What is the difference between Mn and Mw?

Mn (number-average molecular weight) is the arithmetic mean: each chain counts equally regardless of size. Mw (weight-average molecular weight) weights each chain by its mass, so larger chains contribute more. Mw is always >= Mn.

What does dispersity index (D) mean?

D = Mw / Mn. A value of exactly 1.0 means all chains are the same length (monodisperse). Living polymerization techniques typically yield D near 1.0–1.1, while free-radical polymerization gives D around 1.5–2.0.

Should I enter mole fractions or number of moles?

Enter the number of moles (or any proportional count) for each fraction in the Ni column. The tool normalizes them internally. Absolute mole values or relative counts both work correctly.

Can I paste data from a spreadsheet?

Yes. Copy two columns (Mi and Ni) from your spreadsheet, then click "Paste CSV" and paste the data. Comma- or tab-separated values are both accepted.

What units should molecular weight be in?

Any consistent unit works (g/mol is standard). Mn and Mw will be in the same unit you enter. The dispersity index is dimensionless.

How many fractions can I add?

There is no hard limit. The tool handles large tables efficiently in the browser without any server calls.

Molecular Weight Polymer Calculator

Enter polymer fraction data (molecular weight and mole count or mole fraction) to compute Mn, Mw, and dispersity index (D = Mw/Mn).

Fraction Data

#	M_i (g/mol)	N_i (mole count)

Results

Enter fraction data to see results

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Summary

This calculator determines the number-average molecular weight (Mn) and weight-average molecular weight (Mw) of a polymer from its molecular weight distribution fractions. Enter each fraction's molecular weight along with its mole count, mole fraction, or any proportional number count, and the tool instantly computes Mn, Mw, and the dispersity index D = Mw/Mn. A dispersity of 1.0 indicates a perfectly monodisperse polymer, while values above 1 reflect broader distributions.

How it works

Add rows for each fraction: enter the molecular weight (g/mol) and mole count, mole fraction, or proportional number count (Ni).
The calculator normalizes your inputs automatically — no need to pre-normalize fractions.
Number-average Mn = sum(Ni * Mi) / sum(Ni) is computed from mole counts.
Weight-average Mw = sum(wi * Mi) is computed once weight fractions are derived from Ni * Mi.
Dispersity index D (also called PDI) = Mw / Mn; a value near 1.0 means narrow distribution.
Results update live as you type; export the table as CSV with one click.

Use cases

Characterize synthetic polymer samples from GPC/SEC data.
Verify molecular weight distribution calculations in polymer chemistry coursework.
Quickly compute PDI from a known fraction table during lab analysis.
Compare dispersity between different polymerization runs.
Derive weight-fraction distributions from mole-count or mole-fraction polymer data.
Teach students the difference between Mn and Mw with interactive fraction editing.

Frequently Asked Questions

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