Peptide Content, Purity and Assay Guide

The weight on a vial label is not always the weight of actual peptide. This guide explains purity, assay and gross weight, and the arithmetic behind the peptide content calculator. It is not medical advice.

The three terms

Gross weight: everything that was weighed, including peptide, water, salts and counter-ions.
Assay (net peptide content): the percentage of the gross weight that is peptide material.
Purity: the percentage of that peptide material that is the target sequence, usually measured by HPLC.

The formulas

Net peptide = gross weight × assay %
Absolute peptide = net peptide × purity %

Worked example (mathematical example only)

Gross 10 mg, assay 80%, purity 98%. Net peptide = 10 × 0.80 = 8 mg. Absolute peptide = 8 × 0.98 = 7.84 mg.

Why it matters for other calculations

If you enter the gross weight into a concentration calculation, the result describes the powder, not the peptide. Using the net or absolute value instead changes the answer by the same percentage.

Assumptions

Labs define and report these values in different ways, so check how your certificate defines each one.
Both percentages must be 100% or less.
The calculator multiplies the percentages as shown. It does not model individual impurities.

Common mistakes

Treating purity and assay as the same number.
Using purity alone and ignoring assay.
Entering 0.98 instead of 98 in a percentage field.

Peptide content calculator · Molecular weight guide · Concentration guide

Mathematical calculation only. Not medical advice.