How the peptide calculator works
Peptides ship as a freeze-dried powder in a vial labeled in milligrams. Before use you add bacteriostatic water (reconstitution). The math then has one idea: concentration. Dividing the milligrams in the vial by the millilitres of water gives mg per mL, and everything else follows from that. The vial holds the same amount of peptide whatever you add; water only sets how much peptide each unit on the syringe carries.
The formula
| Quantity | Formula |
|---|---|
| Concentration (mg/mL) | peptide mg / water mL |
| Micrograms per unit | concentration x 1000 / 100 = concentration x 10 |
| Units to draw | dose mcg / micrograms per unit |
| mL to draw | units / 100 |
| Doses per vial | vial mg x 1000 / dose mcg |
Worked example
A 5 mg vial with 2 mL of water: 5 / 2 = 2.5 mg/mL, which is 2,500 mcg per mL. One unit is 0.01 mL, so each unit carries 25 mcg. A 250 mcg dose is 250 / 25 = 10 units, or 0.1 mL. The vial holds 5,000 / 250 = 20 doses. Add 1 mL instead of 2 and the same dose becomes 5 units; add 4 mL and it becomes 20 units.
Insulin syringe sizes
| Syringe | Capacity in units | Smallest tick |
|---|---|---|
| 0.3 mL | 30 units | 0.5 or 1 unit |
| 0.5 mL | 50 units | 1 unit |
| 1 mL | 100 units | 2 units |
Pick a water volume that lands the dose on a whole number of units that fits comfortably on the syringe. Doses under 5 units are hard to measure accurately; if you are there, use less water. Doses over the syringe capacity need less water or a larger syringe. The "water that makes this dose a round number" table above lists the volumes that give 5, 10, 20, 25, or 50 units.
Common mistakes
Mixing up mg and mcg is the big one: 1 mg is 1,000 mcg, so a "2.5" dose of tirzepatide is 2,500 mcg, while a "250" dose of BPC-157 is 0.25 mg. The second is reading a 0.3 mL syringe as if it were 1 mL; the markings are still units, but the full barrel is 30 units, not 100. The third is assuming more water means a bigger dose. It does not; the dose is set by the units you draw at the concentration you made.
Bacteriostatic water versus sterile water
Bacteriostatic water contains 0.9% benzyl alcohol, which keeps a multi-dose vial from growing bacteria over the weeks it is in use. Sterile water has no preservative and is meant for single use. Either one gives the same concentration for this calculator. Most reconstituted peptides are stored in the refrigerator and used within 28 days, and the vial should be swirled, not shaken.