Last updated: September 2026

Peptide Reconstitution Calculator

Quick answer: A 10 mg vial reconstituted to 1,000 mcg/mL needs 10 mL of bacteriostatic water (10,000 ÷ 1,000 = 10 mL). Formula: BAC water (mL) = vial content (mg × 1000) ÷ target concentration (mcg/mL).

About this calculator

The peptide reconstitution calculator answers one question: how much bacteriostatic water do I add to my vial? Enter your vial size (in mg or mcg) and your target concentration (mcg/mL) — the calculator gives you the exact BAC water volume using the formula BAC water (mL) = vial content (mg × 1000) ÷ target concentration (mcg/mL). Once you have your answer, hit Transfer to pre-fill the Peptide Dosage Calculator with your concentration and calculate per-injection draw volumes. Free, instant, no login.

How to use this calculator

You need two things: the amount of peptide in the vial (printed on the label — e.g., 10 mg) and the concentration you want to achieve in mcg/mL.

Enter the vial size first. If your vial is labeled in mcg instead of mg, tap the unit toggle to switch. Then set your target concentration — 1000 mcg/mL is a common starting point. The calculator instantly shows how many milliliters of BAC water to draw up and inject into the vial.

Once reconstituted, tap Transfer to Dosage Calculator to carry your concentration through to the dosage calculator and compute how many units to draw per injection.

Worked example — 10 mg vial at 1000 mcg/mL

Reconstitution

Vial size: 10 mg (10,000 mcg)

Target concentration: 1,000 mcg/mL

BAC water to add = 10,000 ÷ 1,000 = 10 mL

Worked example — 5 mg vial at 500 mcg/mL

Vial size: 5 mg (5,000 mcg)

Target concentration: 500 mcg/mL

BAC water to add = 5,000 ÷ 500 = 10 mL

Common reconstitution volumes by vial size

Vial size	Target conc.	BAC water to add
5 mg	500 mcg/mL	10 mL
5 mg	1,000 mcg/mL	5 mL
10 mg	1,000 mcg/mL	10 mL
10 mg	2,000 mcg/mL	5 mL
2 mg	500 mcg/mL	4 mL

How to add BAC water to a peptide vial

Draw up the calculated BAC water volume into a sterile syringe. Insert the needle into the vial through the rubber stopper and let the water run slowly down the inside wall of the vial — do not aim directly at the powder. Once all the water is in, gently swirl (do not shake). The powder should dissolve completely within a minute. If it does not, let the vial sit refrigerated for 15–30 minutes and swirl again.

Store reconstituted vials refrigerated at 2–8°C and discard after 30 days, or per the manufacturer's guidance for your specific peptide.

Step-by-step reconstitution guide

1. Draw up BAC water

Fill syringe to the calculated volume

2. Run down the wall

Don't aim at the powder — let water slide down the inside wall

3. Swirl — never shake

Gently roll or swirl until fully dissolved. Do not vortex.

BAC water volume to concentration chart

These examples use a 10 mg vial and a 1 mg sample dose. Adding water changes concentration and draw volume, but not the vial's total amount. Values illustrate the formulas and are not dosing guidance.

BAC water added	Concentration (10 mg vial)	1 mg dose draw	Units (U-100)
1 mL	10 mg/mL	0.10 mL	10 units
2 mL	5 mg/mL	0.20 mL	20 units
2.5 mL	4 mg/mL	0.25 mL	25 units
4 mL	2.5 mg/mL	0.40 mL	40 units
5 mL	2 mg/mL	0.50 mL	50 units

For each row, concentration equals 10 mg divided by water volume. The sample draw is 1 mg divided by concentration, then multiplied by 100 to convert mL to U-100 units. More water lowers concentration and makes the same dose occupy a larger volume.

Worked examples: water, concentration and units

5 mg vial, 2 mL water

5 mg ÷ 2 mL = 2.5 mg/mL (2,500 mcg/mL). 250 mcg ÷ 2,500 mcg/mL = 0.10 mL.

0.10 mL × 100 = 10 units.

5 mg vial, 1 mL water

5 mg ÷ 1 mL = 5 mg/mL. A 250 mcg dose is 0.25 mg; 0.25 mg ÷ 5 mg/mL = 0.05 mL.

