How peptide reconstitution works: mg, mL, units and concentration

This page explains the words and the arithmetic. It does not tell you what to take. The amount you draw up is whatever your prescriber wrote; the STACK reconstitution calculator only converts that amount into a volume once you have entered your vial and diluent.

What reconstitution means

Some medicines are supplied as a dry powder that has to be mixed with a liquid before use. Nursing texts define reconstitution as “the process of adding a liquid diluent to a dry ingredient to make a specific concentration of liquid” [1]. The powder is the drug; the liquid (often sterile or bacteriostatic water) is the diluent; the result is a solution with a known strength [1]. That strength is what every later calculation depends on [1].

For an approved product the label states the exact diluent and volume. The Egrifta SV label, for example, says to use only the diluent provided, to reconstitute one 2 mg vial with 0.5 mL of it, to mix by rolling the vial gently for 30 seconds without shaking, to inspect for particles or discoloration, and to discard unused solution [5]. For a compounded product the pharmacy’s label plays the same role. STACK does not replace either.

The four terms you will see

mg (milligram) is the amount of drug in the vial. Vials are labeled by mass, such as “2 mg” or “5 mg” [5].

mcg (microgram) is a smaller mass unit. The standard equivalence is 1 mg = 1000 mcg [2]. Prescriptions for some peptides are written in mcg; the vial is often labeled in mg. Converting between them is the first place errors happen, so STACK shows both.

mL (milliliter) is a volume of liquid. It is the same thing as a cubic centimeter: 1 mL = 1 cc [3].

mg/mL (concentration) is mass divided by volume. It is fixed the moment you finish mixing and it does not change as the vial empties [1].

What a “unit” on an insulin syringe is

Insulin syringes are not marked in mL. They are marked in insulin units, and the marking assumes a specific insulin strength. Humulin R’s label says: “Doses of insulin are measured in units. U-100 insulin contains 100 units/mL (1 mL=1 cc)” [3]. So on a syringe marked for U-100, 100 units of scale corresponds to 1 mL of liquid, and one unit mark corresponds to 0.01 mL [3]. Current Humulin R Instructions for Use likewise list “a U-100 insulin syringe and needle” as the supply that matches a 100 units/mL vial [4].

Two cautions follow from that. First, the “units” on the syringe are a volume scale, not a measure of how much peptide is in it; the peptide amount depends on your concentration. Second, the scale is only correct for U-100 syringes. The Humulin label warns that failing to use the proper syringe “can lead to a mistake in dosage” [3]. STACK’s calculator assumes a U-100 scale and says so.

A worked example (neutral numbers, not a dose)

Take a 5 mg vial and add 2 mL of bacteriostatic water.

• Concentration: 5 mg ÷ 2 mL = 2.5 mg/mL, which is the same as 2500 mcg/mL [2].
• Volume for a given mass: mass ÷ concentration. 0.25 mg ÷ 2.5 mg/mL = 0.1 mL.
• Syringe reading: 0.1 mL × 100 units/mL = 10 units on a U-100 syringe [3].

So in this vial, 10 units on a U-100 syringe holds 0.25 mg (250 mcg). The same 10 units in a vial mixed with 1 mL of water instead would hold twice as much, because the concentration would be 5 mg/mL. The number on the syringe never tells you the amount by itself.

These are illustrative numbers chosen because the arithmetic is clean. They are not an amount anyone should take. Enter your own vial size, your own diluent volume, and the amount your prescriber wrote, and the calculator does this same division.

Why the diluent volume matters (and one subtlety)

The nursing example above uses a 500 mg antibiotic vial reconstituted with 2 mL of water, and the label states the result is 225 mg/mL rather than 250 mg/mL, because the powder itself occupies some volume [1]. The text is explicit that the volume of diluent “must also be considered” and that the concentration printed on the label is what to calculate from [1]. For a few milligrams of lyophilized peptide the powder’s volume is negligible, which is why the simple division works; for a labeled product, use the concentration the label states.

What STACK does with this

The reconstitution calculator stores the vial mass, the diluent volume, and the resulting mg/mL. When you log a dose you enter the amount exactly as your prescriber wrote it, in mg or mcg, and STACK shows the matching mL and U-100 units so you can check them against your syringe. It also decrements the vial inventory. It will not suggest an amount, a schedule, or a change. If the concentration you calculated does not match what your pharmacy label says, or if any step is unclear, stop and call your prescriber or pharmacist.

Try it: Peptide calculator

Turn a vial, a diluent volume and the dose you were given into a concentration, a draw volume and a U-100 syringe reading. A unit converter — it never suggests a dose.

More free tools

Sources

Every URL below was fetched and checked on 2026-09-24. Bracketed numbers in the text link here.

1. OtherReconstituted Medication (5.10) — Nursing Skills 2e, Open RN / WisTech Open
2. OtherEquivalencies (5.4) — Nursing Skills 2e, Open RN / WisTech Open
3. LabelHumulin R (insulin human) injection, U-100 — patient information, FDA-approved label (2003)
4. LabelHumulin R (insulin human) injection — prescribing information and Instructions for Use, DailyMed
5. LabelEgrifta SV (tesamorelin) for injection — prescribing information, FDA (2019)