A lyophilized peptide vial is labelled in milligrams. An insulin syringe is marked in units. Nothing on either label tells you how to get from one to the other, and that gap is where most dosing mistakes happen. The arithmetic is short, so here it is in full.

Step 1: work out the concentration

concentration (mg/mL) = vial contents (mg) / bacteriostatic water added (mL)

A 5 mg vial reconstituted with 2 mL of bacteriostatic water gives 2.5 mg/mL. The number printed on the vial is a total, not a concentration. Treating "5 mg" as "5 mg per mL" is the single most common error.

Step 2: work out the volume

volume (mL) = target dose (mg) / concentration (mg/mL)

A 0.25 mg dose at 2.5 mg/mL is 0.1 mL.

Step 3: convert to syringe units

A U-100 insulin syringe holds 1 mL across 100 units, so:

units = volume (mL) x 100

0.1 mL is 10 units. Collapsing all three steps into one line:

units = (target mg / vial mg) x water added (mL) x 100

Choosing the dilution volume

The same vial can be reconstituted several ways, and the choice changes how easy the syringe is to read.

More diluent spreads the same dose across more gradations, so a one-gradation misread costs proportionally less. The tradeoffs are a larger injection volume and a vial that may not physically hold the water. Check the vial capacity before choosing.

How many doses are in the vial

total units in vial = water added (mL) x 100

doses = total units / units per dose

A vial reconstituted with 2 mL holds 200 units. At 10 units per dose that is 20 doses; at 20 units per dose it is 10. When a titration step doubles the dose, the vial empties twice as fast, and whatever is left over at the end that falls short of one full dose is waste. Planning the step-up date and the vial change date together is what keeps that waste small.

Four mistakes worth naming

1. Reading the vial label as a concentration.

2. Using unit numbers from a U-100 syringe on a U-40 or U-50 syringe. The gradations mean different volumes.

3. Not writing down how much water went in. Once you forget the dilution, the remaining vial has no known concentration.

4. Carrying the old unit count forward after a dose change.

Keep the numbers somewhere

None of this is hard maths, but the dilution differs per vial and the dose changes on a schedule, which is exactly the shape of problem where arithmetic done from memory goes wrong. I keep the concentration and the per-injection unit count in the Peptide Tracker & Calculator and re-check the syringe reading with its reconstitution calculator whenever the dose steps up. A spreadsheet does the job too; the point is that the dilution volume is recorded next to the dose rather than recalled later.

This covers the arithmetic only. Doses and titration schedules come from your prescriber.