Peptide Reconstitution Calculator

How do you calculate a peptide reconstitution?

A reconstitution calculation takes three divisions and one multiplication. First, concentration equals the milligrams of peptide in the vial divided by the milliliters of water you add, so a 5 mg vial mixed with 2 mL is 2.5 mg per mL. Second, the volume to draw equals your dose divided by the concentration, so a 0.25 mg dose is 0.25 divided by 2.5, which is 0.1 mL. Third, syringe units equal that volume multiplied by the units per milliliter on the barrel. Standard insulin syringes use 100 units per mL, so 0.1 mL reads as 10 units.^[1] The dose in this example is only an illustration of the arithmetic. The calculator converts the dose your prescriber gives you and never chooses one. Changing the water changes the concentration and the draw volume, but the total peptide in the vial stays the same.

How much bacteriostatic water should you add to a peptide vial?

There is no single correct volume of water. Water changes the concentration, not the amount of peptide, so more water gives a larger and easier to read draw, and less water gives a smaller draw. A practical way to choose is to start from the draw you want. If you want to draw 10 units on a U-100 syringe for a 0.3 mg dose from a 10 mg vial, the water is 10 times 10, divided by 0.3 times 100, which is 3.33 mL. Using 3 mL instead gives 9 units. Bacteriostatic water is sterile water with a preservative. One manufacturer label describes it as water for injection with 0.9 or 1.1 percent benzyl alcohol, supplied in a multiple-dose container from which repeated withdrawals may be made.^[3] Check the product label and ask your pharmacist which diluent your product calls for.

Concentration

Milligrams of peptide per milliliter after mixing.

U-100 syringe

An insulin syringe marked at 100 units per milliliter.

Bacteriostatic water

Sterile water with a preservative such as benzyl alcohol, sold in multiple-dose vials.^[3]

How many units is 0.1 mL on an insulin syringe?

On a U-100 insulin syringe, 0.1 mL is 10 units. Standard insulin contains 100 units in 1 mL, and every small notch on a standard 1 mL insulin syringe is 1 unit, so one unit is 0.01 mL.^[1] Units are a volume mark, not an amount of peptide, which is why the same volume reads differently on another scale. A U-40 syringe matches insulin with 40 units per mL, such as the veterinary insulin Vetsulin, so 0.1 mL is 4 units there.^[6] Mixing up units, milliliters, and milligrams causes real harm. In a 2024 alert, the FDA described patients told to draw a 5-unit dose, which is 0.05 mL on a U-100 syringe, who instead drew 50 units, and it reported patients taking five to 20 times the intended dose.^[2] Always confirm the scale printed on your syringe before you read a mark.

What if your dose lands between syringe lines or will not fit?

A dose can fall between unit lines, or it can need more volume than the barrel holds. Take a 5 mg vial mixed with 2.5 mL of water and a 0.25 mg dose. The concentration is 2 mg per mL, the volume is 0.125 mL, and that is 12.5 units on a U-100 syringe. You cannot draw half a line accurately, and rounding to 13 units would deliver about 0.26 mg. The cleaner fix is to change the water. Using 2 mL puts the same dose on exactly 10 units. If the dose needs more units than the barrel holds, for example 150 units on a 100 unit barrel, you can add less water, split the draw, or use a larger syringe. The calculator flags both cases and always checks the barrel size you select.

How do you draw a measured dose out of a vial?

Once the vial is mixed, you draw the measured volume the same way for any vial of injectable medicine, and the calculator tells you which line that volume sits on. Wipe the rubber top clean with an alcohol pad. Pull back the plunger to the line on the syringe for your dose, which fills the syringe with air, then push that air into the vial so a vacuum does not form. Turn the vial upside down, keep the needle tip in the liquid, and pull the plunger back to the line for your dose. Tap the syringe to move air bubbles to the top, then push gently to return the bubbles to the vial before you pull back to the final dose line.^[5] Your clinician or pharmacist can show you the technique in person before your first injection, and can confirm which line to read on your syringe.

1. Clean the topWipe the rubber top of the vial with an alcohol pad and let it dry.
2. Add airPull the plunger back to your dose line, then push that air into the vial.
3. Draw the doseTurn the vial upside down and pull the plunger back to the dose line.
4. Clear bubblesTap the syringe so bubbles rise, then push them back into the vial.

How long can you keep a mixed multi-dose vial?

Follow the labeling for your specific product and ask your pharmacist, because storage time depends on the medicine. The FDA recommends that vials containing multiple doses of sterile medications be used within 28 days after the first use, because the risk of contamination rises after 28 days, and it advises discarding the vial after that time even if medication remains.^[4] The bacteriostatic water label adds that you should not store reconstituted solutions of drugs for injection unless the manufacturer of the drug says otherwise.^[3] A calculator cannot know your product's rules, so write the mixing date on the vial and check it against the instructions that came with it. If you are unsure whether a vial is still usable, ask your pharmacist instead of guessing. Storage rules differ between products, so your product's own leaflet outweighs any general guidance on this page.