How to Read an Insulin Syringe
A complete guide to understanding insulin syringe markings, U-100 vs U-40 differences, syringe capacity sizes, and how to draw the correct dose accurately and safely.
Insulin Concentrations — U-100 vs U-40
| Concentration | Units per mL | Common Use | Syringe Color |
|---|---|---|---|
| U-100 | 100 units/mL | Standard worldwide (human & analogue insulins) | Orange cap |
| U-40 | 40 units/mL | Veterinary use (cats, dogs); some countries outside USA | Red cap |
| U-200 | 200 units/mL | Tresiba U-200, Humalog U-200 KwikPen (pen only) | N/A — pen only |
| U-300 | 300 units/mL | Toujeo (pen only) | N/A — pen only |
| U-500 | 500 units/mL | Humulin R U-500 (severe resistance); special U-500 syringe required | Yellow cap |
U-40 dose error example: If you draw 10 units on a U-40 syringe but inject U-100 insulin, you actually inject 25 units — 2.5× the intended dose. This can cause severe hypoglycemia. U-200, U-300, and U-500 insulins are only available in pens or special syringes specifically calibrated for those concentrations.
U-100 Syringe Sizes
U-100 syringes come in three barrel sizes. Choose the smallest syringe that holds your full dose — smaller barrels have finer graduation marks and are easier to read accurately.
| Syringe Size | Max Dose | Graduation | Best For |
|---|---|---|---|
| 0.3 mL (30-unit) | 30 units | 1 unit per mark | Small doses (≤ 30 units); children; fine control |
| 0.5 mL (50-unit) | 50 units | 1 unit per mark | Medium doses (31–50 units) |
| 1.0 mL (100-unit) | 100 units | 2 units per mark | Large doses (51–100 units); note: half-unit marks not visible |
Tip: If your dose is 28 units, use a 30-unit syringe rather than a 100-unit syringe — the markings are much easier to read precisely.
How to Read the Syringe
- Hold the syringe horizontally at eye level.
- Locate the flat edge of the rubber plunger — this is the dose reference point (not the dome).
- Count the major numbered lines (e.g., 10, 20, 30 on a 30-unit syringe).
- Count the minor lines between numbers — each represents 1 unit on a 30 or 50-unit syringe, and 2 units on a 100-unit syringe.
- Align the flat edge of the plunger tip with your target dose line.
Quick Check: mL vs Units
10 units = 0.1 mL · 50 units = 0.5 mL · 100 units = 1.0 mL
How to Draw Insulin
- Wash hands thoroughly.
- If using cloudy insulin (NPH, pre-mixed), roll the vial gently between palms 10 times — do not shake.
- Wipe the vial top with an alcohol swab and let dry.
- Draw air into the syringe equal to your dose amount.
- Insert the needle into the vial and inject the air (prevents vacuum).
- Invert the vial and draw insulin to your dose, pulling the plunger slowly.
- Check for air bubbles — tap the syringe and push bubbles out, then re-draw to correct dose.
- Remove needle from vial and inject promptly.
Sources
- American Diabetes Association. Standards of Medical Care — 2024.
- FDA. "Insulin U-500 — Special Syringe Required." FDA Drug Safety Communication, 2016.
Last reviewed: June 2025