Reading a syringe

General information about the equipment, not medical advice. The directions that came with your medicine and the person who prescribed it come first, every time.

Find out which scale you are holding

Look at the barrel. If the numbers run 10, 20, 30 up to 50 or 100 and the word units appears, it is an insulin syringe. If they run 0.2, 0.4, 0.6 and so on, or 1, 2, 3, it is graduated in millilitres.

The two are not interchangeable and there is no conversion you can do in your head reliably, because it depends on the concentration of what is in the vial. Match the syringe to what the label says.

What U-100 means

U-100 describes the concentration the scale assumes: one hundred units per millilitre. On a 1mL U-100 syringe, the 100 mark and the 1mL mark are the same place on the barrel, and a single unit is one hundredth of a millilitre.

U-40 syringes exist for products at forty units per millilitre, which in Australia is mostly veterinary. Using one scale for the other concentration gives you the wrong amount of liquid. This is the single most important thing on this page.

Reading the line, not the tip

The measurement is taken at the flat top of the rubber stopper inside the barrel, not at the pointed tip of it. On a small dose that difference is a meaningful share of what you drew.

Hold the syringe at eye level rather than looking down at it. Reading a scale from an angle shifts where the line appears to sit, which matters more the smaller the dose is.

Half unit syringes exist if you need finer resolution than a whole unit. Whether you need one is a question for your prescriber, not a preference.