0.05 mL × 100 = 5 units.

10 mg vial, 2 mL water

10 mg ÷ 2 mL = 5 mg/mL. 1 mg ÷ 5 mg/mL = 0.20 mL.

0.20 mL × 100 = 20 units.

2 mg vial, 1 mL water

2 mg ÷ 1 mL = 2 mg/mL (2,000 mcg/mL). 500 mcg ÷ 2,000 mcg/mL = 0.25 mL.

0.25 mL × 100 = 25 units.

15 mg vial, 3 mL water

15 mg ÷ 3 mL = 5 mg/mL. 2.5 mg ÷ 5 mg/mL = 0.50 mL.

0.50 mL × 100 = 50 units.

Choose water for a readable draw

A 1 mg vial with 1 mL water is 1 mg/mL (1,000 mcg/mL). A 100 mcg sample is 0.10 mL, or 10 units. With 2 mL water, concentration is 500 mcg/mL; 100 mcg ÷ 500 mcg/mL = 0.20 mL.

0.20 mL × 100 = 20 units.

Convert units back to dose

Eight U-100 units equal 0.08 mL. In a 10 mg vial with 2 mL water, concentration is 5 mg/mL. 0.08 mL × 5 mg/mL = 0.4 mg.

0.4 mg × 1,000 = 400 mcg.

Avoid the mg and mcg mix-up

250 mcg equals 0.25 mg. At 2.5 mg/mL, 0.25 mg ÷ 2.5 mg/mL = 0.10 mL.

0.10 mL × 100 = 10 units.

Common BAC water mistakes

Mixing mg and mcg: 1 mg equals 1,000 mcg, so convert both dose and concentration to matching units before dividing. Exceeding syringe capacity: a U-100 insulin syringe typically holds 1 mL; a computed draw above that capacity needs a different plan confirmed with a qualified prescriber.

Reusing equipment: use a new sterile needle and syringe for each draw and follow the product label and local instructions. Assuming every diluent is suitable: bacteriostatic water contains benzyl alcohol and is not for use in neonates. Product suitability, storage, and beyond-use time depend on the specific product; arithmetic cannot determine them.

Frequently asked questions

Does it matter how much BAC water I add?

Yes — the amount determines concentration, which directly changes how many units you draw per dose. Less BAC water = higher concentration = smaller injection volumes. More BAC water = lower concentration = larger injection volumes. There is no universally correct amount; it depends on your dose range and syringe preference.

What is bacteriostatic water?

Bacteriostatic water is sterile water preserved with 0.9% benzyl alcohol, which inhibits bacterial growth. It is the standard diluent for multi-dose peptide vials because the preservative keeps the reconstituted solution safe across multiple needle punctures. Do not use plain sterile water for multi-dose vials — it has no preservative.

What target concentration should I use?

1000 mcg/mL (1 mg/mL) is a common starting point because the maths is simple and the resulting injection volumes are manageable on a standard 1 mL insulin syringe. Higher concentrations mean smaller injections; lower concentrations mean larger ones. The right choice depends on your dose and syringe size.

Can I use sterile water instead of bacteriostatic water?

Sterile water has no preservative and is only suitable for single-use reconstitution. Bacteriostatic water is preferred for multi-dose vials. If your vial will be used in a single injection, sterile water is acceptable, but BAC water is the safer default for any multi-dose use.

How long does reconstituted peptide last?

Most peptides are stable for approximately 30 days when refrigerated at 2–8°C after reconstitution with bacteriostatic water. Stability varies by peptide — some are more fragile. Freeze lyophilised (unreconstituted) vials for long-term storage; once reconstituted, refrigerate and use within 30 days.

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How this is calculated

This calculator works out your concentration (peptide amount ÷ bacteriostatic water added), then the draw volume for a target dose and the matching insulin-syringe units (volume in mL × 100 for a U-100 syringe). It is arithmetic only — nothing is stored — and it is not medical advice. Confirm your protocol with your prescriber.

References

Bacteriostatic Water for Injection, USP — label (0.9% benzyl alcohol; for reconstitution) — FDA / DailyMed, 2024

Citations are provided for transparency and education. They do not imply the cited authors or bodies endorse this tool. Always confirm dosing with your prescriber.