Advanced Polydispersity Index Calculator

Calculator Inputs

Enter molecular weight fractions and counts. Use at least two valid rows.

Sample Name

Molecular Weight Unit

Decimal Places

Row	Molecular Weight	Molecules / Relative Count	Action
1
2
3
4
5

This tool calculates Mn, Mw, Mz, PDI, distribution span, fractions, and a quick interpretation of molecular weight breadth.

Example Data Table

Row	Molecular Weight (g/mol)	Molecules / Relative Count
1	10,000	50
2	20,000	80
3	30,000	60
4	40,000	30
5	50,000	10

This example produces approximately Mn = 24,347.8261 g/mol, Mw = 29,285.7143 g/mol, Mz = 33,414.6341 g/mol, and PDI = 1.2028.

Formula Used

Number average molecular weight: Mn = Σ(N_iM_i) / ΣN_i

Weight average molecular weight: Mw = Σ(N_iM_i²) / Σ(N_iM_i)

Z average molecular weight: Mz = Σ(N_iM_i³) / Σ(N_iM_i²)

Polydispersity index: PDI = Mw / Mn

Here, M_i is the molecular weight of fraction i, and N_i is the number of molecules or a relative count for that fraction.

How to Use This Calculator

Enter a sample name for your report.
Select the molecular weight unit that matches your dataset.
Add each molecular weight fraction in a separate row.
Enter the corresponding molecule count or relative amount.
Use the add row button if your dataset has more fractions.
Press the calculate button to display Mn, Mw, Mz, and PDI.
Review the processed distribution table and interpretation label.
Download the result as CSV or PDF for reporting.

FAQs

1) What does the polydispersity index show?

PDI shows how broad a polymer’s molecular weight distribution is. A value near 1.0 suggests a narrow spread, while larger values show more variation across chain sizes.

2) Why are Mn and Mw both needed?

Mn emphasizes molecule count, while Mw gives more influence to heavier chains. Comparing them reveals whether large molecules significantly widen the overall distribution.

3) Can I use relative counts instead of exact molecule numbers?

Yes. Relative counts, peak areas, normalized fractions, or scaled amounts work if they are proportional across all rows. The dispersity ratios remain unchanged.

4) What is considered a low PDI?

Values close to 1.0 usually indicate a narrow distribution. Many controlled polymer systems target low PDI values, though acceptable limits depend on chemistry, process, and application.

5) What usually causes a high PDI?

High PDI can result from incomplete control during polymerization, chain transfer, branching, blending different materials, degradation, or broad product recovery during separation steps.

6) Is PDI unitless?

Yes. Because PDI is the ratio of Mw to Mn, the molecular weight units cancel out. You can use g/mol, kg/mol, or Da consistently.

7) Can this calculator be used for oligomers too?

Yes. The same weighted-average principles apply to oligomer distributions, provided your dataset represents meaningful fractions with positive molecular weights and counts.

8) What happens if one row is blank or zero?

Completely blank rows are ignored. Rows with zero or negative values are invalid and will trigger an input warning because they distort the weighted molecular averages.