A reconstitution calculator does one division, and its answer is only as true as the two numbers it was given. Mass in the vial divided by volume of diluent gives concentration. The arithmetic is not where anything goes wrong. What goes wrong is the mass term, which people take from the label, and the volume term, which people take from the syringe graduation. Both are approximately right, and the approximation is the error.
What the calculator computes
Concentration equals mass over volume. Given a vial mass and a diluent volume, the calculator returns mg/mL; given a target concentration, it returns the volume that produces it; given a concentration and a required amount, it returns the volume to transfer. Three rearrangements of one equation. The site's calculator does exactly this and nothing more, which is the honest scope for the tool.
| Input | Where it usually comes from | Where it should come from |
|---|---|---|
| Mass | The vial label | Label mass × net peptide content, from the certificate |
| Volume | The syringe graduation | The same, with its accuracy limit understood |
| Molecular weight | A calculation from the sequence | The observed mass on the certificate |
Why is the label mass the wrong input?
Because it is the mass of material, not of peptide, and the difference is a fifth or more. A 5 mg vial at 78% net peptide content holds 3.9 mg of peptide; reconstituted in 2 mL, the calculator says 2.5 mg/mL and the solution is 1.95 mg/mL. Every dilution from that stock inherits the error, and it is invisible because nothing in the arithmetic flags it. The fix is to enter the peptide mass — label mass multiplied by the content fraction — rather than the label mass. The content figure is on the certificate, and it changes lot to lot.
How accurate is the volume term?
Less than the graduations suggest, and the error grows as the volume shrinks. A 1 mL graduated syringe reads to perhaps 0.01 mL, but its accuracy at small volumes is a few percent of the reading, and the dead volume in the hub is a fixed quantity that matters little at 1 mL and a lot at 0.05 mL. Adding 2 mL to a vial is accurate to within a percent or two. Transferring 20 µL from that stock with the same syringe is not, and a dilution series built on such transfers compounds the slip. The general arithmetic covers this; the practical rule is that the smaller the volume, the more it deserves a calibrated pipette rather than a syringe.
What the calculator cannot know
Four things sit outside the equation, and each one moves the true concentration.
Net peptide content, as above. The displaced volume of the solid, which is negligible at milligram scale and not at fifty. Losses to the vial and the tube, which are negligible at a concentrated stock and can dominate at a dilute working solution. And degradation between reconstitution and use, which a calculator cannot see because it has no notion of time.
A calculator that asked for all of these would be more accurate and would never be used. The trade is reasonable as long as the user knows which one they are making.
Should the calculation be done in mg/mL or in molarity?
In whichever unit the downstream work uses, and the answer is usually molarity for anything involving a receptor. A mg/mL figure describes the material; a molar figure describes the number of molecules, which is what binding responds to. Converting needs the molecular weight, and that number should be the observed mass on the certificate, not a figure summed from the sequence — modifications change it, and the salt form does not. Record both units on the stock. It costs one line.
What is the best concentration to reconstitute to?
Concentrated enough to keep for months frozen in aliquots, dilute enough to stay clear. Those two constraints usually leave a comfortable range, and a stock near 1 mg/mL suits most short peptides. Going higher risks aggregation, particularly for hydrophobic or amyloid-prone sequences; going lower wastes vial and exposes the stock to adsorption. The calculator will happily return any number; the range is a judgement about the compound.
Why does the volume added not equal the final volume?
Because the dissolved solid occupies space. Add 2 mL to 5 mg and the result is marginally over 2 mL — well under a percent, and smaller than the pipetting error. Add 2 mL to 100 mg and the difference becomes a few percent, which is no longer negligible against the other terms. At large masses the honest method is to reconstitute to a final volume in a volumetric vessel, so the volume term is the volume in the flask rather than the volume that went in. A calculator assumes the two are equal, and at ordinary vial sizes it is right to.
What should I write on the vial afterwards?
The date, the diluent, the concentration — in the unit you will use — and the lot number if it is not already printed. All four at the moment of reconstitution, not from memory later. A stock labelled "1 mg/mL, 26 Sep, lot 2409-A, bac. water" can be used, diluted, compared to a certificate and traced months later. A stock labelled "BPC" cannot. The calculator produced the number; the label is where it has to live.
All products referenced here are supplied for laboratory and research use only. They are not drugs, foods, supplements or cosmetics, and are not for human or veterinary use.
