Insulin is a hormone that helps regulate blood sugar levels in the body. It is produced by the pancreas and works to move glucose from the blood into cells to be used for energy. People with diabetes often need to inject or infuse insulin to control their blood sugar levels.
Insulin dosing is measured in units. But what exactly is 1 unit of insulin? Here’s a closer look at what 1 unit means.
The Standards for Insulin Unit Measurements
Standards for insulin concentrations and dosing units have evolved over the years as insulin preparations have changed. Back in the 1980s and earlier, animal-sourced insulins were used. These had a potency of 40 units per milliliter (U/mL). In the 1980s, purified human insulins became available, standardized at 100 U/mL. Today, synthetic human insulins made using recombinant DNA technology are used, and they also have a potency of 100 U/mL.
So currently in the United States, the most common insulin concentrations are:
- 100 units/mL (U-100)
- 500 units/mL (U-500)
The 100 units/mL concentration is by far the most commonly used. The higher 500 units/mL concentration is typically only used in people with significant insulin resistance who require very high doses.
What 1 Insulin Unit Means for U-100 Insulin
For standard U-100 insulin at a concentration of 100 units/mL:
- 1 unit of insulin is the amount of insulin in 1/100th of 1 mL (0.01 mL) of the solution.
- So 1 unit = 0.01 mL of U-100 insulin
Another way to think of it is:
- 1 mL of U-100 insulin contains 100 units of insulin
- So 1 unit of insulin is 1/100th of the amount in 1 mL
Most insulin syringes for injecting U-100 insulin are marked in single unit increments up to 100 units. Insulin pens also allow U-100 insulin to be dialed and injected in 1 unit increments.
What 1 Unit Means for U-500 Insulin
For the more concentrated U-500 insulin:
- 1 unit of U-500 insulin is the amount of insulin in 1/500th of 1 mL (0.002 mL) of the solution
- So 1 unit = 0.002 mL of U-500 insulin
Because U-500 insulin is more concentrated and administered in larger volumes, the syringes used for it are different. They are marked in 5 unit increments up to 100 units.
How Much Insulin is in an Insulin Pen or Vial?
Insulin pens and vials are labeled with the total number of insulin units they contain. For example:
- A 3 mL pen or vial of U-100 insulin contains 300 units (3 mL x 100 units/mL).
- A 3 mL pen or vial of U-500 insulin contains 1500 units (3 mL x 500 units/mL).
The total units and volume (mL) will always be listed on an insulin package so you know precisely how much insulin is in it.
Factors That Impact Insulin Dosing
While insulin unit measurements are standardized, the number of units a person needs can vary significantly based on a number of factors:
- Type of insulin – Long-acting insulins stay in the body longer so less units are needed. Short or rapid acting insulins act faster so more units may be required.
- Time of day – More insulin may be needed per meal during the day versus at nighttime.
- Carbohydrate intake – More insulin is required to cover higher carb meals.
- Insulin sensitivity – Someone very insulin sensitive may need less insulin than someone who is insulin resistant.
- Activity levels – Exercise and activity lower blood sugar, so less insulin may be required.
- Current blood glucose level – Higher blood sugars often need more insulin to lower.
- Individual differences – The right insulin dose varies between individuals.
Doctors prescribe initial insulin dosing recommendations based on weight, but the dose is adjusted over time based on an individual’s unique response. Blood sugar testing and tracking is essential to find the right dose for an individual over time.
Calculating an Insulin Dose
To calculate an insulin dose, there are some key pieces of information needed:
- Blood sugar level
- Carbohydrates to be eaten
- Correction factor and/or insulin-to-carb ratio prescribed
- Insulin type and time of administration
With a prescriber’s guidance, some basic math can then help determine how many units are needed for a meal or to correct for high blood sugar:
Bolus Insulin Dose = Food Insulin + Correction Insulin
Food Insulin is the dose to cover the carbs eaten. This is calculated by dividing the grams of carbs by the insulin-to-carb ratio. For example, for 60g carbs with a 1:10 ratio:
60g carbs/10g per 1 unit insulin = 6 units of insulin
Correction Insulin is added insulin to lower an elevated blood sugar. This is calculated by dividing the difference between current and target glucose by the correction factor. For example, for a 200 mg/dL sugar with a 100 mg/dL target and 1:50 correction factor:
(200 – 100) / 50 = 2 units correction insulin
So the full bolus dose would be:
Food Insulin (6 units) + Correction Insulin (2 units) = 8 units total insulin
Of course, this is a simplified example. An individual’s prescribed ratios and factors as well as insulin type must be incorporated into real-life calculations.
