What is the polydispersity index (PDI)?

PDI is the ratio Mw/Mn. It quantifies the breadth of the molecular weight distribution in a polymer sample. A PDI of 1.0 means all polymer chains have identical molecular weights (monodisperse). A PDI above 1 means chains vary in length.

What PDI value is considered monodisperse?

Strictly monodisperse means PDI = 1.0. In practice, polymers with PDI ≤ 1.1 are considered nearly monodisperse and are common products of living polymerization techniques such as RAFT or ATRP.

Can PDI ever be less than 1?

No. By definition, Mw ≥ Mn for any real distribution, so PDI ≥ 1. A value below 1 indicates an error in the molecular weight measurements.

What are typical PDI values for common polymers?

Free-radical polymerization typically gives PDI of 1.5–2.0. Condensation polymers often approach PDI = 2.0. Anionic living polymerization can achieve PDI of 1.01–1.1. Biological proteins are monodisperse with PDI ≈ 1.0.

How is PDI measured experimentally?

PDI is most commonly determined by gel permeation chromatography (GPC), also called size exclusion chromatography (SEC), which separates polymer chains by size and calculates Mw, Mn, and PDI from the elution profile.

Is PDI the same as dispersity (D)?

Yes. The IUPAC recommended term since 2009 is dispersity, symbolized D (or Đ). PDI and D are equivalent and refer to the same quantity Mw/Mn.

Is the DLS polydispersity index the same as Mw/Mn PDI?

No. In dynamic light scattering (DLS), PDI is a dimensionless number from 0 to 1 derived from the cumulants fit of the autocorrelation function, where below 0.1 is monodisperse and above 0.7 is broadly polydisperse. This calculator computes the polymer PDI as Mw/Mn, which is ≥ 1. The two share a name but use different scales and definitions.

Polydispersity Index Calculator

Enter weight-average (Mw) and number-average (Mn) molecular weights to instantly calculate the polydispersity index (PDI) of your polymer sample.

Molecular Weight Inputs

Both values must be positive. Mw must be ≥ Mn.

Weight-average molecular weight, M_w (g/mol)

Number-average molecular weight, M_n (g/mol)

Quick Examples

Result

Enter Mw and Mn to see the PDI

PDI Interpretation Guide

PDI = 1.0 — Perfectly monodisperse (all chains identical)

1.0 < PDI ≤ 1.2 — Narrow distribution (living polymerization)

1.2 < PDI ≤ 1.8 — Moderate distribution (typical synthetic polymer)

PDI > 1.8 — Broad distribution (condensation or degraded polymer)

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Summary

The polydispersity index (PDI), also called dispersity (D), is the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn). A PDI of 1.0 indicates a perfectly monodisperse polymer where all chains are identical in length, while larger values indicate broader distributions. Most commercially produced polymers have PDI values between 1.5 and 2.0, and living polymerization techniques can produce near-monodisperse polymers with PDI close to 1.0.

How it works

Enter the weight-average molecular weight (Mw) in g/mol.
Enter the number-average molecular weight (Mn) in g/mol.
Mw must always be greater than or equal to Mn — the calculator validates this.
Click "Calculate PDI" or let the result update automatically.
The tool computes PDI = Mw / Mn and classifies the distribution as narrow, moderate, or broad.

Use cases

Assess the uniformity of synthetic polymers after gel permeation chromatography (GPC).
Compare batch-to-batch consistency in polymer manufacturing.
Evaluate living vs. conventional radical polymerization products.
Report PDI in academic publications and lab reports.
Screen polymer samples for pharmaceutical or biomedical applications requiring narrow distributions.
Determine whether a polymer is suitable for precision applications such as drug delivery or photolithography.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu