20 ÷ 5 = 4 mg/mL. Concentration is vial content divided by diluent volume, so 20 mg of peptide in 5 mL of bacteriostatic water gives 4 mg/mL. On a U-100 barrel one unit is 0.01 mL, so one unit of this solution carries 0.04 mg — 40 µg. That is the number to write on the label, because you will not reconstruct it from memory at an awkward moment.
The part that matters: this is not exotic. It is just the difference between doing it deliberately and doing it approximately.
Room temperature before drawing is worth the ten minutes. Cold solution is more viscous, draws slower, and is more likely to pull a bubble past the plunger seal.
Concentration and unit conversion at a glance
| Vial | Diluent | Concentration | 0.25 mg | 0.5 mg | 1 mg | 2.5 mg |
|---|
| 5 mg | 1 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 5 mg | 2 mL | 2.5 mg/mL | 10 u | 20 u | 40 u | 100 u |
| 10 mg | 1 mL | 10 mg/mL | 2.5 u | 5 u | 10 u | 25 u |
| 10 mg | 2 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 10 mg | 3 mL | 3.33 mg/mL | 7.5 u | 15 u | 30 u | 75 u |
Units are U-100 insulin units, where 1 unit = 0.01 mL. Divide dose by concentration for millilitres, then multiply by 100.
It helps to be literal here: photograph the vial against a matte black card with a single point light source off to one side, not with a flash from the front.
One limitation: technique reduces risk, it does not remove it.
Do the arithmetic twice, ideally with someone else doing it independently.