mcg vs mg vs Units

Three different things get casually called the same names here: a weight of peptide, a volume of liquid, and the marks on a syringe. Keeping them straight removes most reconstitution confusion.

Weight: mg and mcg

These measure how much peptide there is. The conversion is simply a thousand:

1 mg = 1,000 mcg · 1 mcg = 0.001 mg

Vials are usually labelled in mg (e.g. a 10 mg vial); doses are usually discussed in mcg (e.g. 250 mcg). 250 mcg is 0.25 mg.

Volume: millilitres (mL)

This is how much liquid you draw, not how much peptide it contains. The link between weight and volume is concentration (mcg per mL), which comes from how much water you added.

Syringe units

Insulin syringes are marked in units, not mL. On a standard U-100 syringe, 100 units = 1 mL, so each unit is 0.01 mL. Units are a volume mark — see how to read an insulin syringe for the scale.

Note: a syringe "unit" is not an "IU" (international unit). IU measures biological activity for certain substances; a syringe unit is just a volume graduation. Same word, different meaning.

Putting it together

Example

Vial10 mg = 10,000 mcg

Water added2 mL → 5,000 mcg/mL

Dose (weight)250 mcg

Draw (volume)0.05 mL

On the syringe5 units

Weight in, volume and units out — with concentration as the bridge. The calculator handles every conversion for you.

Open the dosage calculator

Frequently asked questions

How many mcg are in a mg?

1 milligram (mg) equals 1,000 micrograms (mcg). So 0.25 mg is 250 mcg, and 5,000 mcg is 5 mg. It's just a factor of 1,000.

What is the difference between a dose in mcg and units on a syringe?

A dose in mcg is a weight of peptide. Units on an insulin syringe are a volume mark — on a U-100 syringe, 100 units is 1 mL. To turn mcg into units you need the mix's concentration, because the same weight occupies a different volume depending on how dilute the solution is.

Is a syringe 'unit' the same as an IU (international unit)?

No. On an insulin syringe, 'units' are volume graduations: 1 unit is 0.01 mL on a U-100 syringe. An IU (international unit) is a measure of biological activity used for some substances and is not a volume. They share the word 'unit' but mean different things.

How do I convert mcg to syringe units?

Divide the dose by the concentration to get millilitres, then multiply by 100. For example, 250 mcg ÷ 5,000 mcg/mL = 0.05 mL = 5 units. The calculator does this automatically.