Potency Calculator
Manufacturing · Calculator
API Potency Calculator (Assay and Water Correction)
Calculates how much API to dispense for a batch, correcting for assay, water or LOD, salt form and overage. Shows how much diluent to reduce to keep the batch weight constant.
Show the calculation steps
For learning and checking only. Always follow your approved master formula, batch manufacturing record and site SOP for potency calculation and diluent compensation.
Why Potency Correction Is Needed
An API is never exactly 100% pure drug. Its assay may be slightly below 100%, it may contain water, and the label claim may be expressed as the base while the material is a salt. If the master formula quantity were dispensed without correction, every tablet would contain less drug than the label claim. Potency correction adjusts the quantity of API dispensed so that each unit gets the correct amount of drug, and a matching amount of diluent is removed so the batch weight stays the same.
Potency Calculation Formula
As-is assay (%) = Assay (%) × (100 − Water or LOD %) ÷ 100
Qty to dispense = Theoretical qty × 100 ÷ As-is assay
Diluent to reduce = Qty to dispense − Theoretical qty
Worked example
A batch of 1,00,000 tablets of 500 mg each is to be made. The API assay is 99.2% on the anhydrous basis and the water content (KF) is 0.5%.
- Theoretical quantity = 500 mg × 1,00,000 = 50.0000 kg
- As-is assay = 99.2 × (100 − 0.5) ÷ 100 = 98.704%
- Quantity to dispense = 50.0000 × 100 ÷ 98.704 = 50.6565 kg
- Reduce the diluent (for example, microcrystalline cellulose) by 0.6565 kg
Frequently Asked Questions
What is potency correction in pharmaceutical manufacturing?
Potency correction is the adjustment of the quantity of API dispensed for a batch, based on its actual assay, water content and salt form, so that each finished unit contains the labelled amount of drug.
What is the difference between assay on as-is, dried and anhydrous basis?
As-is assay is calculated on the sample as received. Dried basis corrects for loss on drying, and anhydrous basis corrects for water measured by Karl Fischer titration. To dispense correctly, a dried or anhydrous assay must first be converted to the as-is basis.
What is a salt conversion factor?
When the label claim is expressed as the base but the API is a salt, the salt factor is the molecular weight of the salt divided by the molecular weight of the base. It is only needed when the assay is reported on the salt rather than the base.
Why is the diluent reduced after potency correction?
Dispensing extra API increases the total batch weight. Reducing the diluent by the same amount keeps the batch weight, and therefore the tablet weight, the same as in the master formula.
What is an overage?
An overage is a deliberate, justified excess of API added to compensate for a known loss during manufacture. Overages must be justified in the product's registration, and ICH Q8(R2) discourages using them to compensate for degradation or to extend shelf life.
Where should potency calculations be recorded?
Potency calculations are recorded in the batch manufacturing record and checked by a second person. Clear, contemporaneous records are part of good documentation practices.