Understanding IU, Reconstitution & Measuring Doses.
A foundational reference on the three measurements every researcher confuses — International Units, milligrams, and millilitres — and the arithmetic that connects them.
Biological activity
Mass / weight
Liquid volume
01 — Definition
What is an IU?
IU stands for International Unit. It is not the same as mg or mL.
An IU is a measurement based on biological activity, while mg measures weight and mL measures liquid volume.
That means you cannot assume that 1 IU = 1 mg or 1 IU = 1 mL. The correct conversion depends entirely on the specific compound.
- IUBiological potency
- mgMass of substance
- mLVolume of solution
02 — Volume
mL & Syringe Units
When measuring liquid, the key volume unit is mL (millilitres). On a U-100 insulin syringe, the printed scale is based on a fixed relationship between units and volume:
"Syringe units" are a volume marking on the barrel — not automatically the same thing as the drug dose itself. Misunderstanding this is the single most common cause of dosing error.
Visual Reference
U-100 Insulin Syringe — Fill Levels
Lowest practical research draw
Common low-dose research volume
Half-barrel — half a millilitre
Full U-100 syringe — exactly 1 mL
Fig. 01 — U-100 syringe: 100 units = 1.0 mL across the full barrel.
03 — Process
What Reconstitution Means
Reconstitution means adding a sterile liquid to a lyophilised powder to create a usable solution. Once mixed, the powder is dissolved into a known volume, allowing you to calculate the final concentration — the amount of substance per mL.
Lyophilised peptide in a sealed vial.
Bacteriostatic water introduced slowly down vial wall.
Fully dissolved solution at a calculable concentration.
04 — Arithmetic
The Two Formulas
→ Every 1 mL contains 5 mg of substance.
→ On a U-100 insulin syringe, 0.2 mL = 20 units.
05 — Reference
Worked Example Table
Vial contains 10 mg. Reconstituted with 2 mL. Resulting concentration: 5 mg/mL.
| Volume Drawn | Substance Dose | U-100 Syringe |
|---|---|---|
| 0.1 mL | 0.5 mg | 10 units |
| 0.2 mL | 1.0 mg | 20 units |
| 0.3 mL | 1.5 mg | 30 units |
| 0.4 mL | 2.0 mg | 40 units |
| 0.5 mL | 2.5 mg | 50 units |
06 — Pitfalls
Common Mistakes to Avoid
Safety Note
Always Confirm Before Drawing
Four data points should be verified — out loud, on paper, or in writing — before any dose is drawn.
Total amount of substance present in the vial
Total mL of solvent introduced during reconstitution
Resulting amount of substance per mL of solution
Confirm the exact syringe (e.g. U-100 insulin syringe)
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