How many units is X mg?
The most-asked question in the category has a two-word answer — "it depends" — and a one-line formula. Here's the formula, then a converter that does it for you.
The formula
units = (dose in mg ÷ concentration in mg/mL) × 100, where concentration = milligrams in the vial ÷ millilitres of water added. That's it. Every chart on the internet is this line evaluated at one particular vial and water.
1 mg at common concentrations
- 2.5 mg/mL (5 mg in 2 mL) → 40 units
- 5 mg/mL (10 mg in 2 mL, or 5 mg in 1 mL) → 20 units
- 10 mg/mL (10 mg in 1 mL, or 20 mg in 2 mL) → 10 units
- 20 mg/mL (100 mg in 5 mL) → 5 units
Why people get this wrong
Three reasons, in order of how often we see them. Units are volume, not amount. One unit on a U-100 syringe is 0.01 mL, full stop; it says nothing about compound until you know the concentration. mg and mcg. A thousand micrograms is a milligram; the vial is labelled in one and the dose is discussed in the other. The water changes everything. Two identical vials reconstituted with different volumes give different units for the same dose, and someone's chart from a forum used their water, not yours.
The converter
The calculator above is the formula with guardrails: it flags doses under 5 units (hard to measure), doses over your syringe's capacity, and doses that land between tick marks. Pick any compound or just use the vial and water fields as a plain mg-to-units converter.
Questions people actually type
How many units is 1 mg?
At 5 mg/mL: 20 units. At 2.5 mg/mL: 40 units. At 10 mg/mL: 10 units. There is no answer without the concentration.
How many units is 0.25 mg?
At 2.5 mg/mL: 10 units. At 5 mg/mL: 5 units.
How many units is 250 mcg?
250 mcg = 0.25 mg. At 2.5 mg/mL: 10 units. At 5 mg/mL: 5 units.
How many mL is 10 units?
10 units on a U-100 insulin syringe is 0.1 mL. 50 units is 0.5 mL. 100 units is 1 mL.
Is a unit the same on every insulin syringe?
On any U-100 syringe, yes: 1 unit = 0.01 mL. The syringe size (0.3, 0.5, 1 mL) changes the capacity and the tick spacing, not the size of a unit.