The number average molecular weight for a polymer sample is given by the following equation:
where:<br />
 is a calibration constant,<br />
 is the bridge imbalance output voltage,<br />
 is the polymer-solvent solution concentration
It is necessary to calibrate a vapor phase osmometer and it is important to note that K is found for a particular solvent, operational temperature, and type of commercial apparatus. A calibration can be carried out using a standard of known molecular weight. Some possible solvents for VPO include toluene, tetrahydrofuran, or chloroform. Once the experiment is performed, concentration and output voltage data can be graphed on a plot of (ΔV/c) versus c. The plot can be extrapolated to the y-axis in order to obtain the limit of (ΔV/c) as c approaches zero. The equation above can then be used to calculate K.