Polydispersity Index Calculator
The polydispersity index (PDI), now formally called dispersity (symbol D) by IUPAC, is the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn): PDI = Mw / Mn. It quantifies the width of a polymer's molecular weight distribution. A perfectly monodisperse polymer (all chains exactly the same length) has PDI = 1.0. Real polymers always have PDI greater than 1. Enter Mw and Mn from your GPC/SEC report to get PDI and an interpretation of the distribution breadth.
Polydispersity index formula
PDI = Mw / Mn
Mw is the weight-average molecular weight and Mn is the number-average molecular weight, both in g/mol. PDI is always greater than or equal to 1. IUPAC recommends the symbol D (dispersity) instead of PDI, defined identically as Mw/Mn. The older literature sometimes uses the symbol U for uniformity.
PDI values by polymerization method
- Perfect monodisperse standard (e.g., protein): PDI = 1.00.
- Living anionic polymerization: PDI = 1.01 to 1.10.
- RAFT, ATRP, NMP controlled radical: PDI = 1.05 to 1.30.
- Free radical polymerization: PDI typically 1.5 to 2.0.
- Step-growth polymerization (Carothers, high conversion): PDI approaches 2.0.
- Commercial polyolefins (Ziegler-Natta): PDI = 3 to 20 or higher.
Polydispersity index: frequently asked questions
What is polydispersity index (PDI)?
Polydispersity index (PDI), also called dispersity (symbol D), is the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn): PDI = Mw / Mn. It measures the breadth of the molecular weight distribution. PDI = 1.0 for a perfectly monodisperse polymer.
What is a good PDI value for a polymer?
PDI close to 1.0 indicates a narrow, uniform distribution. Step-growth polymers typically have PDI close to 2.0 at high conversion. Living polymerization methods (RAFT, ATRP, anionic) can achieve PDI below 1.1. Most commercial polymers have PDI between 1.5 and 3.0.
What is the difference between Mw and Mn?
Mn is the number-average molecular weight: (sum of Ni x Mi) / (sum of Ni). It gives equal weight to each chain. Mw is the weight-average molecular weight: (sum of Ni x Mi^2) / (sum of Ni x Mi). It gives more weight to larger chains. Mw is always greater than or equal to Mn.
How are Mw and Mn measured?
Gel permeation chromatography (GPC/SEC) is the standard method, reporting Mn, Mw, and PDI simultaneously. Mn can also be measured by end-group analysis (NMR or titration) and osmometry. Mw can be measured by light scattering. The ratio gives the PDI.
Why does PDI matter for polymer properties?
Narrow PDI (close to 1) gives more predictable and homogeneous properties: sharper melting point, lower melt viscosity at a given Mw, better film-forming properties. Broad PDI (high dispersity) can be advantageous for some processing applications where a range of chain lengths improves flow.
Official sources
- IUPAC: Glossary of Basic Terms in Polymer Science (IUPAC Recommendations 1996).
- NIST: NIST Polymer Science Division.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.