mg, mcg, mL
and units

The four units

Two measure mass, two measure volume, and mixing the pairs up is the whole problem.

• Milligram (mg) — mass. How much peptide the vial contains.
• Microgram (mcg, sometimes µg) — mass, one thousandth of a milligram. Doses are usually quoted here.
• Millilitre (mL) — volume. How much solvent you added, and how much liquid you draw.
• Unit (U) — volume, as marked on an insulin syringe. On a U-100 syringe, 100 units is 1 mL.

A unit is not a measure of drug. It is a mark on a barrel that happens to be one hundredth of a millilitre. Two syringes drawn to the same unit mark hold the same volume and can hold entirely different amounts of peptide, depending on how much solvent went into each vial.

Mass: milligrams and micrograms

The only conversion here is a factor of a thousand.

1 mg = 1000 mcg

So 0.25 mg is 250 mcg, 2.5 mg is 2500 mcg, and 500 mcg is 0.5 mg. Multiply by 1000 going from milligrams to micrograms, divide by 1000 coming back.

Volume: millilitres and units

An insulin syringe marked U-100 is graduated at 100 units per millilitre. This is fixed by the syringe, not by what is in it.

1 mL = 100 units   and so   1 unit = 0.01 mL

Multiply millilitres by 100 for units; divide units by 100 for millilitres. All three common insulin syringes are U-100 — they differ in capacity, not in scale.

Mass to volume: the concentration step

This is the conversion the other two exist to serve, and the one that depends on a decision you made when you reconstituted the vial.

concentration (mg/mL) = peptide (mg) ÷ solvent (mL)

Once you have a concentration, mass and volume convert freely in both directions:

volume (mL) = dose (mg) ÷ concentration (mg/mL)

dose (mg) = volume (mL) × concentration (mg/mL)

A worked example, end to end

A 5 mg vial, 2 mL of solvent added, and a 250 mcg dose wanted.

1. Concentration. 5 mg ÷ 2 mL = 2.5 mg/mL
2. Dose into the same unit. 250 mcg ÷ 1000 = 0.25 mg
3. Volume. 0.25 mg ÷ 2.5 mg/mL = 0.1 mL
4. Units. 0.1 mL × 100 = 10 units

Draw to the 10 mark on a U-100 syringe. The vial holds twenty such doses, because 5 mg ÷ 0.25 mg = 20.

The same vial, reconstituted differently

Nothing above fixes the solvent volume — it is your choice, and it changes every number downstream except the dose itself. The same 5 mg vial and the same 250 mcg dose, with three different amounts of solvent:

Every row delivers exactly 250 mcg. What changes is how easy that is to measure. Five units is a short draw where a half-unit misread is a ten per cent error; twenty-five units is a comfortable one, but needs a syringe that holds it and uses the vial up in fewer doses per millilitre.

Where it goes wrong

• Treating units as a dose. "Ten units" means nothing without the concentration. The same ten units is 250 mcg from one vial and 1000 mcg from another.
• Skipping the mg/mcg step. Dividing 250 by 2.5 gives 100, not 0.1 — a factor of a thousand out, because the dose was in micrograms and the concentration in milligrams.
• Reusing an old number after re-reconstituting. Change the solvent volume and every unit figure you wrote down is wrong.
• Assuming a syringe is U-100. All three common insulin syringes are, but the scale is printed on the barrel and is worth reading once.

Do it automatically

The reconstitution calculator runs all four steps and shows where the draw lands on the syringe. It is free, needs no account, and works entirely in your browser.

This page is arithmetic and definitions. It explains how the units relate to each other. It does not recommend a dose, a solvent volume or a compound, and it is not medical advice. Those decisions belong with a qualified healthcare professional.

Related

• Reconstitution calculator — the conversions above, done for you
• Reading an insulin syringe — what the graduations mean and how the sizes differ
• TideTracker — inventory, protocols and dose logging, kept on your iPhone