Polydispersity Index or Index of polydispersity (PDI) is defined as the ratio of the weight-average molar mass (M_{w})
When the total molecular weight of group of molecules having particular molecular weights are multiplied with their respective molecular weight the products are added and the sum is dived by the total weight of the all molecules, the result obtained is called weight average molecular weight(M_{w}). to the number-average molar mass (M_{n}).
When the total molecular weight of all the molecules of a sample is divided by the total number of molecules, the result obtained is called the number average molecular weight(M_{n}).

PDI = M_{w}/M_{n}

The weight-average molar mass is always larger than that of the number-average molar mass. If the weight-average molar mass is equal to the number-average molar mass (i.e. PDI = 1), then the sample is monodispersive (e.g. natural polymers and synthetic polymers made by anionic polymerization). If the weight-average molar mass is greater than the number-average molar mass (i.e. PDI > 1), then the sample is polydispersive. Man-made polymers are always polydispersive particles. Number-average molar mass is more sensitive to lower molecular weight samples, while Weight-average molar mass is influenced more by higher molecular weight samples.

For well-controlled synthetic polymers, PDI values are close to 1 (1.02–1.10). For some typical addition polymerizations or step-growth polymerizations, PDI values can be much higher than 1 (5 to 20).

**Monodispersive Polymers:** Monodispersive polymers are uniform polymers in which all molecules have the same degree of polymerization or relative molecular mass.

**Polydispersive Polymers:** Polydispersive polymer is non-uniform and contains polymer chains of unequal length, and so the molecular weight is not a single value - the polymer exists as a distribution of chain lengths and molecular weights.