How the reconstitution math works
Peptides usually ship as a dry powder in a sealed vial. Adding bacteriostatic water turns the powder into a liquid you can measure. The math has three steps, and it's the same for every peptide.
2. Volume to draw (mL) = amount wanted (mcg) ÷ concentration
3. Units on a U-100 syringe = volume (mL) × 100
Worked example
A 10 mg vial is 10,000 mcg. Add 2 mL of bacteriostatic water and the concentration is 10,000 ÷ 2 = 5,000 mcg per mL, or 50 mcg per unit.
To measure 250 mcg: 250 ÷ 5,000 = 0.05 mL, which is 5 units on a U-100 syringe. The vial holds 10,000 ÷ 250 = 40 measures of that size.
Units vs. mL on a U-100 syringe
Insulin-style syringes are marked in units, not mL. On a U-100 syringe, 100 units equal 1 mL, so 1 unit is 0.01 mL. The "units" here are just volume markings; they have nothing to do with how strong the peptide is.
| Syringe | Holds | Typical markings |
|---|---|---|
| 0.3 mL | 30 units | Every 1 unit (some every half unit) |
| 0.5 mL | 50 units | Every 1 unit |
| 1 mL | 100 units | Every 2 units |
Where to find your vial size
The vial size is the total amount of peptide in the vial, printed on the label, usually in mg. That's the number that goes in "Peptide in the vial."
Choosing how much water to add
The amount of water doesn't change how much peptide is in the vial. It only changes how concentrated the liquid is:
- Less water means a stronger solution, so each measure is a small number of units. Small draws are harder to measure accurately.
- More water means a weaker solution, so the same amount takes more units. That's easier to measure but uses more of the syringe.
Many people work backward: pick a round number of units that's easy to read on the syringe, then use the "How much water to add" tab to find the matching water volume.
Calculators by peptide
Same math, with quick picks for each peptide's vial sizes, a reconstitution table and its FDA status:
Is the peptide you're measuring legal?
The math is the same for every peptide, but the rules aren't. Several peptides changed status with the FDA in 2026. See our peptide FDA status tracker for where each one stands, with sources.
Frequently asked questions
Does adding more water change how much peptide is in the vial?
No. The total stays the same. More water only makes each unit weaker, so you draw more units for the same amount.
How many units are in 1 mL on an insulin syringe?
On a U-100 syringe, 1 mL is 100 units and 1 unit is 0.01 mL.
Why does the calculator warn me when the answer is under 2 units?
Most syringe markings are 1 or 2 units apart, so very small volumes are hard to measure accurately. Adding more water turns the same amount into more units.
What's the difference between mg and mcg?
1 mg (milligram) equals 1,000 mcg (micrograms). Vials are usually labeled in mg, while measured amounts are often written in mcg, so mixing them up is the most common math mistake. The calculator converts for you.
Why bacteriostatic water instead of sterile water?
Bacteriostatic water contains 0.9% benzyl alcohol, which slows bacterial growth so a vial can be drawn from more than once. Sterile water has no preservative and is meant for single use.