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Peptide reconstitution calculator
Enter the peptide mass printed on your vial, the volume of bacteriostatic water you added, and the amount you intend to draw. The calculator returns the resulting concentration, the volume in millilitres, and the equivalent reading on a U-100 insulin syringe. It runs entirely in your browser and stores nothing.
How the calculation works
Reconstitution arithmetic is three divisions, and it is worth understanding rather than trusting blindly:
- Concentration = vial mass ÷ diluent volume. A 5 mg vial in 2 mL is 2.5 mg/mL, or 2,500 mcg/mL.
- Volume = target amount ÷ concentration. Drawing 250 mcg from 2,500 mcg/mL is 0.1 mL.
- Syringe units = volume × units-per-mL. On a U-100 barrel that is 0.1 × 100 = 10 units.
That is the entire calculation. There is no hidden model, no compound-specific adjustment, and no judgement about the number you entered. See reconstitution explained for the longer version.
Using it accurately
- Enter the mass from the label, not from memory — 5 mg and 10 mg vials look identical.
- Enter the diluent volume you actually added, measured, not intended.
- Check your syringe's calibration. U-100 is standard, but U-40 syringes exist and change every unit figure. See reading an insulin syringe.
- Record the result, not just the reading. A unit figure without its concentration is uninterpretable later.
Why the result belongs in a record
The output of this calculator is only valid for one specific vial at one specific dilution. Reconstitute the next vial with a different volume and every number changes. That is why the vial's concentration has to live in your log alongside every entry drawn from it — a point covered in how to track a peptide cycle.
PepCycle stores the vial once and converts automatically thereafter, in both directions, so the arithmetic never has to be repeated at seven in the morning.
Frequently asked questions
How do I calculate peptide reconstitution?
Divide the vial's peptide mass by the volume of bacteriostatic water added to get the concentration. Divide the amount you intend to draw by that concentration to get the volume in millilitres. Multiply by 100 for units on a U-100 insulin syringe.
How much bacteriostatic water should I add?
That is your choice and this calculator will not make it for you. More water gives a dilute solution with larger, easier-to-read volumes; less gives a concentrated one with smaller volumes. Whatever you choose, record it — that number makes every later entry interpretable.
Does this calculator work for GLP-1 pens?
No, and it should not be used for them. A pre-filled pen has a fixed concentration set at manufacture and a device that meters the amount. Reconstitution maths only applies to lyophilised powder you dissolve yourself. See our guide to pens versus vials.
Is my data sent anywhere?
No. The calculation runs entirely in your browser using JavaScript on this page. No values are transmitted, stored on a server, or logged.
Related pages
Stop recalculating this every time
PepCycle stores each vial once — mass, diluent volume, concentration — and converts automatically for every entry after that, in both directions, including entries you backdate.
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Last reviewed . All calculations run in your browser; no values are transmitted or stored.