How to Convert Insulin Units to mL?

Insulin is an injectable medication used to treat high blood sugar in people with either type 1 or type 2 diabetes.

But while most medications are conveniently measured in milligrams or milliliters, insulin is dosed in units.

When you use your insulin pen or draw insulin from a vial into an insulin syringe, the number you see is neither in milligrams nor milliliters. It’s the number of international units of insulin. It is quite confusing and ambiguous for most patients, but there’s actually a reason for this particularity.

So, let’s zoom in on insulin units. What’s an international insulin unit? Why and how to convert it to milliliters or milligrams?

Insulin is a temperature-sensitive medicine; Keep it Cool!

What is an international insulin unit?

When insulin was discovered in 1921, scientists could not measure an insulin molecule's mass or volume. So, they came up with another model unique to insulin dosage.

One international unit of insulin was defined as the amount of insulin necessary to lower fasting blood sugar in laboratory rabbits by 2.5 mmol/L (45 mg/dL).

Nowadays, thanks to advanced technology, we know that one unit of insulin is the biological equivalent of 0.0347 mg of pure crystalline insulin. However, we still use the international unit because dosage and calculations would be too complicated using 0.0347 mg as a reference.

Related article: How to mix insulin in a syringe?

Insulin concentration: U-100 vs U-40 vs U-500

Insulin concentration is another important thing to understand. You may have seen numbers like U-100, U-40, or U-500 on your insulin vials or pens. These numbers represent the insulin concentration (or the insulin strength). In other words, they represent the number of units of insulin per milliliter of fluid.

For example:

U-100 insulin has 100 units of insulin per milliliter of fluid.
U-40 insulin has 40 units of insulin per milliliter of fluid.
U-500 insulin has 500 units of insulin per milliliter of fluid.

Simply put, the higher the number, the stronger the insulin. Before insulin pens were invented, different concentrations of insulins required different syringes, and you had U-100 syringes, U-40 syringes, and U-500 syringes. This situation made it complicated for diabetic patients to understand insulin therapy, leading to numerous mistakes with sometimes severe consequences.

That's why insulin concentration has now been standardized in the United States, and most human insulins are U-100. Some patients with extreme insulin resistance may be prescribed U-500 insulin. U-40 insulin isn't used anymore for humans, but you can still find this strength in veterinary insulins used for dogs and cats or in some countries.

If you’re traveling with diabetes and need to purchase insulin in a different country, always make sure to check the insulin concentration before use! If it's a different one than yours, you must absolutely be able to make the conversion. Otherwise, you'd inject too much or too little insulin, which can be extremely dangerous.

In the United States, the only insulin sold for humans that's not a U-100 is Humalog U-200 (insulin lispro 200U/ml), but it's only sold with the KwikPen (not in vials), so there is no possible confusion.

How to convert insulin units to ml?

Reasons why you would need to convert insulin units to milliliters are very limited. First, because of the standardization of insulin strength, almost all human insulins are now U-100 (containing 100 units of insulin per milliliter of fluid).

However, knowing how to convert insulin units to mL may be helpful in the following situations:

You find yourself with insulin with a different concentration (when purchased in another country, for example).
You’re out of insulin syringe and need to draw insulin with a regular syringe that shows mL instead of units.
You’re using human insulin for your pets (which can sometimes be recommended by your veterinarian) and need to convert U-100 insulin units to milliliters to find the correct dose U-40 dosage.

Insulin unit to mL conversion

Converting insulin units to milliliters is quite simple. As said above, the concentration of insulin (the U-number) indicates the number of insulin units in one milliliter of fluid.

Let's take an example with standard U-100 insulin:

mL to units:

1 mL = 100 units of insulin
2 mL = 200 units of insulin (100*2)
5 mL = 50 units of insulin (100*0.5)

Units to mL:

1 unit of insulin = 0.01 mL (1/100)
20 units of insulin = 20 * 0.01 mL = 0.2 mL
35 units of insulin = 35 * 0.01 mL = 0.35 mL

Let's take another example with U-40 veterinary insulin:

mL to units:

1 mL = 40 units of insulin
2 mL = 80 units of insulin (40*2)
5 mL = 20 units of insulin (40*0.5)

Units to mL:

1 unit of insulin = 0.025 mL (1/40)
20 units of insulin = 20 * 0.025 mL = 0,5 mL
35 units of insulin = 35 * 0.025 mL = 0.875 mL

How many milliliters are in a unit of insulin?

It depends on the insulin strength. For U-100 insulin, one unit of insulin equals 0.01 mL. For U-40 insulin, one unit of insulin equals 0.025 mL. And for a U-500 insulin, one unit of insulin equals 0.002 mL.

How many units of insulin are in a milliliter?

Again, it depends on the insulin concentration. For U-100 insulin, there are 100 units in one milliliter. For U-40 insulin, there are 40 units in one milliliter. And for U-500 insulin, there are 500 units in one milliliter.

How many units of insulin are in a vial?

Again, how many units of insulin are in a vial depends on the insulin concentration and the size of the vial, of course!

The simple math to find out how many insulin units are in your vial is to multiply the concentration U-number by the number of milliliters in the vial. Examples:

A 10mL vial of U-100 insulin contains 100*10 = 1000 units of insulin
A 10 mL vial of U-40 insulin contains 40*10 = 400 units of insulin
A 5 mL vial of U-100 insulin contains 100*5 = 500 units of insulin

Use an Insulin Vial Protector!

How to convert insulin units to mg?

There’s not much point in converting insulin units to milligrams. But the conversion is simple, as one international unit of insulin = 0.0347 mg of pure crystalline insulin.

However, it does not mean that 0.0347 mg of insulin you draw from a vial equals one unit of insulin. Insulin sold at the pharmacy is dissolved or suspended with other components whose weight must be considered, too (but it is only possible to find out if you're the manufacturer!).

Have you ever had to convert insulin units? Why? How did you do? Share your experiences, doubts, and stories in the comment section below!