Peptide Reconstitution & Dosing Calculator (mg to mcg to Insulin-Syringe Units)

Key takeaways

• The calculator does three things: concentration (how many mcg are in each mL after you add water), the volume you draw for a dose, and that volume expressed as units on a U-100 insulin syringe.
• 100 units = 1 mL. That is true for every standard insulin syringe, whatever its size.
• "Units" measure volume, not peptide. The same 10 units can hold 100 mcg or 1,000 mcg depending on how the vial was reconstituted, which is why mixing up concentrations is the most common dosing mistake.
• For prescribed medication, the dose and reconstitution volume on your pharmacy label override this calculator. The tool does not recommend doses.

How the math works

Three numbers decide every peptide dose:

1. How much peptide is in the vial, in milligrams (mg). This is on the label of a lyophilized (freeze-dried) vial: 5 mg, 6 mg, 10 mg and so on.
2. How much bacteriostatic water you add, in milliliters (mL). This sets the concentration. It does not change how much peptide you have, only how spread out it is.
3. The dose you need, in micrograms (mcg). Most peptide doses are written in mcg; 1 mg = 1,000 mcg.

From those:

• Concentration (mcg/mL) = vial mg × 1,000 ÷ water mL
• Volume per dose (mL) = dose mcg ÷ concentration
• Insulin-syringe units = volume mL × 100

Worked example

A 6 mg vial of sermorelin reconstituted with 3 mL of bacteriostatic water:

• Concentration = 6 × 1,000 ÷ 3 = 2,000 mcg/mL
• A 300 mcg dose = 300 ÷ 2,000 = 0.15 mL
• 0.15 mL × 100 = 15 units on an insulin syringe
• Doses per vial = 6,000 ÷ 300 = 20 doses

If the same vial had been reconstituted with 1 mL instead, the concentration would be 6,000 mcg/mL and the same 300 mcg dose would be only 5 units — a small, hard-to-measure volume. That is why more water is usually better for small doses, within the limit of what fits in the vial.

Units to mg converter

Use this when your vial is already in solution (as compounded GLP-1 vials usually are) and the label gives a concentration in mg/mL.

Syringe sizes and the units scale

All three use the same U-100 scale, so 10 units is 0.1 mL on each of them. A 30-unit syringe is simply easier to read for a 5-unit dose than a 100-unit syringe where the same dose is barely a mark and a half.

Reconstitution, briefly

Add bacteriostatic water slowly down the side of the vial, never squirting onto the powder; swirl gently, do not shake; wait until the solution is clear. Refrigerate the reconstituted vial and use it within the beyond-use date on your label. The full step-by-step, with water volumes by compound, is in how to reconstitute peptides; injection sites and technique are in how to inject peptides.

Frequently asked questions

+−How many units are in 1 mL on an insulin syringe?

100 units. Standard insulin syringes use the U-100 scale, where 100 units equals 1 mL, 50 units equals 0.5 mL and 10 units equals 0.1 mL. The units measure volume; how much peptide is in that volume depends on how the vial was reconstituted.

+−How much bacteriostatic water should I add to a peptide vial?

There is no single right amount; it only changes the concentration. Adding more water makes each dose a larger, easier-to-measure volume. A common choice is 1–3 mL for a 5–10 mg vial. For prescribed medication, use the volume on your pharmacy's instructions.

+−How do I convert mg to mcg?

Multiply by 1,000. A 5 mg vial contains 5,000 mcg; a 250 mcg dose is 0.25 mg.

+−Why does my calculated dose exceed the syringe?

The vial is too dilute for the dose. Either reconstitute with less water so the concentration is higher, or use a larger syringe. A 1 mL (100-unit) syringe is the practical maximum for a subcutaneous injection.

+−Can I use this calculator for semaglutide or tirzepatide?

The math is identical for any reconstituted vial. Compounded GLP-1 vials are usually dispensed already in solution at a labeled concentration, in which case you only need the units converter below. Always follow the dose on your pharmacy label.