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Reconstitution

How to reconstitute peptides: the math, step by step

Reconstitution turns a freeze-dried powder into a liquid with a known strength. The chemistry is simple and the math is three lines. Most errors come from mixing up units, not from the arithmetic. This guide walks through both, without telling you what dose to take.

Medically reviewed by Juan Pablo Frías, MD · · DisclosureUpdated 6 min read
Concentration
vial mg ÷ water mL
The water changes strength, not the amount in the vial.
Volume to draw
dose mg ÷ mg per mL
U-100 units
mL × 100
1 unit = 0.01 mL on a U-100 syringe.
Storage after mixing
Product-specific
Follow the product label; research vials have no label value. See "Storage after mixing" below.

What reconstitution means

Many peptides are supplied as a lyophilized (freeze-dried) powder because they are more stable dry than in solution. Before the contents can be measured with a syringe, a liquid called a diluent is added to the vial. The result is a solution with a known concentration, usually written in milligrams per millilitre (mg/mL).

The single most important idea: the amount of water you add does not change how much peptide is in the vial. A 10 mg vial holds 10 mg whether you add 1 mL or 3 mL. The water only decides how concentrated the solution is, and therefore how much liquid you need to draw for a given amount.

Which diluent a product needs is set by its label. For prescription products, the pharmacy or the package insert names it. Bacteriostatic water is the most common diluent discussed online; our bacteriostatic water guide explains what it is and how it differs from sterile water.

The three numbers you need

  1. Vial strength in mg. Printed on the vial or label. Check whether it is the total contents or a per-mL figure; prescription products sometimes state both.
  2. Volume of diluent in mL. Whatever volume was actually added. Write it on the vial label so you never have to guess later.
  3. Syringe scale. Almost all insulin syringes sold for human use are U-100, meaning 100 units per mL. U-40 syringes also exist. Read our insulin syringe guide before you trust a unit count.

With those three numbers, any dose that a prescriber or a published trial specifies can be turned into a volume and a syringe reading. Without any one of them, the answer is a guess.

The formula

concentration (mg/mL) = vial mg ÷ water mL
volume (mL) = dose mg ÷ concentration
units = volume mL × 100 (U-100 syringe)
The same three lines run inside our reconstitution calculator, which shows its working under every answer.

If the dose is written in micrograms (mcg), convert it first: 1 mg = 1,000 mcg, so 250 mcg = 0.25 mg. The mcg to mg converter does this with the working shown.

Worked examples

These numbers illustrate the arithmetic only. They are not dose recommendations for any peptide.

Example 1: 10 mg vial, 2 mL of water, 2 mg drawn

  • Concentration: 10 mg ÷ 2 mL = 5 mg/mL
  • Volume: 2 mg ÷ 5 mg/mL = 0.40 mL
  • Units: 0.40 mL × 100 = 40 units on a U-100 syringe

Example 2: 5 mg vial, 2 mL of water, 250 mcg drawn

  • Convert: 250 mcg = 0.25 mg
  • Concentration: 5 mg ÷ 2 mL = 2.5 mg/mL
  • Volume: 0.25 mg ÷ 2.5 mg/mL = 0.10 mL
  • Units: 0.10 mL × 100 = 10 units
One 10 mg vial, three water volumes
Water addedConcentrationUnits per 1 mgUnits per 0.25 mg
1 mL10 mg/mL10 units2.5 units
2 mL5 mg/mL20 units5 units
3 mL3.33 mg/mL30 units7.5 units

The table shows the trade-off. More water makes each unit hold less peptide, which makes small amounts easier to measure but makes larger amounts take up more of the syringe. Less water does the opposite: tiny volumes that are hard to read accurately. Neither is "correct"; what matters is that the concentration is known and written down.

Try your own numbers in the reconstitution calculator. It flags a volume that would overflow the syringe you picked and shows how many draws a vial holds.

