For many people with diabetes, insulin injections are the key to keeping their blood sugar at normal levels. To find out how much insulin you need, it may help to calculate the insulin sensitivity factor. However, this calculation alone does not always determine how much insulin you need.

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The pancreas makes the hormone insulin. Insulin helps the body use sugar as an energy source. It also helps balance your blood glucose levels.

People with type 1 diabetes don’t make insulin. People with type 2 diabetes cannot properly use the insulin their bodies make. Taking insulin is necessary for people with type 1 diabetes, but it can also be important for people who have type 2 diabetes.

Getting the right amount of insulin can seem a bit tricky at first. It requires doing some math to get the dose just right, along with considering various factors such as your diet, your level of physical activity, and more.

The insulin sensitivity factor tells you how many points, in milligrams per deciliters (mg/dL), your blood sugar will drop for each unit of insulin that you take.

The insulin sensitivity factor is also sometimes called a “correction factor.” You need to know this number to correct a blood sugar level that’s too high. This is most useful for people with type 1 diabetes.

You can calculate your insulin sensitivity factor in two different ways.

One way tells you your sensitivity to regular insulin such as Humulin or Novolin. The other tells you your sensitivity to rapid-acting insulins such as insulin aspart (NovoLog) and insulin lispro (Humalog).

Regular insulin

For regular insulin, use the “1800 rule.” This tells you how much your blood sugar will drop for each unit of regular insulin.

For example, if you take 30 units of regular insulin daily, divide 1800 by 30. This equals 60.

This means your insulin sensitivity factor is 1:60, or that one unit of regular insulin will lower your blood sugar by about 60 mg/dL.

Rapid-acting insulin

For rapid-acting insulin, use the “1500 rule.” This tells you how much your blood sugar will drop for each unit of rapid-acting insulin.

For example, if you take 30 units of rapid-acting insulin daily, divide 1500 by 30. This equals 50. This means your insulin sensitivity factor is 1:50, or that one unit of rapid-acting insulin will lower your blood sugar by about 50 mg/dL.

An insulin dose that’s too high could lower your blood sugar too much. This can cause hypoglycemia. Hypoglycemia occurs when your blood sugar falls below 70 mg/dL. Hypoglycemia can lead to a loss of consciousness and seizures.

An insulin dose that’s too low may not bring your blood sugar to the target level. The resulting high blood sugar is called hyperglycemia. Hyperglycemia can lead to serious complications over time that can affect your:

  • heart
  • kidneys
  • eyes
  • nerves
  • other organs

You’ll need to know how sensitive you are to insulin to know the right insulin dose to take. In other words, you’ll need to know how much insulin you need to lower your blood sugar by a certain amount.

Insulin sensitivity isn’t the same for everyone. Some people with diabetes are more sensitive to insulin than others.

People with type 1 diabetes are generally more sensitive to insulin than those with type 2 diabetes. Your sensitivity to insulin can vary during the day based on your activity level and your body’s rhythm of daily hormone secretion. Illness can also affect your insulin sensitivity.

You can determine your insulin sensitivity factor by finding your insulin sensitivity factor. However, it’s important to remember that this figure may not always accurately represent how much insulin you need.

Your blood sugar can be affected by a lot of things throughout the day, so speaking with your doctor to make sure your insulin dosage takes this into account is important.

Once you know how sensitive you are to insulin, you can figure out how much insulin you need to give yourself to lower your blood sugar by a certain amount.

For example, if your blood sugar is 200 mg/dL and you’d like to use your rapid-acting insulin to lower it to 125 mg/dL, you’d need your blood sugar to drop by 75 mg/dL.

From the insulin sensitivity factor calculation, you know that your rapid-acting insulin sensitivity factor is 1:60. In other words, one unit of rapid-acting insulin lowers your blood sugar by about 60 mg/dL.

So, how much insulin do you need to lower your blood sugar by 75 mg/dL?

You’ll need to divide the number of mg/dL you want to lower, which is 75, by the number from your insulin sensitivity factor calculation, which is 60. The answer of 1.25 tells you that you need to take 1.25 units of rapid-acting insulin to lower your blood sugar by 75 mg/dL.

These are rough calculations that are used by people with type 1 diabetes. To be certain, consult your doctor on the amount of insulin you need. If you have type 2 diabetes, you’ll definitely need to check with your doctor for guidance.

Here you’ll find answers to common questions about insulin and blood sugar.

How do I prevent a blood sugar spike?

The best way to manage your diabetes is to keep your blood sugar from spiking.

If you have type 1 diabetes, you can accomplish this by using long-acting insulin once or twice per day and rapid-acting insulin before each meal.

This method will involve counting your carbohydrates at meals and dosing your premeal insulin based on your individual correction factor. You may also want to talk with your doctor about continuous blood glucose monitoring to help get better control and avoid hypoglycemia.

How often do I check my blood sugar?

You should check your blood sugar after taking extra insulin to ensure that your blood sugar drops appropriately.

The effectiveness of regular insulin peaks after approximately 3 hours. With rapid-acting insulin, it’s after about 90 minutes.

Speak with your doctor to learn when the best time is for you to check your blood sugar depending on the type of insulin you’re using and other factors.

Where can I get more help with this if I need it?

If you like using your smartphone, you can use an app to help you calculate your insulin sensitivity factor and dosage.

Search for insulin sensitivity or insulin correction calculators on your iPhone or Android device. Find one that seems easy to use and play around with it until you feel comfortable.

You may also be able to find online resources, such as the American Association of Diabetes Educators (AADE) website. However, it’s crucial to always consult with your doctor as every person’s diabetes needs may vary.

Understanding your insulin sensitivity is important for maintaining your blood sugar. You can determine this using a mathematical formula. Apps can also help.

This method only applies to decreasing your blood sugar when it’s already high. The reality is that there will be times when your blood sugar will be too high. This method may help you safely bring down your blood sugar to a more reasonable level.

However, speaking with your doctor before using this method is also important. Other factors can influence how much insulin you need such as how long it’s been since you’ve last received insulin, how physically active you are, what you’ve eaten, and more.