How to Calculate Assay Percentage in Pharmaceutical Analysis

Assay calculation is one of the most fundamental calculations in pharmaceutical analysis. It tells you what percentage of the labelled amount of active ingredient is actually present in your sample — whether you are testing a tablet, capsule, injection, or bulk API.

Despite being routine, it is also a source of errors. This guide walks through the correct formula, explains each variable, and gives a fully worked example.

The Assay Formula

For HPLC assay against an external reference standard, the standard formula is:

Where:

• AT — Peak area of the sample (test) solution
• AS — Peak area of the reference standard solution
• WS — Weight of reference standard taken (mg)
• WT — Weight of sample taken (mg)
• PS — Purity of reference standard (%)
• DT — Dilution factor of the test solution
• DS — Dilution factor of the standard solution

Worked Example

An assay of 99.0% is within the typical acceptance criteria of 98.0–102.0% for pharmaceutical products. This result would pass.

Common mistakes to avoid

• Forgetting purity correction — Always include PS in your formula. Using 100% when purity is 99.2% introduces a systematic error.
• Wrong weight entry — Enter the actual weighed amount, not the theoretical target weight.
• Mismatched dilutions — If standard and sample are at different concentrations, include DT and DS. Not accounting for different dilution factors is a common source of out-of-specification results.
• Single injection — Always run at least duplicate injections of both standard and sample and use the average peak area.