Lesson 20 of 36 · 8 min
mg, mcg, mL & syringe units.
Four labels show up again and again. The easiest way to avoid bad calculations is to understand what each one measures before doing any math.
The four concepts
mg
Milligrams. An amount of substance.
mcg
Micrograms. A smaller amount of substance. 1 mg = 1,000 mcg.
mL
Millilitres. A volume of liquid.
U-100 units
A volume scale on a U-100 insulin syringe. 100 units = 1 mL.
The units map
Mass ≠ volume ≠ syringe markings.
These numbers answer different questions. Concentration and the syringe scale connect them.
- MassHow much substance?
mg / mcg
1 mg = 1,000 mcg
- Requires concentration (mg/mL)VolumeHow much liquid?
mL
Mass ÷ concentration = volume
- Check the syringe scaleSyringe markingsWhere on the syringe?
U-100 units
U-100 insulin syringe only: 1 mL = 100 units.
Other syringe scales may differ.
Then concentration connects amount to volume.
Concentration tells you how much substance exists in a given liquid volume. A common way to express it is mg per mL.
Arithmetic example
10 mg + 2 mL → 5 mg/mL
Concentration
5 mg/mL
Same concentration
5,000 mcg/mL
0.1 mL contains
0.5 mg / 500 mcg
This is a unit-conversion example only. It is not a recommended target amount.
What does “10 units” mean?
On a U-100 insulin syringe, 100 units represents 1 mL of liquid. That means the syringe markings can be converted directly into volume.
10 units
0.10 mL
30 units
0.30 mL
50 units
0.50 mL
Why this matters in the real world
FDA has documented 5–20× dosing errors from exactly this kind of unit confusion.
In reports involving compounded semaglutide supplied in vials, patients and clinicians confused milligrams, millilitres and syringe “units.” FDA described overdoses involving five to twenty times the intended amount. The arithmetic is not advanced; the failure was mixing up what each number represented.
mg / mcg
amount of drug
mL
liquid volume
U-100 units
a syringe volume scale
Try it
The calculator does this math transparently.
Enter the vial amount, liquid volume and a target amount. The Pep Report shows the concentration, draw volume and U-100 syringe units without choosing a target amount for you.
Open calculatorFurther reading
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