This tool converts a vial mass and a solvent volume into a solution concentration. It is arithmetic about a solution and nothing else: it does not know what the material is for, and it produces no recommendation of any kind.
Enter a mass and a volume.
Calculated on the label mass. The label mass is the mass of material in the vial — peptide plus counter-ion plus residual water — so the true peptide concentration is lower wherever net peptide content is below 100%. See net peptide content explained.
How the concentration is calculated
Concentration is mass divided by volume. A vial labelled 5 mg reconstituted in 2 mL of solvent gives 2.5 mg/mL, or 2,500 µg/mL. There is nothing more to it than that, and the arithmetic is worth doing explicitly because the two figures people conflate — the label mass and the peptide mass — are not the same number.
Why the label mass overstates the peptide
The figure printed on a vial is the mass of solid in it. That solid is peptide, plus the counter-ion left from purification, plus whatever water the material has taken up. TFA and acetate counter-ions covers how much the salt form contributes, and water content in lyophilised peptides covers the Karl Fischer measurement that establishes the rest.
A vial can be 98% pure by HPLC and 78% peptide by mass at the same time, with both numbers honest. If the certificate reports net peptide content, multiply the label mass by it before using the figure above and the concentration you get will be the real one.
Working in molarity instead
Where a protocol is specified in molar terms rather than mass terms, the conversion needs the molecular weight, which is on the certificate next to the measured mass. Millimolar concentration is the mass concentration in milligrams per millilitre divided by the molecular weight in daltons, multiplied by one thousand. Enter a molecular weight above and the tool reports it.
Use the molecular weight for the form you actually have. An amidated peptide and its free-acid counterpart differ by roughly one dalton, and a copper complex differs from its free peptide by tens — which is why the calculated mass on a certificate matters as much as the measured one.
Before reconstituting
Lyophilised material is hygroscopic. Bring a vial to room temperature before opening it, or condensation forms on cold solid and the mass you are calculating from is no longer the mass you think it is. Weighing lyophilised peptides covers why a balance reading drifts upward while you are taking it.
Solvent choice changes stability rather than arithmetic, and it is compound-specific: choosing a reconstitution solvent covers the decision. Where a vial will be entered more than once, aliquoting and vial entry covers limiting the number of times material is exposed, and light, oxygen and temperature covers what degrades once it is in solution.
What this tool does not do
It computes a concentration from two numbers you supply. It does not establish that the label mass is accurate, it does not know the net peptide content of your vial, and it has no view on what the solution is for. Suitability for use in a person or an animal is outside what any certificate or calculator addresses — endotoxin, sterility and the limits of research grade covers why.
All material is supplied for laboratory research use only. It is not a drug, not a supplement, and not for use in humans or animals.
