Pen Dial Units Explained
A dial unit measures liquid, not peptide. Once that is clear the rest is one division. This works through the conversion with tables for the pen sizes in common use.
The fixed relationship
One dial unit = 0.01 ml. A 200-unit pen holds 2 ml. A 300-unit pen holds 3 ml.
That is a property of the pen mechanism and does not change between compounds or strengths. It is the direct equivalent of "100 units is 1ml" on a U-100 insulin syringe, and it causes the same confusion for the same reason: the number on the device counts volume, and people read it as dose.
The conversion
Peptide per unit = concentration (mg/ml) × 0.01
And concentration itself, where it is not printed:
Concentration = total content (mg) ÷ pen volume (ml)
Worked through
A 20mg pen holding 3ml. Concentration is 20 ÷ 3 = 6.67mg/ml. One unit is 0.01ml, so one unit carries 6.67 × 0.01 = 0.0667mg, which is 66.7mcg. Ten units carries 667mcg.
The same 20mg in a 2ml pen would be 10mg/ml, and one unit would carry 100mcg. Same total peptide, different pen, half again as much per unit. This is why the pen size matters and why a dial figure never transfers between products.
Reference tables
200-unit pen (2ml)
| Total content | Concentration | Per unit | 10 units | 20 units |
|---|---|---|---|---|
| 5 mg | 2.5 mg/ml | 25 mcg | 250 mcg | 500 mcg |
| 10 mg | 5 mg/ml | 50 mcg | 500 mcg | 1000 mcg |
| 20 mg | 10 mg/ml | 100 mcg | 1000 mcg | 2000 mcg |
300-unit pen (3ml)
| Total content | Concentration | Per unit | 10 units | 20 units |
|---|---|---|---|---|
| 10 mg | 3.33 mg/ml | 33.3 mcg | 333 mcg | 667 mcg |
| 20 mg | 6.67 mg/ml | 66.7 mcg | 667 mcg | 1333 mcg |
| 40 mg | 13.3 mg/ml | 133 mcg | 1333 mcg | 2667 mcg |
| 50 mg | 16.7 mg/ml | 167 mcg | 1667 mcg | 3333 mcg |
Checking a published figure
Where a spec panel lists content, dial graduation, per-unit amount and total units, those four numbers must agree. Content divided by total units should reproduce the per-unit figure exactly.
If it does not, one of the numbers is wrong, and that is worth resolving before relying on any of them. It is the cheapest sanity check available and it catches transcription errors that are otherwise invisible.
Dual-strength pens
Some compounds are supplied at two strengths in the same pen body. Retatrutide and tirzepatide are both available at 20mg and 40mg, and the pens are physically identical.
The dial behaves the same. What changes is the concentration, and therefore what each unit carries. A 40mg pen delivers exactly twice the peptide per unit as a 20mg pen of the same volume. Read the label rather than the pen body.
Where the number is not printed
Not every product page publishes the pen volume. Where it is missing, the arithmetic still works from total content and concentration: volume equals content divided by concentration. A 20mg pen stated at 6.67mg/ml is a 3ml pen, and therefore 300 units.
Common slips
- Reading dial units as micrograms.
- Carrying a dial figure across from a different strength of the same compound.
- Assuming pen volume from pen size, when two strengths share a body.
- Mixing mg and mcg partway through a calculation.
- Applying a vial concentration to a pen, or the reverse.
For the vial equivalent of all this, see concentration and syringe maths.