Between meals, during fasting, and through the night around 40% to 50% of a daily insulin dose is used to replace insulin and is labeled basal or background insulin replacement. This dosage of basal insulin should be consistent every day. 50% to 60% of the daily insulin dose is used to correct high blood sugar and food or carbohydrate coverage and is labeled bolus insulin replacement. An insulin to carb ratio is how the bolus dose is determined and shows how 1 unit of insulin removes the numbers of carbohydrate grams. 1 unit of fast-acting insulin can remove 12 to 15 grams of carbohydrates typically. Depending on a person’s insulin sensitivity this can vary from 4 to 30 grams of carbohydrates or more. Physical stress and activity and even the time of day can affect sensitivity to insulin. It can also vary wildly from person to person. The bolus dose can also correct high blood sugar and is known as the insulin sensitivity factor. Typically blood glucose will drop 50 mg/dL with one unit of fast-acting insulin. Individual sensitivity to insulin and various other factors can vary this blood sugar correction from 15 to 100 mg/dL or more.
Example #1: Meal Carbohydrate Coverage
A meal’s total grams of CHO ÷ grams of CHO removed by 1 unit of insulin = CHO insulin dose.
This is the formula to determine the carbohydrate coverage insulin dose. The denominator of the Insulin to CHO ratio or the bottom number is what is used in the second part of the equation, the grams of CHO removed by 1 unit of insulin. To put this formula in action;
At lunch, you plan on eating 60 carbohydrates. 1/10 is your CHO insulin ratio. The formula would look like: 60 ÷ 10. And 6, the result of the equation, would be the CHO insulin dose needed to cover the carbohydrates in the meal.
Most of the times patients enter their carbohydrates and blood glucose values in the bolus wizard, and it automatically calculates the amount of insulin to be given.
Example #2: Correction dose for high blood sugar
The difference between actual and target blood glucose ÷ the correction factor for high blood sugar = the correction does for high blood sugar
This is the formula to determine the correct dose for high blood sugar. To put this equation into action;
120 mg/dL is the target for pre-meal blood sugar, but 220 mg/dL is your actual level before lunch. Your correction factor for high blood sugar is 50 because the assumption in this equation is that your blood sugar will lower by 50 mg/dL with 1 unit of insulin. The formula would look like (220-120) ÷ 50. The result of the equation, 2, would be the number of rapid-acting insulin units needed to lower your current blood sugar levels to the desired level or the “correct” range.
Example #3: Mealtime total dose
Insulin dose for blood sugar correction + CHO insulin dose = Mealtime total insulin dose
This is the formula to determine the total dosage amount of insulin required at mealtime. To put this formula into action using the above numbers in example 1 and 2:
CHO insulin dose is 6 units of rapid-acting insulin. The insulin dose for high blood sugar correction is 2 units of rapid-acting insulin. The formula would look like: 2 + 6. The result of the equation, 8, is the total mealtime rapid-acting insulin dosage.
Example #4: Recommended formulas used for insulin doses
There are 3 methods introduced in this example, full insulin replacement dosage, bolus dosage, and basal or background dosage. This formula is to provide an example, but it is important that before you use this in your own life that you talk with your healthcare provider to determine your own needs. If you are highly resistant to insulin, these calculations could be too low for you. If you are still generating a lot of insulin on your own and new to diabetes, these calculations may be too high for you.
? Requirement for total daily insulin
Weight in pounds ÷ 4 = requirement of total daily insulin in units. Or, 0.55 x weight in kilograms = requirement of total daily insulin in units.
This is the calculation used to determine the daily insulin requirement for the body in pounds or kilograms.
The equations for bolus and basal doses require this information. To put these equations into action;
You weigh 160 pounds. 160 lbs. ÷ 4 = 40. You will need to have 40 units of rapid-acting insulin per day.
You weigh 70 kg. 0.55 x 70 kg = 38.5. You will need to have 38.5 units of rapid-acting insulin per day.
Keep in mind, if you are less insulin resistant, you may need a lower dose, and if you are highly resistant, you may be a higher dose.
? Basal or Background Insulin Dose:
40% or 50% of your total insulin dose per day = basal dose
This is the formula for finding your background or basal dose. To put this equation into action;
You weigh 160 lbs. and your daily insulin dose is 40 units. 50%(40 units). The resulting answer, 20, is the number of rapid-acting insulin units, if you use a pump, or long-acting insulin units, like detemir or glargine.
? Bolus Insulin Dose:
500 ÷ Total insulin dose per day = how many carbohydrates 1 unit of insulin covers
This is the called the “Rule of 500” and is the ratio of carbohydrate coverage. The assumption of this equation is that, throughout the day, your insulin response is constant. Because the reality is your insulin response fluctuates throughout the day you need to adjust this ratio along with it. You do not need to adjust your background or basal dosage. To put this formula into action;
You weigh 160 pounds. Your total daily insulin dose is 40 units. 500 ÷ 40. The resulting answer, 12, is the number of grams of carbohydrates covered by 1 unit of insulin. This answer is then written as a ratio, 1: 12.
? Correction factor for high blood sugar:
1800 ÷ total daily insulin dose = how much blood sugar will lower from 1 unit of insulin, rounded to the nearest whole number.
This is the correction factor formula and is also known as the “Rule of 1800.” To put this formula in action;
You weight 160 pounds. Your total daily insulin dose is 40 units. 1800 ÷ 40. 45, the answer of the equation, is how low the blood sugar will be dropped with 1 unit of insulin. In this equation, for convenience, the resulting number is typically rounded up or down to the closest whole number making the stated answer 40-50 mg/dL.
This information is an estimation and modifications may be required to keep your blood sugar stable. In addition, there are many forms of therapy for insulin. Your diabetes team and health care providers should work with you to determine your unique insulin need and dosage regimen.
Ajay Varanasi, MD, FACP, FACE
Director, Tampa Bay Diabetes, Thyroid & Endocrinology Center
Assistant Professor, Morsani School of Medicine, University of South Florida
Adjunct Clinical Professor, Lake Erie College of Osteopathic Medicine
