Concentration and Syringe Maths
Almost every dosing error with research peptides comes from the same confusion: treating syringe units as though they measure peptide. They measure liquid. This works the arithmetic through from first principles, with tables you can read off directly.
The one relationship to memorise
On a U-100 insulin syringe, 100 units equals 1 millilitre. That is fixed. It is a property of the syringe and has nothing to do with what is dissolved in it.
| Units | Millilitres |
|---|---|
| 100 | 1.0 ml |
| 50 | 0.5 ml |
| 20 | 0.2 ml |
| 10 | 0.1 ml |
| 5 | 0.05 ml |
How much peptide sits in those units depends entirely on how much diluent you added to the vial. The syringe cannot know that, which is why the syringe cannot tell you.
The formula
Concentration (mg/ml) = total peptide in the vial (mg) ÷ diluent volume added (ml)
And to find what a given draw contains:
Peptide in a draw = concentration (mg/ml) × volume drawn (ml)
That is the whole of it. Everything below is those two lines applied.
Worked from both directions
Forward. A 10mg vial, 2ml of diluent added. 10 ÷ 2 = 5mg/ml. Drawing 10 units is 0.1ml, so 5 × 0.1 = 0.5mg, which is 500mcg.
Backward. You want 250mcg from that same vial. 250mcg is 0.25mg. 0.25 ÷ 5 = 0.05ml, which is 5 units.
Reference tables
5mg vial
| Diluent | Concentration | 10 units | 20 units | 50 units |
|---|---|---|---|---|
| 1 ml | 5 mg/ml | 500 mcg | 1000 mcg | 2500 mcg |
| 2 ml | 2.5 mg/ml | 250 mcg | 500 mcg | 1250 mcg |
| 3 ml | 1.67 mg/ml | 167 mcg | 333 mcg | 833 mcg |
| 5 ml | 1 mg/ml | 100 mcg | 200 mcg | 500 mcg |
10mg vial
| Diluent | Concentration | 10 units | 20 units | 50 units |
|---|---|---|---|---|
| 1 ml | 10 mg/ml | 1000 mcg | 2000 mcg | 5000 mcg |
| 2 ml | 5 mg/ml | 500 mcg | 1000 mcg | 2500 mcg |
| 3 ml | 3.33 mg/ml | 333 mcg | 667 mcg | 1667 mcg |
| 5 ml | 2 mg/ml | 200 mcg | 400 mcg | 1000 mcg |
30mg vial
| Diluent | Concentration | 10 units | 20 units | 50 units |
|---|---|---|---|---|
| 2 ml | 15 mg/ml | 1.5 mg | 3 mg | 7.5 mg |
| 3 ml | 10 mg/ml | 1 mg | 2 mg | 5 mg |
| 6 ml | 5 mg/ml | 0.5 mg | 1 mg | 2.5 mg |
Choosing a diluent volume
There is no correct answer, but there is a principle: put your intended measurement in the middle of the syringe, not at the bottom.
Reading a syringe carries roughly a one to two unit uncertainty. At 50 units that is a two to four percent error. At 4 units it is twenty-five to fifty percent. Same syringe, same hand, radically different consequence.
So if a calculation lands you on 3 or 4 units, add more diluent and recalculate. Dilution costs nothing and buys accuracy. The limit is the vial's capacity and how long the solution will sit, since time in solution is what degrades peptides.
Units and micrograms
Two conversions worth having: 1 mg = 1000 mcg, and on a U-100 syringe 1 unit = 0.01 ml. Nearly every arithmetic slip traces to one of those two.
Where pens differ
None of this applies to a pre-filled pen. The concentration was fixed at manufacture and dose is set by dialling units on the mechanism, where the relationship between a dial unit and a volume is a property of the device. That removes the calculation and its errors, at the cost of being unable to choose your concentration. See pens compared with vials.
Common slips
- Reading syringe units as micrograms.
- Changing vial size without recalculating for the new diluent volume.
- Mixing mg and mcg mid-calculation.
- Assuming a concentration carries over from a different product.
- Measuring at the very bottom of the syringe scale where error is proportionally largest.