mg, mcg and "units" — the three-way confusion that causes most beginner mistakes
This trips up basically everyone and the error mode is the dangerous kind, so it's worth being extremely explicit.
**mg and mcg are mass.** 1 mg = 1000 mcg. That part is just arithmetic and people usually get it.
**"Units" on a syringe are not mass at all.** On a standard U-100 insulin syringe, 100 units = 1 mL. Units are a *volume* marking. They only correspond to a mass once you know the concentration of what's in the vial.
That's the whole problem. Someone says "10 units" and it means nothing on its own — 10 units of a vial reconstituted one way is a completely different mass from 10 units of the same vial reconstituted another way.
So any time you see a number of units quoted without the reconstitution volume and the vial contents, that number is unusable. Not "roughly right" — genuinely unusable.
The conversion needs three things: how much peptide is in the vial, how much liquid you added, and how many units you're reading. Miss any one and the answer is undefined.
This is exactly the arithmetic the Peptide Library calculator handles, and honestly it's worth using it rather than doing it in your head, because the failure mode of getting it wrong is a ten-fold error rather than a small one.