A four-point probe is an electrical measurement technique and device used to determine the sheet resistance (Rs) and bulk resistivity (ρ) of conductive or semiconducting materials without the error caused by lead and contact resistance.

The system uses four equally spaced (s), sharp metal pins aligned in a straight line to apply a controlled DC current (I) through the two outer probes (probes 1 and 4). A high-impedance voltmeter measures the resulting voltage drop (V) across the two inner probes (probes 2 and 3). Once the system captures the exact current (I) and the exact voltage (V), it computes the resistance (R = V/I). It then applies geometric correction factors based on how thick or wide the sample is to output the final sheet resistance or bulk resistivity.