Units vs mg vs mL
Milligrams measure how much substance there is. Millilitres and syringe units measure how much liquid there is. Converting one to the other needs a third number, the concentration, and that number is different for every vial and every amount of water added.
Three different kinds of measurement
| Unit | What it measures | Where you find it |
|---|---|---|
| mg (milligram) | Mass: the amount of the substance | Vial label, prescription |
| mL (millilitre) | Volume of liquid | Water added, tuberculin syringes |
| units (U-100 syringe) | Volume, in steps of 0.01 mL | Insulin syringe barrel |
| mg/mL | Concentration: amount per volume | Pre-mixed vial labels, or worked out after mixing |
Syringe units and mL are the same kind of thing, so they convert with a fixed factor: 100 units is 1 mL on a U-100 syringe. Milligrams are a different kind of thing. There is no fixed number of mg in a mL, in the same way there is no fixed number of grams of sugar in a cup of coffee. It depends how much was stirred in.
Concentration is the bridge
Take a vial holding 10 mg. Add 2 mL of water and the concentration is 5 mg/mL. Each 0.1 mL, which is 10 units, holds 0.5 mg. Add 1 mL instead and the concentration is 10 mg/mL, so the same 10 units now holds 1 mg. The syringe reading is the same. The amount in it has doubled.
The same 1 mg at different concentrations
This is why a question such as "how many units is 1 mg" cannot be answered without the vial details. The table shows the units that contain 1 mg at several concentrations.
| One way to get there | Concentration | Units that hold 1 mg | 10 units holds |
|---|---|---|---|
| 5 mg vial + 5 mL | 1 mg/mL | 100 units | 0.1 mg |
| 5 mg vial + 2 mL | 2.5 mg/mL | 40 units | 0.25 mg |
| 10 mg vial + 2 mL | 5 mg/mL | 20 units | 0.5 mg |
| 10 mg vial + 1 mL | 10 mg/mL | 10 units | 1 mg |
| 20 mg vial + 1 mL | 20 mg/mL | 5 units | 2 mg |
From 100 units down to 5 units, for the same milligram. Any chart that gives one answer has assumed a concentration, whether or not it says so. The 1 mg to units page lists the answer for each common concentration, and the 20 units to mg page does the reverse.
Where insulin fits
The word "units" causes much of the confusion because it started with insulin. Insulin is dosed in units of activity, and U-100 insulin is made at 100 units per mL, so the syringe was printed to read the dose directly. With U-100 insulin, 10 on the barrel is 10 units of insulin.
With any other liquid, 10 on the barrel is 0.1 mL and nothing more. People still say "units" because that is what is printed on the syringe, but it is a volume. See IU vs mg for the separate meaning of units as a measure of biological activity.
What you need before converting
- The amount in the vial, in mg, from the label.
- The volume of water added, in mL, as written on your pharmacy instructions and as actually measured.
- The amount to convert, from your prescription.
If the vial came already mixed, the label gives the concentration and you can skip the first two. Reading a vial label shows where to look.
With those numbers, the calculator gives units to draw, or use the unit converter for a single mg, mL or units conversion.
Questions
How many units is 1 mg?
It depends on the concentration. At 5 mg/mL (for example 10 mg in 2 mL), 1 mg is 20 units on a U-100 syringe. At 10 mg/mL it is 10 units. At 2.5 mg/mL it is 40 units. Units = mg ÷ concentration × 100.
Are units and mL the same thing?
They measure the same thing, volume, on different scales. On a U-100 syringe 100 units is 1 mL, so 1 unit is 0.01 mL.
How do I convert mg to mL?
Divide the mg by the concentration in mg/mL. For a liquid at 5 mg/mL, 1 mg is 0.2 mL. Without the concentration the conversion cannot be done.
Why does my chart give a different number of units from someone else's?
The two charts assume different concentrations, usually because a different vial size or a different amount of water was used. Work from your own vial label and the water volume on your own instructions.