Peptide reconstitution math, explained with examples
Reconstitution math comes down to three steps: concentration = vial strength ÷ liquid added, volume = dose ÷ concentration, and units = volume × 100 on a U-100 syringe. For a 5 mg vial with 2 mL added, a 250 mcg dose is 0.1 mL, or 10 units.
The rest of this guide walks through each step. If you would rather not do it by hand, the reconstitution calculator does the same arithmetic.
This is unit arithmetic, not dosing advice. The dose, the liquid and the volume to add should come from your prescriber, pharmacist or the product label.
Step 1: get everything into the same units
Vials are labelled in milligrams (mg). Doses are often written in micrograms (mcg). Mixing the two is the most common source of a tenfold or thousandfold error.
- 1 mg = 1,000 mcg
- 0.25 mg = 250 mcg
- 2.5 mg = 2,500 mcg
Convert the dose and the vial strength to the same unit before doing anything else.
Step 2: work out the concentration
Concentration is how much substance is in each millilitre once the liquid has been added.
Concentration = vial strength ÷ volume of liquid added
| Vial | Liquid added | Concentration |
|---|---|---|
| 5 mg | 1 mL | 5 mg/mL (5,000 mcg/mL) |
| 5 mg | 2 mL | 2.5 mg/mL (2,500 mcg/mL) |
| 10 mg | 2 mL | 5 mg/mL (5,000 mcg/mL) |
| 10 mg | 3 mL | 3.33 mg/mL (3,333 mcg/mL) |
Adding more liquid does not change how much peptide is in the vial. It only spreads the same amount through a larger volume.
Step 3: work out the volume for your dose
Volume (mL) = dose ÷ concentration
With a 2,500 mcg/mL solution and a 250 mcg dose: 250 ÷ 2,500 = 0.1 mL.
Step 4: convert the volume to syringe units
A U-100 insulin syringe is marked in units, where 100 units = 1 mL. Each unit is 0.01 mL.
Units = volume (mL) × 100
So 0.1 mL is 10 units, 0.25 mL is 25 units and 0.5 mL is 50 units.
Syringe units are a measure of volume. They are not the same as international units (IU), which measure biological activity and differ for every substance. See the glossary for the difference.
A shortcut: mcg per unit
Once you know the concentration, divide it by 100 to get how many mcg each unit mark holds.
| Concentration | mcg per unit |
|---|---|
| 1 mg/mL | 10 mcg |
| 2 mg/mL | 20 mcg |
| 2.5 mg/mL | 25 mcg |
| 5 mg/mL | 50 mcg |
At 2.5 mg/mL each unit is 25 mcg, so 250 mcg is 10 units and 500 mcg is 20 units.
Checking your answer
Two quick sanity checks catch most mistakes:
- Does it fit the syringe? A 0.3 mL syringe holds 30 units, a 0.5 mL holds 50 and a 1 mL holds 100. A result above that means something is off, or the solution is too dilute for that syringe.
- Do the doses add up? Vial strength ÷ dose gives the number of doses in the vial. Doses × volume per dose should equal the liquid you added.
If a number looks surprising, stop and confirm with a pharmacist before drawing anything.
Keeping the result
The concentration stays the same for the life of that vial, so it is worth writing down once rather than recalculating each time. In PepVault you can record each vial's strength and status alongside your dose log, so the numbers are there when you need them.
This guide is about record keeping and unit arithmetic. It is not medical advice. See the medical disclaimer.
