Purity to recovered mass
The calculation the index performs on every record with a peptide-content figure: from the gravimetric mass balance and the purity to how many milligrams of the named compound are actually in the vial.
Defaults are the index medians: water 4.20 %, counter-ion 6.10 %, purity 98.99 %. Change any field and every downstream figure recomputes.
The arithmetic
total peptide content (%w/w) = 100 − water − counter-ion − residual target-compound content = total peptide content × purity ÷ 100 recovered target (mg) = gross fill (mg) × target content ÷ 100 against label claim (%) = recovered ÷ label claim × 100
Why this is the calculation that matters
Purity is a ratio of chromatographic peak areas. It answers "of everything that eluted and absorbed at 214 nm, what fraction was the main peak", and it answers no other question. In particular it says nothing whatsoever about how much material is in the vial. A vial containing 6 mg of 99.5 % pure peptide and 4 mg of water is a 99.5 % pure product and a 60 % fill.
The four components in the mass balance are exhaustive and mutually exclusive: everything in the cake is water, counter-ion, residual solvent and inorganic residue, or peptide. They must sum to 100 %. The index closes this balance to within 0.02 percentage points on every record that carries a content figure, and the build asserts it — a record where the balance does not close is a bug, not a finding.
Where the numbers come from on a real record
Water is Karl Fischer. Counter-ion is ion chromatography. Residual is usually estimated rather than measured. Total peptide content is then by difference, which is why an unmeasured residual propagates directly into it. Target-compound content multiplies that by chromatographic purity, on the assumption that the impurities have the same response factor as the target — an assumption that is wrong in detail and adequate in practice, and one the method page takes apart properly.
Across this index, 2,825 of 6,722 records carry no content figure at all, because peptide content is an extra-cost test that most submitters do not order. For those records the recovered-mass column is blank and the label claim is unchecked.