Four units appear in almost every peptide calculation, and the reason they cause trouble is not that any one of them is complicated. It is that two of them measure a completely different kind of thing from the other two, and the notation does not make that obvious.
| Unit | Measures | Family |
|---|---|---|
| mg (milligram) | Mass of peptide | Amount of substance |
| mcg (microgram) | Mass of peptide | Amount of substance |
| mL (millilitre) | Volume of liquid | Amount of liquid |
| units (U-100) | Volume of liquid | Amount of liquid |
Inside each family, conversion is a fixed multiplication. Between families, it is not possible at all without knowing the concentration.
mg and mcg: a factor of one thousand
A milligram is a thousand micrograms. Vials are commonly labelled in mg while doses are commonly written in mcg, so this conversion sits in the middle of nearly every calculation, and being off by a factor of 1,000 does not look like a small error, which is at least a mercy.
| mcg | mg |
|---|---|
| 50 mcg | 0.05 mg |
| 100 mcg | 0.1 mg |
| 250 mcg | 0.25 mg |
| 500 mcg | 0.5 mg |
| 1,000 mcg | 1 mg |
| 5,000 mcg | 5 mg |
mL and units: a factor of one hundred
A U-100 insulin syringe is graduated at 100 units per millilitre. So one unit is 0.01 mL, and the conversion holds regardless of what is in the barrel, because the markings describe volume, not content.
| Units | mL |
|---|---|
| 5 units | 0.05 mL |
| 10 units | 0.10 mL |
| 25 units | 0.25 mL |
| 50 units | 0.50 mL |
| 100 units | 1.00 mL |
The U-100 part matters. U-40 and U-50 syringes are graduated on a different scale, and applying a U-100 conversion to them produces a wrong answer that looks entirely plausible. The scale is printed on the barrel.
Concentration is the bridge
To get from an amount of peptide to an amount of liquid, you need to know how much peptide is in each millilitre. That figure is concentration, expressed in mg/mL, and it comes from the reconstitution:
Concentration (mg/mL) = vial strength (mg) ÷ diluent volume (mL)
This is why a dose in mcg has no fixed unit equivalent. 250 mcg is 5 units from one vial and 15 units from another, depending entirely on how each was reconstituted. Anyone who quotes a unit figure without stating the concentration has left out the load-bearing half of the sentence.
The full chain, in order
- 1Start with the dose your healthcare professional has set
Usually in mcg. This is the only number in the chain that is not arithmetic.
- 2Convert mcg to mg
Divide by 1,000. 250 mcg becomes 0.25 mg.
- 3Find the concentration
Vial strength divided by diluent volume. A 5 mg vial in 2 mL is 2.5 mg/mL.
- 4Convert mg to mL
Dose in mg divided by concentration. 0.25 ÷ 2.5 = 0.1 mL.
- 5Convert mL to units
Multiply by 100. 0.1 mL is 10 units on a U-100 barrel.
A quick sanity check
Two checks catch most slips before they matter. First: does the volume make sense against the barrel? A calculated draw of 200 units does not fit in a 1 mL syringe, so something upstream is wrong. Second: does the dose count match the vial? Vial strength divided by dose should give a whole-ish number of doses. A 5 mg vial against a 250 mcg dose is 20, which is plausible. If that arithmetic returns 2,000, an mg/mcg conversion went the wrong way.
