How to read an insulin syringe

Read an insulin syringe at the top edge of the black rubber plunger tip, holding it at eye level. On a U-100 syringe 100 units is 1 mL; the short lines are 1 unit each on a 0.3 mL or 0.5 mL barrel and 2 units each on a 1 mL barrel.

Last edited 2026-09-28 · written by PepPrep · not medical advice

An insulin syringe has more printed lines than any other syringe you will hold, and the trick is knowing which ones to ignore. Here is what each part means and how to read it without guessing.

The four things printed on the barrel

• U-100 — the scale. 100 units to the millilitre. If it says U-40, it is a veterinary syringe with a different scale; put it back.
• The long lines with numbers — every 5 or 10 units depending on the barrel size.
• The short lines between them — 1 unit each on a 0.3 mL or 0.5 mL barrel, 2 units each on a 1 mL barrel. Count the short lines between two numbers once, and you know the barrel.
• The gauge and length — usually on the packet, not the barrel. 31G × 8 mm is the common one.

Where to read the level

Read at the top edge of the black rubber plunger tip — the flat end nearest the needle, not the pointed cone in the middle and not the bottom of the rubber. Hold the syringe at eye level; looking down at it from above makes the reading sit a line lower than it really is.

Bubbles

A bubble takes up room that should be liquid, so a syringe reading 20 units with a bubble in it holds less than 20 units. Draw a little more than you need, tap the barrel so the bubble rises to the needle end, then push the plunger until the bubble is gone and the top of the rubber sits exactly on your line. The measuring guide covers the other ways volume quietly goes missing.

If the lines are hard to see

Three fixes, in order of cost: better light aimed at the syringe (a desk lamp), a clip-on syringe magnifier, and a smaller barrel with more space between the lines (see which size to buy). A 1 mL barrel in dim light is the hardest combination to read.

Supplies mentioned: Syringes & Needles

Equipment only: nothing on this page is advice about what to inject, how much, or whether a product is fit to use. That comes from the product's labelling and whoever supplied it.