The polydispersity index (PDI) is a measure of the heterogeneity of particle sizes in the sample and is derived from fitting the autocorrelation function. 

The polydispersity of a sample is the standard deviation of the distribution divided by the mean radius. To obtain the polydispersity index, this value is squared. The PDI calculation is specified in the ISO 22412 on DLS measurements and for the cumulants analysis defined as:

where m₂ is and G is the mean decay rate proportional to the mean translational diffusion coefficient D which is used to calculate rH. 

For the size distribution model, PR.Panta Control reports the hydrodynamic radii and PDI for up to three discreet peaks in the tabulated results. The PDI of each size distribution peak is defined as:

where is the intensity-weighted average radius of the peak, the radius i, and the intensity-weighted amount of particles with size .

Since the calculation of the PDI differs between the two fitting models, the output PDI values may also differ. In PR.Panta Control the PDI is automatically calculated for both models for each acquisition.