A conversion reference that actually covers the cases people get wrong

Written out because the errors cluster in predictable places.

**Mass** 1 mg = 1000 mcg. 1 mcg = 0.001 mg.

**Volume** 1 mL = 1000 µL (mcL). U-100 syringe: 100 units = 1 mL, so 1 unit = 0.01 mL. U-40 syringe: 40 units = 1 mL, so 1 unit = 0.025 mL. **Different scale entirely.**

**Concentration to mass-per-unit** (U-100) mcg per unit = concentration in mg/mL × 10

So 5 mg/mL → 50 mcg per unit. 2 mg/mL → 20 mcg per unit. 10 mg/mL → 100 mcg per unit.

That ×10 shortcut only holds for U-100. It's the single most useful line here and the one that silently breaks if the syringe is different.

**Worked backwards** To get 250 mcg from a 5 mg/mL solution: 250 ÷ 50 = 5 units.

**Where people slip** - Reading a mcg figure as mg or vice versa. - Assuming U-100 without checking. - Applying a unit count from a different concentration. - Rounding a fractional unit to something the syringe can't actually resolve.

Evidence tier: arithmetic.