The term Ohms-cm (Ohms centimeter) refers to the measurement of the "bulk" or "volume" resistivity of a semi-conductive material. Ohms-cm is used for measuring the conductivity of a three dimensional material such as a silicon ingot or a thick layer of a material. The term "Ohms-per-square" is used when measuring sheet resistance, i.e., the resistance value of a thin layer of a semi-conductive material. To calculate Ohms-cm using the Jandel RM2 Test Unit, one needs to know the thickness of the wafer (if it is a homogeneous material) or the thickness of the top layer that's being measured, to be able to calculate Ohms-cm. The equations for calculating bulk resistivity are different from those used to calculate sheet resistance, however, if one already knows the sheet resistance, bulk resistivity can be calculated by multiplying the sheet resistance in Ohms-per-square by the thickness of the material in centimeters.