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How to convert mg to units on an insulin syringe

Published 3 min read

There is no fixed number of units in a milligram: units = (dose in mg / concentration in mg per mL) x units per mL of the syringe, so on a U-100 barrel that is draw volume in millilitres times 100. Any answer given without a concentration is guessing.

What is the mg to units conversion formula?

units = (dose in mg / concentration in mg per mL) x units per mL of the syringe

The middle term is the one people skip. A U-100 insulin syringe is graduated at 100 units per millilitre, so on that barrel the line simplifies to units equals draw volume in millilitres times one hundred.

A worked example

Take a 10 mg vial reconstituted with 2 mL of bacteriostatic water. The concentration is 10 divided by 2, or 5 mg per mL.

  • A 1 mg dose is 1 / 5 = 0.2 mL, which is 20 units on U-100.
  • A 2.5 mg dose is 2.5 / 5 = 0.5 mL, which is 50 units.
  • A 0.25 mg dose is 0.25 / 5 = 0.05 mL, which is 5 units.

Change the water to 1 mL and the same vial is 10 mg per mL. Now the 1 mg dose is 0.1 mL, or 10 units. Same vial, same dose, half the reading, because the water volume changed. This is why how you reconstitute the vial determines what the syringe shows.

Micrograms per unit is the useful number

Once a vial is mixed, the figure worth writing on the lid is micrograms per unit:

mcg per unit = concentration in mcg per mL / units per mL of the syringe

At 5 mg per mL, that is 5000 divided by 100, so 50 mcg per unit on a U-100 barrel. Every dose you might use is then a single multiplication rather than a fresh calculation, and you can check the calculator against your own arithmetic in one step. The calculator on the homepage reports this figure next to the draw volume for exactly that reason.

Rounding is part of the answer

A barrel can be read to about half a unit. If the arithmetic gives 10.4 units, the honest answer is that you can measure 10.5, and that 10.5 units delivers a slightly larger dose than you asked for: at 25 mcg per unit, 262.5 mcg rather than 260 mcg. A calculator that quietly prints 10.4 is describing a precision the equipment does not have. Ours reports both the exact figure and the reading you can actually make, and says when the two differ.

Very small draws are a related problem. Under about two units, a half unit misread is a large proportion of the dose. The arithmetic still works; it is the measurement that gets unreliable, which is why the calculator flags it rather than silently returning 1.2 units.

In short: mg to units is units = (dose / concentration) x syringe units per mL, which on a standard U-100 syringe is simply draw volume in mL times 100.

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