Reconstitution Volume Reference — 2 mg, 5 mg, and 10 mg Vials

Published 2026-09-15 · Lyophile Editorial

Reconstitution math for a 2 mg, 5 mg, or 10 mg vial comes down to one relationship: concentration = mass ÷ diluent volume. U-100 syringe units, micrograms per tick mark, volume per draw — all of it is arithmetic downstream of that single division.

Worked once before the tables:

5 mg peptide in 2 mL diluent = 2500 mcg/mL = 25 mcg per unit on a U-100.

That last number is the one that matters at the bench. It converts what you draw into what you have.

Reconstitution concentration table — 2 mg, 5 mg, 10 mg vials

Divide mg by mL, then multiply by 1000 to move from mg/mL to mcg/mL. The last column is your bench shortcut: mcg per unit on a U-100 insulin syringe, where 1 unit = 0.01 mL.

| Vial size | Diluent added | Concentration (mg/mL) | Concentration (mcg/mL) | mcg per U-100 unit | |---|---|---|---|---| | 2 mg | 1 mL | 2.0 | 2000 | 20 | | 2 mg | 2 mL | 1.0 | 1000 | 10 | | 2 mg | 5 mL | 0.4 | 400 | 4 | | 5 mg | 1 mL | 5.0 | 5000 | 50 | | 5 mg | 2 mL | 2.5 | 2500 | 25 | | 5 mg | 2.5 mL | 2.0 | 2000 | 20 | | 5 mg | 5 mL | 1.0 | 1000 | 10 | | 10 mg | 1 mL | 10.0 | 10000 | 100 | | 10 mg | 2 mL | 5.0 | 5000 | 50 | | 10 mg | 5 mL | 2.0 | 2000 | 20 | | 10 mg | 10 mL | 1.0 | 1000 | 10 |

Read the extremes and the tradeoff is obvious. A 10 mg vial in 2 mL gives 50 mcg per unit — convenient for larger-mass work, coarse for anything small. A 2 mg vial in 5 mL gives 4 mcg per unit: fine resolution, but you draw a lot of volume per unit of mass. Pick the diluent volume that puts your target draw in the middle of the barrel, not at either end.

How do I calculate peptide concentration after reconstitution?

Concentration equals total peptide mass divided by total diluent volume, expressed in the same units. A 10 mg vial with 2 mL of bacteriostatic water added comes to 5 mg/mL, or 5000 mcg/mL, which is 50 mcg per unit on a U-100 insulin syringe.

The formula, step by step:

1. Convert mass to micrograms: 10 mg × 1000 = 10,000 mcg.
2. Divide by diluent volume in mL: 10,000 mcg ÷ 2 mL = 5000 mcg/mL.
3. Convert to per-unit: 5000 mcg/mL × 0.01 mL/unit = 50 mcg/unit.

Step 3 is the one people skip, and it is the one that converts a concentration into something you can actually draw. A U-100 syringe is calibrated so that 100 units = 1 mL, meaning each unit mark is 0.01 mL. Multiply concentration by 0.01 and you have micrograms per unit mark.

One practical note on volume displacement: the lyophilized powder occupies some volume, so the final liquid volume is slightly greater than the diluent you inject. In a 2 mg to 10 mg vial the displacement is small, but it is not zero. If your math needs to be exact, add diluent to the marked graduation rather than trusting the injected volume alone.

Volume-per-unit reference for a U-100 syringe

The table below inverts the concentration math. Find your concentration, read across to the volume you need to draw. Volumes are given in U-100 units and in mL, since some work is done with a 1 mL tuberculin syringe graduated in hundredths.

| Concentration (mcg/mL) | 10 mcg | 25 mcg | 50 mcg | 100 mcg | 250 mcg | |---|---|---|---|---|---| | 1000 | 1 unit / 0.01 mL | 2.5 units / 0.025 mL | 5 units / 0.05 mL | 10 units / 0.10 mL | 25 units / 0.25 mL | | 2000 | 0.5 unit / 0.005 mL | 1.25 units / 0.0125 mL | 2.5 units / 0.025 mL | 5 units / 0.05 mL | 12.5 units / 0.125 mL | | 2500 | 0.4 unit / 0.004 mL | 1 unit / 0.01 mL | 2 units / 0.02 mL | 4 units / 0.04 mL | 10 units / 0.10 mL | | 5000 | 0.2 unit / 0.002 mL | 0.5 unit / 0.005 mL | 1 unit / 0.01 mL | 2 units / 0.02 mL | 5 units / 0.05 mL |

The 0.5-unit and 0.2-unit cells are the failure mode. A standard U-100 insulin syringe has graduations every 1 unit, with half-unit marks on some models. You cannot reliably draw 0.2 units. If your target mass lands below 1 unit at your chosen concentration, dilute further — more diluent, lower concentration, larger draw — or accept that the mass sits below the resolution of the tool. That is a measurement limitation, not a math error.

How much bacteriostatic water for a 5 mg vial?

A 5 mg vial reconstituted with 2 mL of bacteriostatic water yields 2500 mcg/mL, or 25 mcg per unit on a U-100 syringe. The 2 mL figure is common because it produces a workable mid-barrel draw across a range of masses without pushing the concentration so high that small targets fall below syringe resolution.

Bacteriostatic water is sterile water containing 0.9% benzyl alcohol as a bacteriostatic preservative, per the standard USP monograph for Bacteriostatic Water for Injection. The benzyl alcohol is what allows multiple withdrawals from a single vial over weeks rather than the single-use constraint of plain sterile water. It is not a sterilant. It inhibits bacterial growth; it does not eliminate it. Handle vials aseptically and store per the supplier's published specification, typically refrigerated or at controlled room temperature.

Reconstitution steps, plain English:

1. Wipe the stopper of the peptide vial and the diluent vial with an alcohol swab. Let both dry.
2. Draw your target diluent volume into a syringe using a needle sized for the stopper.
3. Inject the diluent down the inside wall of the peptide vial, not directly onto the cake. Aiming at the glass reduces foaming.
4. Do not shake. Swirl gently, or let the vial sit, until the cake is fully dissolved. Foam is denatured surface area.
5. Note the concentration on the label: mass, diluent volume, mcg/mL, and mcg per unit.

Incomplete dissolution read as full dissolution is the most common failure mode. A faint haze or a few undissolved particles at the vial base means the concentration is lower than your math assumes. Swirl again and re-inspect before drawing.

Where to source bacteriostatic water

• BAC Water Depot — bacteriostatic water in sealed multi-dose vials
• Med Lab Supply — sterile diluents and lab consumables
• Ozone Solutions — general lab water and reagent supply

Where to source research peptides

• Alpha Amino USA — per-lot HPLC/MS COA, US-shipped
• Bachem — catalog peptides with published specifications
• Sigma-Aldrich — research-grade peptide inventory with datasheets

For research use only — not clinical guidance.