Reconstituting a lyophilised peptide is simple arithmetic — but it is arithmetic done in mixed units (milligrams, micrograms, millilitres and microlitres), and that is where errors creep in. This guide sets out the core formulas, works through examples and highlights the mistakes we see most often.
If you just want the numbers, use our free reconstitution calculator. Everything below explains what it is doing.
The units you need
- mg (milligram) = 1,000 mcg (microgram, also written µg)
- mL (millilitre) = 1,000 µL (microlitre)
- Concentration is amount per volume, usually expressed as mg/mL or mcg/mL. Note that 1 mg/mL is the same as 1,000 mcg/mL, and also the same as 1 mcg/µL.
Most mistakes come from mixing mg and mcg in a single calculation. Convert everything to the same unit first.
Step 1: choose a diluent
Use the diluent stated in the product documentation. For the peptides in our range, the specification typically reads "Reconstitute using bacteriostatic water or sterile laboratory-grade water". Some peptides with unusual solubility need a different solvent or pH; if the documentation specifies one, follow it. Our storage and handling guide covers why solutions are less stable than dry powder.
Step 2: calculate concentration
Concentration (mcg/mL) = peptide in vial (mg) × 1,000 ÷ diluent volume (mL)
Worked example — a 5 mg vial of BPC-157 reconstituted with 2 mL of diluent:
- 5 mg × 1,000 = 5,000 mcg
- 5,000 mcg ÷ 2 mL = 2,500 mcg/mL (2.5 mg/mL)
Step 3: calculate volume per aliquot
Volume per aliquot (mL) = target amount (mcg) ÷ concentration (mcg/mL)
Continuing the example, for 250 mcg per aliquot:
- 250 mcg ÷ 2,500 mcg/mL = 0.1 mL (100 µL)
And the number of full aliquots in the vial:
- 5,000 mcg ÷ 250 mcg = 20 aliquots
Choosing a diluent volume that gives convenient numbers
You can choose the diluent volume to make downstream measurement easier. The same 5 mg vial gives:
| Diluent added | Concentration | Volume for 250 mcg |
|---|---|---|
| 1 mL | 5,000 mcg/mL | 0.05 mL (50 µL) |
| 2 mL | 2,500 mcg/mL | 0.10 mL (100 µL) |
| 2.5 mL | 2,000 mcg/mL | 0.125 mL (125 µL) |
| 5 mL | 1,000 mcg/mL | 0.25 mL (250 µL) |
Larger diluent volumes give more dilute solutions and larger, easier-to-measure aliquot volumes; smaller volumes concentrate the peptide. Check that your chosen concentration is within the peptide's solubility and suits your pipetting precision.
Common pitfalls
- mg/mcg slips. Writing "5" for a 5 mg vial in a formula expecting micrograms gives an answer 1,000× too small.
- Vial fill vs actual content. The nominal fill (for example "10 mg") refers to the stated quantity in the vial. As explained in our CoA guide, lyophilised powder also contains water and counter-ions, so if exact peptide mass matters, check how the fill is defined and whether peptide content was measured.
- International Units. Some proteins and hormones are specified in International Units (IU) rather than mass. IU-to-mass conversion depends on the specific substance and its biological activity, so never apply a generic conversion factor — use the figures stated in the product documentation.
- Rounding too early. Keep full precision through the calculation and round only the final measured volume.
Good reconstitution practice
- Let the vial reach room temperature before opening to avoid condensation.
- Add diluent slowly down the inside wall of the vial rather than directly onto the powder.
- Swirl gently or roll the vial; avoid vigorous shaking, which can cause foaming and aggregation.
- Allow time for complete dissolution and check visually that the solution is clear.
- Label the vial with compound, batch number, concentration and date.
- Aliquot for single use and store according to the documentation.
Try the calculator
Our reconstitution calculator applies exactly these formulas: enter the peptide in the vial, the diluent volume and your target amount per aliquot to get concentration, volume per aliquot and the number of full aliquots per vial. It runs entirely in your browser.
These calculations are for laboratory and research work only and are not intended to guide human or veterinary use.