mcg to mg to units cheat sheet, covering the cases people actually get wrong
Written out because the errors cluster in predictable places.
Mass: 1mg = 1000mcg. 1mcg = 0.001mg.
Volume: 1ml = 1000 microliters. U-100 syringe, 100 units = 1ml so 1 unit = 0.01ml. U-40 syringe, 40 units = 1ml so 1 unit = 0.025ml. Different scale entirely.
Concentration to mcg per unit (U-100 only): mcg per unit = mg/ml x 10. So 5mg/ml is 50mcg per unit, 2mg/ml is 20mcg per unit, 10mg/ml is 100mcg per unit. That x10 shortcut is the single most useful line here and the one that silently breaks if the syringe is different.
Worked backwards: 250mcg from a 5mg/ml solution is 250 / 50 = 5 units.
Where people slip: reading mcg as mg or vice versa, assuming U-100 without checking, applying a unit count from a different concentration, and rounding a fractional unit to something the syringe can't actually deliver.