Correct label-claim API demand for overage and raw-material assay to get a gross weigh-out.
On paper, the amount of active ingredient a batch needs is simple: dose per unit times the number of units. In practice, two adjustments almost always apply. First, an overage — extra API added to cover expected degradation or losses during processing. Second, an assay correction, since raw API rarely arrives at exactly 100% purity; the certificate of analysis will state its actual assay value, and you need to weigh out more than the theoretical amount to compensate for the difference.
This calculator applies both corrections in sequence: it first works out the theoretical, or net, API requirement including overage, then divides that by the assay percentage to arrive at the gross weight you'd actually need to dispense from the raw material.
It's worth double-checking the assay value against the current certificate of analysis for the specific lot you're dispensing from, since assay can vary lot to lot, and using the wrong figure here directly changes how much API ends up in the batch.
Of all the raw materials in a formulation, API is usually the most expensive and the most tightly controlled. Under-weighing risks a sub-potent batch failing release testing, while over-weighing wastes an expensive material. A calculator that automatically layers overage and assay correction on top of the theoretical dose removes one of the easiest places for a manual calculation slip to happen.
An adjustment for raw material purity — since API is rarely 100% pure, you weigh out more than the theoretical amount to compensate.
To account for expected degradation or losses during manufacturing, so the finished product still meets its labeled potency at release.