Insulin is an injectable medication used to treat high blood sugar in people with type 1 or type 2 diabetes.
But while most medications are measured in milligrams or milliliters, insulin is dosed in units.
When you use your insulin pen or draw insulin from an insulin vial into an insulin syringe, the number you see is not in milligrams or milliliters. It is the number of international units of insulin . This is quite confusing and unclear for most patients, but there is a reason for this peculiarity.
So let's take a closer look at insulin units. What is 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 the mass or volume of an insulin molecule, so they developed a different model for insulin dosing.
One international unit of insulin was defined as the amount of insulin required to lower fasting blood glucose in laboratory rabbits by 2.5 mmol/L (45 mg/dL).
Today, 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 dosing and calculations would be too complicated using 0.0347 mg as a reference.
Insulin concentration: U-100, U-40 versus U-500
Insulin concentration is another important point to understand. You may have seen numbers like U-100, U-40, or U-500 on your insulin vials or pens. These numbers represent insulin concentration (or insulin strength) . In other words, they indicate the number of units of insulin per milliliter of fluid.
An example:
- U-100 insulin has 100 units of insulin per milliliter of liquid.
- U-40 insulin has 40 units of insulin per milliliter of liquid.
- U-500 insulin has 500 units of insulin per milliliter of liquid.
Simply put , the higher the number, the stronger the insulin . Before insulin pens were invented, different syringes were needed for different insulin concentrations, and there were U-100 syringes, U-40 syringes, and U-500 syringes. This situation made it complicated for diabetics to understand insulin therapy, leading to numerous errors with sometimes serious consequences.
For this reason, insulin concentration has 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 is no longer used for humans, but this strength can still be found in veterinary insulins for dogs and cats or in some countries.
If you are travelling with diabetes and need to buy insulin in another country, always make sure you check the insulin concentration before using it ! If it is different from yours, it is essential that you be able to do the conversion. Otherwise, you will inject too much or too little insulin, which can be extremely dangerous.
In the United States, the only insulin sold for humans that is not U-100 is Humalog U-200 (insulin lispro 200U/ml), but it is only sold with the KwikPen (not in vials) so there can be no confusion.
How do you convert insulin units into ml?
There are only a few reasons why you would need to convert insulin units to milliliters. First, due to the standardization of insulin strength, almost all human insulins are now U-100 (containing 100 units of insulin per milliliter of liquid).
However, it can be helpful to know how to convert insulin units to ml in the following situations
- You have insulin with a different concentration (e.g. if you bought it in another country).
- You no longer have an insulin syringe and must draw up insulin using a regular syringe that displays mL instead of units.
- You use human insulin for your pets (which is sometimes recommended by your veterinarian) and need to convert U-100 insulin units to milliliters to find the correct U-40 dose.
Conversion of insulin units into milliliters
Converting insulin units to milliliters is very simple. As already mentioned, the insulin concentration (the U-number) indicates the number of insulin units in one milliliter of liquid.
Let’s take an example with standard insulin U-100:
ml in units:
- 1 ml = 100 insulin units
- 2 ml = 200 insulin units (100*2)
- 5 ml = 500 insulin units (100*5)
Units to ml:
- 1 insulin unit = 0.01 ml (1/100)
- 20 insulin units = 20 * 0.01 ml = 0.2 ml
- 35 insulin units = 35 * 0.01 ml = 0.35 ml
Let’s take another example with the veterinary insulin U-40:
ml in units:
- 1 ml = 40 insulin units
- 2 ml = 80 insulin units (40*2)
- 5 ml = 200 insulin units (40*5)
Units to ml:
- 1 insulin unit = 0.025 ml (1/40)
- 20 insulin units = 20 * 0.025 ml = 0.5 ml
- 35 insulin units = 35 * 0.025 ml = 0.875 ml
How many milliliters are contained in one unit of insulin?
This depends on the strength of the insulin. For U-100 insulin, one unit of insulin is equal to 0.01 ml . For U-40 insulin, one unit of insulin is equal to 0.025 ml . And for U-500 insulin, one unit of insulin is equal to 0.002 ml .
How many units of insulin are contained in one milliliter?
Here too, it depends on the insulin concentration. U-100 insulin has 100 units in one milliliter. U-40 insulin has 40 units in one milliliter. And U-500 insulin has 500 units in one milliliter.
How many units of insulin are in a vial?
How many units of insulin are in a vial depends, of course, on the insulin concentration and the size of the vial!
To find out how many insulin units are in your insulin vial, simply multiply the concentration U number by the number of milliliters in the vial . Examples:
- A 10 ml 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
How do you convert insulin units to mg?
It doesn't make much sense to convert insulin units into milligrams. But the conversion is easy because one international unit of insulin = 0.0347 mg of pure crystalline insulin.
However, this does not mean that 0.0347 mg of insulin that you take out of a vial is equal to one unit of insulin. Insulin sold in the pharmacy is dissolved or suspended with other components, the weight of which must also be taken into account (but you can only find this out if you are the manufacturer yourself!).
Have you ever had to convert insulin units? Why? How did it go for you? Share your experiences, doubts and stories in the comments section below!
