mg is mass, mL is volume, IU is biological activity, a syringe unit is a volume mark. What each label means and how they relate. Not medical advice.
Key points
- 01Milligrams measure mass. A milligram is an amount of substance, and it is the number a vial label puts front and centre. It says nothing about how much liquid that substance is dissolved in, which is why an mg figure on its own never tells you what to draw.
- 02Millilitres measure volume. A millilitre is an amount of liquid. Diluent volumes, syringe capacities, and pen cartridge sizes are all stated in mL. Volume is the quantity your hand controls at the plunger; mass is the quantity your body responds to.
- 03Concentration is the only bridge between the two. Milligrams per millilitre is the conversion factor that lets you move between mass and volume. Mass equals volume times concentration, and volume equals mass divided by concentration. Every dosing calculation on this site is one of those two lines.
- 04IU measures biological activity, not mass. An international unit is defined separately for each substance by an agreed reference preparation, so there is no universal mg-per-IU figure. An IU of one substance and an IU of another are unrelated quantities that happen to share a name.
- 05A syringe unit is not an IU. On a U-100 insulin syringe, 100 units fill 1 mL, so one unit is 0.01 mL of whatever is in the barrel. That mark is a volume, and it is the single most common mix-up on the page: the barrel says units, the protocol may say IU, and they are different quantities.
- 06Write all four numbers down before you draw. Vial mass in mg, diluent volume in mL, the resulting concentration, and the target dose in whichever label the protocol uses. Four lines on a piece of paper catches nearly every conversion slip before it reaches the syringe.
Frequently asked questions
How do I convert mg to IU?+
There is no general conversion. An international unit is defined per substance against a reference preparation, so the mg-per-IU figure only exists for substances that have one published, and it does not transfer between substances. This is educational information, not medical advice.
Is a syringe unit the same as an IU?+
No. A unit on a U-100 insulin barrel is a volume mark equal to 0.01 mL. An IU is a measure of biological activity. They share a word and mean different things, which is why reading the barrel type first matters.
Why does my vial label show mg but my protocol shows mcg?+
Labels usually state total mass in milligrams while doses are often written in micrograms. One milligram is 1000 micrograms. Converting in the wrong direction is a thousand-fold error, so do that step on paper rather than in your head.
Which number actually decides what I draw?+
Concentration. Once you know milligrams per millilitre, the volume to draw is the dose in mg divided by that concentration, and the units to draw are that volume multiplied by 100 on a U-100 barrel.