How the API Requirement Calculator works

On paper the API for a batch is the dose per unit times the number of units. Two corrections almost always apply. An overage covers expected losses or degradation during processing. An assay correction compensates for the fact that the raw material is not exactly 100% pure, so more has to be weighed to deliver the same amount of active.

The calculator applies the overage first to get the theoretical API, then divides by the assay to get the gross weight to dispense from the container.

Video walkthrough

Net and gross API

Example: 98.5% assay with 3% overage

Label claim 250 mg, batch size 400,000 tablets, overage 3%, assay on the certificate of analysis 98.5%.

1. Theoretical API = 250 mg × 400,000 × 1.03 = 103,000,000 mg = 103.00 kg
2. Gross API to weigh = 103.00 ÷ 0.985 = 104.57 kg

The extra 1.57 kg is purely the assay correction. Without it, the batch would contain about 1.5% less active than planned.

Assay, overage and release limits

• Assay value: take it from the certificate of analysis of the exact lot being dispensed. State whether it is on an as-is or dried (anhydrous) basis and match the basis used in the master formula.
• Overage: only where scientifically justified and approved in the registered formula and batch record. Regulators expect overages to be explained, not used routinely to cover process losses.
• Dispensing tolerance: follow your SOP's weighing tolerance. The calculated gross weight is the target, and the balance reading must be recorded and verified.
• Release check: the batch assay at release must still meet the product specification, commonly 90.0–110.0% or tighter, according to the registered monograph.

Assay and overage errors

• Using the assay from a previous lot or from the vendor brochure instead of the certificate for that lot.
• Multiplying by the assay instead of dividing. A 98.5% assay needs more material, not less.
• Applying overage in the batch size step and again here.
• Mixing as-is and anhydrous assay values.
• Entering the assay as 0.985 in a field that expects a percentage.

Practical tips for dispensing and planning

Treat the gross weight as a dispensing instruction and the theoretical weight as the quantity that should be reconciled. At the end of the batch, the active actually consumed should be traceable to a documented, assay-corrected figure, so keep both numbers in the batch record.

When a container is partly used, dispense from the lot whose certificate you checked and record the lot number next to the assay value. If a second lot with a different assay is needed to complete the batch, recalculate for each lot separately instead of averaging the assays.

For potent or expensive actives, round up only as far as your SOP allows. Dispensing tolerances, balance capability and the rounding rule all affect the final amount that goes into the blender.

Standards and references

The checks on this page relate to the documents below. Standards are revised from time to time, so confirm the current edition and follow your own approved procedures.

• 21 CFR 211.101, Charge-in of components
• 21 CFR 211.186, Master production and control records
• 21 CFR 211.188, Batch production and control records

Questions on API requirement

Why divide by the assay instead of multiplying?

Because a purer material needs less weight. A lower assay means less active per kilogram, so more kilograms must be weighed.

Is overage always required?

No. Many products use none. Add it only when it is justified and approved.

Does this account for water content?

Only if your assay figure is already corrected for it. If the certificate gives assay on an anhydrous basis, apply a water content correction as well.

Related tools and guides: Cost per Tablet · Reading a CoA · How to Calculate Batch Size · Batch Size Calculator · API Overage: How Much Is Too Much? · Unit Converter

By Sajjad Ahmad. For reference and education only; verify limits against your SOPs and the current pharmacopoeia. See the methodology.