Adjusting Insulin Doses
Finding the right insulin dose requires checking blood sugars, being observant for patterns, and making thoughtful adjustments over time. Here are some key points on adjusting insulin doses correctly:
- Make one adjustment at a time.
- Adjust only after 2-3 days of data on a given dose.
- Increase doses by no more than 10-20% if needed.
- Reduce doses by no more than 20-30% if needed.
- Discuss significant adjustments with your healthcare provider.
With careful titration, the optimal dose for an individual for a given time of day and meal can be determined. But ongoing monitoring and future adjustments will likely be needed as circumstances change.
Special Considerations for Concentrated U-500 Insulin
U-500 insulin is 5 times more concentrated than standard U-100 insulin. This means special care is required in measurement and administration:
- U-500 must only be used with U-500 designed syringes and needles. Never use a U-100 syringe.
- The dose scale is different, with U-500 syringes going up by 5 unit increments.
- The units marked on a U-500 syringe are one-fifth the volume injected. For example, 10 units on the syringe = 0.02 mL injected.
- To avoid errors, the numbers 500 or U-500 should be written prominently on a prescription.
- The injected dose should be confirmed visually before administration.
With proper U-500 measurement and injection technique, 1 unit of concentrated insulin can be administered just like 1 unit of standard insulin.
The Takeaway on 1 Unit of Insulin
One unit of insulin represents 1/100th of 1 mL of standard U-100 insulin. While the unit definition is fixed, an individual’s ideal dose in units varies based on many factors. Careful dosing, monitoring, and titration is key along with understanding that insulin needs may change over time.
Frequently Asked Questions
How many units of insulin are in an insulin pen?
The number of units in an insulin pen depends on the type of insulin and the size of the pen. For example:
- A 3 mL prefilled pen of 100 units/mL insulin (U-100) contains 300 units of insulin.
- A 3 mL prefilled pen of 500 units/mL (U-500) insulin contains 1,500 units.
How long does 1 unit of insulin last in your body?
How long 1 unit of insulin lasts depends on the type of insulin:
- Rapid-acting insulin – 3 to 5 hours
- Short-acting insulin – 5 to 8 hours
- Intermediate-acting insulin – 12 to 18 hours
- Long-acting insulin – 18 to 24 hours
How much does 1 unit of insulin lower blood sugar?
In general, 1 unit of rapid or short acting insulin will lower blood glucose by 25-50 mg/dL. However, the exact blood sugar lowering effect of 1 unit varies between individuals.
How do I calculate how much 1 unit of insulin will lower my blood sugar?
To determine how much 1 unit impacts your blood sugar, you need to do a correction factor test. This involves fasting, checking blood sugar, taking 1 unit, and checking again every 1-2 hours. Your doctor or diabetes educator should guide you through testing to find your correction factor.
Should 1 unit of insulin be injected into fat or muscle?
1 unit of insulin can be injected into subcutaneous fat or muscle. Injection sites include the abdomen, thighs, upper arms, and buttocks. Where to inject depends partly on the insulin type. Your healthcare provider can advise on the best sites.
Conclusion
Understanding what 1 unit of insulin means is important for both prescribing and administering it properly. While 1 unit represents a fixed amount of 0.01 mL of U-100 insulin, determining an individual’s needs takes time, frequent monitoring, and careful adjustment. With attentive self-care and provider guidance, the correct insulin dosing in units can be determined for maintaining optimal blood sugar control.
