Insulin syringes are marked in units, not millilitres, and that single fact causes more dosing confusion than anything else in this hobby. Here is the conversion written out so you can stop redoing it in your head.
On a U-100 syringe, 100 units equals 1 mL. So:
- 100 units = 1.00 mL
- 50 units = 0.50 mL
- 25 units = 0.25 mL
- 10 units = 0.10 mL
- 5 units = 0.05 mL
- 2 units = 0.02 mL
That table is independent of what is in the syringe. It is pure volume. The drug amount only enters when you multiply by concentration.
Say your vial is at 2.5 mg/mL. Then each unit carries 0.01 mL x 2.5 mg/mL = 0.025 mg. Twenty units is 0.5 mg. At 5 mg/mL, each unit carries 0.05 mg and twenty units is 1 mg. Same barrel marking, double the drug, because the concentration doubled.
Two practical points. U-40 syringes exist and are marked differently (40 units per mL); if you buy syringes by price rather than by label you will eventually get a surprise. And on a 30-unit barrel the graduations are usually one unit apart, so anything below about two units is guesswork, which is an argument for reconstituting more concentrated when doses are small.
I keep the concentration and the resulting units-per-dose in a peptide dose tracker rather than a note on my phone, mostly so the number is attached to the vial it belongs to. If you want the arithmetic done for you, the Peptide Tracker & Calculator takes vial mg, water mL and target dose and gives the unit mark.
Whatever you use, do the conversion once per vial, write it down, and check it against the syringe in front of you rather than the one you used last month.