How the calculation works
Lyophilised (freeze-dried) peptides ship as a powder. Adding bacteriostatic water dissolves it into a solution with a known strength, and from that strength you can work out the volume that holds any amount.
- Concentration = peptide in the vial (mg) ÷ water added (mL)
- Volume to draw = amount needed (mg) ÷ concentration (mg/mL)
- Syringe units = volume (mL) × 100 on a U-100 insulin syringe
Worked example
A 5 mg vial with 2 mL of bacteriostatic water gives 5 ÷ 2 = 2.5 mg/mL (2,500 mcg per mL, or 25 mcg per unit). For 250 mcg (0.25 mg): 0.25 ÷ 2.5 = 0.1 mL, which is 10 units. The vial holds 5,000 ÷ 250 = 20 measures of that size.
Add 1 mL instead and the solution is twice as strong — the same 250 mcg is only 5 units, which is harder to measure precisely. That's why more water is often easier to work with for small amounts.
Quick reference: 5 mg vial
Units to draw on a U-100 syringe.
| Water added | 100 mcg | 250 mcg | 500 mcg | 1 mg |
|---|---|---|---|---|
| 1 mL · 5 mg/mL | 2 u | 5 u | 10 u | 20 u |
| 2 mL · 2.5 mg/mL | 4 u | 10 u | 20 u | 40 u |
| 2.5 mL · 2 mg/mL | 5 u | 12.5 u | 25 u | 50 u |
| 3 mL · 1.67 mg/mL | 6 u | 15 u | 30 u | 60 u |
| 5 mL · 1 mg/mL | 10 u | 25 u | 50 u | 100 u |
Reconstitution notes for the lab
- Let the vial and water reach room temperature, and wipe both stoppers with an alcohol swab.
- Run the water slowly down the inside wall of the vial rather than onto the powder.
- Swirl gently until clear — don't shake, which can foam and degrade the peptide.
- Label the vial with the concentration and the date it was mixed.
- Store reconstituted solutions refrigerated (2–8 °C) and away from light; follow the vendor's guidance.
Frequently asked questions
How does the peptide calculator work?
It divides the peptide in the vial (mg) by the water you add (mL) to get the concentration in mg/mL. The amount you enter is divided by that concentration to get the volume to draw, and the volume is converted to syringe units — on a U-100 insulin syringe, 100 units = 1 mL.
How many units are in 1 mL?
On a standard U-100 insulin syringe, 1 mL is 100 units, so each unit is 0.01 mL. A 0.3 mL syringe holds 30 units and a 0.5 mL syringe holds 50 units — the scale is the same, only the barrel is shorter, which makes small amounts easier to read.
How much bacteriostatic water should I add?
The water doesn't change how much peptide is in the vial — only how concentrated it is. More water means each unit holds less peptide, so small amounts are easier to measure accurately; less water keeps the volume per draw small. The calculator suggests an amount that puts your figure on a whole, easy-to-read mark.
How do I convert mg to mcg?
1 mg = 1,000 mcg. So 0.25 mg is 250 mcg and 2.5 mg is 2,500 mcg. Use the mcg / mg switch to enter whichever the label or protocol uses.
Does the vial really contain the amount on the label?
Not always — which is why VialCheck records lab reports (COAs). A COA shows the measured quantity and purity for a batch. If a vial tests at 9.2 mg instead of 10 mg, every calculation from it is off by 8%. Check the batch on VialCheck before relying on the label.