helps express values that are too large for microohms but too small for whole ohms.

Measurement in the milliohm range requires careful technique. Four-wire (Kelvin) measurement methods are used to minimize errors from test leads and contacts. Instruments such as precision ohmmeters, low-resistance testers, or dedicated shunt bridges are employed. Temperature affects resistance, so many low-value devices specify a resistance at a reference temperature and a temperature coefficient.

Notes: due to the very low magnitudes involved, tolerances are often tight, and manufacturers specify both nominal values and tolerances (for example, ±0.1% to ±1%). The symbol mΩ is preferred in technical writing; occasionally the informal term “mohm” appears, but using mΩ avoids ambiguity. See also ohm, microohm, shunt resistor, and contact resistance.