What unit is resistance measured in?

Resistance is measured in ohms, symbolized by the Greek letter Ω. The ohm is the SI derived unit used to describe how strongly a component or material opposes the flow of electric current.

The unit is named after German physicist Georg Ohm (1789–1854), whose work connected voltage, current, and resistance. Ohm’s law expresses this relationship as R = V/I: one ohm is the resistance that allows a current of one ampere when a voltage of one volt is applied.

Electrical resistance depends on factors including a material’s composition, its length, its thickness, and its temperature. A long, thin metal wire generally has more resistance than a short, thick wire made from the same material. Resistors are deliberately manufactured to provide particular resistance values in circuits.

A common mix-up is confusing resistance with conductance. Resistance is measured in ohms (Ω), while conductance—the ease with which current flows—is measured in siemens (S), the reciprocal of the ohm. Larger values such as kilohms and megohms represent 1,000 and 1,000,000 ohms respectively.