Standard insulin contains 100 units per milliliter. This concentration, called U-100, is by far the most common insulin formulation worldwide and the default for most people with diabetes. But other concentrations exist, and knowing the difference matters for accurate dosing.
What U-100 Means in Practice
The “U” in insulin labeling stands for units per milliliter. U-100 insulin packs 100 units into every 1 mL of liquid. So if your prescribed dose is 10 units, you’re injecting 0.1 mL. A dose of 30 units is 0.3 mL. A full 1 mL syringe holds exactly 100 units.
On a standard 1 mL insulin syringe, every small notch represents 1 unit. This one-to-one relationship between the syringe markings and the actual units makes U-100 straightforward to measure. Most insulin vials hold 10 mL, which means a single vial contains 1,000 units total.
One international unit of insulin equals 0.0347 milligrams of pure insulin, a definition maintained by the National Institute for Biological Standards and Control. You’ll never need to think in milligrams, though. Prescriptions, syringes, and pen devices all work in units.
Higher Concentration Insulins
Some people need large daily doses of insulin, and injecting high volumes of U-100 can be uncomfortable and slow to absorb. That’s where concentrated insulins come in. These pack more units into the same volume of liquid.
U-200 insulin contains 200 units per mL, exactly double the standard concentration. U-300 (sold as Toujeo) is a long-acting insulin with 300 units per milliliter of insulin glargine. U-500 (sold as Humulin R U-500) is the most concentrated option available, with 500 units packed into a single milliliter, five times the standard concentration.
The practical advantage is smaller injection volumes. Someone who needs 150 units of insulin would have to inject 1.5 mL of U-100, but only 0.3 mL of U-500. Smaller volumes are more comfortable and tend to absorb more predictably.
U-40 Insulin for Pets
If you have a diabetic cat or dog, you’ve likely encountered U-40 insulin, which contains 40 units per milliliter. Veterinary insulins like ProZinc and Vetsulin use this lower concentration. It’s less common in human medicine today but remains standard in veterinary care because pets typically need much smaller doses, and U-40 makes those tiny amounts easier to measure accurately.
Why Syringe and Insulin Must Match
This is where units per milliliter stops being an abstract number and becomes a safety issue. Insulin syringes are calibrated to match a specific concentration. A U-100 syringe assumes the liquid inside contains 100 units per mL. If you draw up to the “30” mark on a U-100 syringe, you get 30 units, but only if you’re using U-100 insulin.
If you accidentally use a U-100 syringe with U-500 insulin and draw to the “30” mark, you’re actually pulling 0.3 mL of liquid that contains 150 units. That’s five times the dose you intended. The reverse is equally dangerous: using a U-500 syringe with U-100 insulin would deliver far less than prescribed.
A safety report from ISMP Canada documented exactly this kind of mix-up. A patient had been using a U-100 syringe to draw Humulin R U-500 from a vial. When their doctor asked what dose they were taking, the patient said “30 units” based on the syringe marking. Their actual dose was 150 units. The doctor then prescribed 30 units in a pen device calibrated for U-500, and the patient’s blood sugar spiked because they were suddenly getting only one-fifth of their previous dose.
The rule is simple: the syringe markings only tell you the correct number of units when the syringe concentration matches the insulin concentration. U-100 syringes for U-100 insulin, U-500 syringes for U-500 insulin. Insulin pens eliminate this problem entirely because the dose dial counts units regardless of the concentration inside the cartridge.
Quick Reference by Concentration
- U-40: 40 units per mL (veterinary insulin)
- U-100: 100 units per mL (standard human insulin)
- U-200: 200 units per mL
- U-300: 300 units per mL (Toujeo)
- U-500: 500 units per mL (Humulin R U-500)
Most people will only ever use U-100 insulin. If you’re prescribed a concentrated insulin, it will almost always come in a pre-filled pen that handles the math for you. The concentration matters most when you’re drawing insulin from a vial with a syringe, where using the wrong syringe type can lead to serious dosing errors.