How to use a peptide calculator: a step-by-step guide

Every reconstitution comes down to one relationship: the mass of peptide in the vial, divided by the volume of diluent you add, is your concentration. A peptide calculator does nothing more than rearrange that sentence. Once you can do it by hand, the calculator becomes a way of checking your work rather than a black box you have to trust.

The three numbers you need

1. Peptide mass in the vial. Printed on the label in milligrams, for example 5 mg or 10 mg.
2. Diluent volume. How much bacteriostatic water you draw into the vial, in millilitres.
3. The amount you want per draw. Usually in micrograms (mcg), sometimes in milligrams.

Everything else, including "units" on an insulin syringe, is derived from those three.

Step 1: choose a diluent volume

There is no single correct volume. The trade-off is:

• More water gives a lower concentration, which makes small amounts easier to measure accurately on a syringe.
• Less water gives a higher concentration, which keeps each draw small but makes tiny amounts harder to read.

For a 5 mg vial, 2 mL is a comfortable middle ground. For 10 mg, 2 to 3 mL. If the vial is small (2 mL or 3 mL glass) do not exceed about two thirds of its capacity, or you will have no headspace to swirl.

Step 2: work out the concentration

Divide mass by volume:

5 mg ÷ 2 mL = 2.5 mg/mL, which is the same as 2,500 mcg/mL.

Keep the unit conversion straight: 1 mg is 1,000 mcg. Nearly every mistake we hear about is a factor-of-ten error made at this step.

Step 3: convert to syringe units

Insulin syringes are marked in "units", where 100 units = 1 mL. That means each unit is 0.01 mL.

So at 2,500 mcg/mL:

To go the other way, divide the amount you want by the concentration, then multiply by 100:

250 mcg ÷ 2,500 mcg/mL = 0.1 mL = 10 units

Use the calculator

Enter the vial size and the amount you want per draw. The calculator picks a water volume that lands the draw on a whole unit and shows the concentration, so you can check it against the steps above.

Want to keep a record? The full calculator can save vials and copy a line for the label on the fridge.

Step 4: sanity check the answer

A quick check that catches most errors: the total number of draws in the vial should equal the mass divided by the per-draw amount. A 5 mg vial at 250 mcg per draw contains 20 draws. If your calculator says you get 200, or 2, a unit slipped somewhere.

Common mistakes

• Mixing mg and mcg. Write both numbers in the same unit before dividing.
• Assuming a 1 mL syringe reads in mL. Most read in units. Check the barrel.
• Changing the water volume and reusing the old figure. Recalculate every time you reconstitute.
• Rounding too early. Keep two decimal places until the last step.

Once the vial is reconstituted, label it with the date and the concentration in mcg per unit. Future you will thank present you.