Common mistakes

  • Mixing up mg and mcg. The two differ by a factor of 1,000. A decimal point in the wrong place is a tenfold error; a unit mix-up is a thousandfold one.
  • Using U-100 math on a U-40 syringe. On a U-40 syringe, 40 units is 1 mL, not 0.4 mL. The volume is 2.5 times larger than intended.
  • Forgetting how much water went in. Label the vial with the date, the volume added and the resulting mg/mL.
  • Misreading tick marks. 1 mL syringes are often marked in 2-unit steps, while 0.3 mL syringes may show 1-unit or half-unit steps. Count the marks on your own barrel.
  • Trusting a chart made for a different vial. A chart built for a 5 mg vial is wrong for a 10 mg vial with the same water. Always recompute.
  • Rounding early. Keep full precision until the final unit reading, then round to the nearest mark the syringe can show.

General handling principles

For prescription products, the package insert and the dispensing pharmacist are the authority on how to mix, store and discard the product. The principles below are the general ones that appear across sterile-product instructions. They do not replace product-specific directions.

  • Work on a clean surface with clean hands, and wipe the rubber stoppers with an alcohol swab before piercing them.
  • Use a new, sterile needle and syringe for each vial entry. Never share needles or syringes.
  • Add the diluent slowly, letting it run down the inside wall of the vial rather than spraying it onto the powder.
  • Swirl or roll gently until dissolved. Shaking can damage some peptides and creates foam that makes measuring harder.
  • Do not use a solution that is cloudy, discoloured or contains particles unless the label says this is expected.
  • Store the mixed vial exactly as the label directs. Storage time after mixing differs by product. The Pregnyl (chorionic gonadotropin) label says the reconstituted solution is stable for 60 days when refrigerated and not to be frozen. The Genotropin (somatropin) label says to store the reconstituted cartridge refrigerated for up to 28 days. Research-labelled vials have no label, so no primary source gives their storage time, and we do not guess it.
  • Dispose of used needles in a sharps container.
Research-labelled products are not approved for human use. Vials sold "for research use only" have no FDA-reviewed label, no verified purity and no tested storage data. The math on this page is the same either way, but the safety questions are not. Talk to a licensed clinician before using any peptide.

Per-peptide calculators and vial pages

Each peptide we cover has its own page with the vial sizes commonly sold, status and sourced data where it exists. For the most searched ones, start with retatrutide, tirzepatide or BPC-157, or browse the full peptides A–Z list. Dedicated calculators such as the retatrutide calculator preload the common vial sizes.

Common questions

Does adding more water change the dose?

No. The amount of peptide in the vial is fixed. More water lowers the concentration, so the same dose takes more liquid and more units on the syringe.

How much water should I add to a vial?

There is no universal answer. Use the volume named on the product label or by the pharmacist. If none is given, pick a volume that makes the amounts you need easy to read on your syringe, write it down, and recompute everything from that concentration.

What if the calculated volume is more than my syringe holds?

Either use a larger syringe or reconstitute with less water so the solution is more concentrated. Our calculator warns when a draw would overflow the syringe you selected.

Why does my answer differ from a chart I found online?

Charts assume a specific vial size, water volume and syringe scale. If any of the three differs, the units differ. Recompute with your own numbers.

Keep going

Retatrutide · Investigational · phase 3Tirzepatide · FDA approved (Mounjaro, Zepbound)BPC-157 · PCAC voted 8–6 for 503A (Jul 2026)GHK-Cu · Injectable off Category 2 (Apr 2026)

Sources

  1. 1Pregnyl (chorionic gonadotropin) for injection: prescribing informationDailyMed, U.S. National Library of Medicine · 2026dailymed.nlm.nih.govAccessed 2 Oct 2026
  2. 2Genotropin (somatropin) for injection: prescribing informationDailyMed, U.S. National Library of Medicine · 2026dailymed.nlm.nih.govAccessed 2 Oct 2026

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