Insulin-to-carbohydrate (I:C) ratios, which are used to calculate the insulin doses people with diabetes need for specific amounts of food containing carbohydrate, are an important part of any intensive diabetes management program.

An I:C ratio tells you how many units of insulin you need to take to "cover" a specified number of carbohydrate grams—for example, if your ratio is 1:12, you'll need 1 unit of insulin for every 12 grams of carbohydrate you eat.

Before learning how to calculate I:C ratios, you'll need to know the basics of carbohydrate (CHO, or "carb") counting, including which food groups contain CHO and where to find information about grams of CHO in various foods. Your diabetes management team will usually teach you these basics.

What Is An I:C Ratio?

The I:C ratio is a guide for determining how much insulin you'll need as a bolus dose to help the body process, or metabolize, the amount of carbohydrate you'll be consuming in a meal or a snack. People with insulin resistance will need more insulin for each CHO serving than people who are more insulin-sensitive. An "average" might be 1 unit of insulin for every 10 or 15 grams of CHO for an adult, or 1 unit for every 20 to 30 grams for a school-age child, depending on the calculation method used. Infants and toddlers need individualized determinations by the diabetes care team.

Typically, the daily total of all bolus doses equals about 50 percent of an individual's estimated daily insulin needs, but the amount could range from 40 to 55 percent. Bolus doses are usually given before eating but are sometimes delayed after food ingestion for infants, toddlers, and "picky" eaters. People with delayed gastric emptying will often program and deliver meal boluses after a meal rather than before.

What's Involved in Determining an I:C Ratio?

You might find that methods for calculating I:C ratios vary among different doctors and different diabetes management teams, based on their particular experiences. Whatever the method used, it's generally considered to be a starting point only, as individuals may differ in their specific needs. A phrase commonly used about diabetes is YMMV ("your mileage may vary"), meaning that what works for one person may not work for another.

Anyone trained in calculating in-sulin doses—usually an MD, a PA, or an RN/RD/CDE—can help you establish I:C ratios. With a bit of practice (and a comfortable knowledge of basal insulin calculation formulas), you can also learn how to do this yourself.

Not all formulas fit all people; this is definitely a situation where YMMV. Always check with your diabetes management team before making calculations on your own. In addition to knowing your total daily insulin dose (TDD, the sum of basal insulin plus all meal bolus doses for 24 hours), you might also need to take into account factors that are unique to you as an individual: lifestyle, growth and developmental status, weight, age, and the presence of other medical conditions, for example.

How Do You Know Your I:C Ratio Is Correct?

Whichever method you choose for calculating your insulin-to-carbohydrate ratio, remember that the ratio is a starting point, to be evaluated by blood-glucose testing and then revised and refined as needed to reach your blood-glucose targets.

To evaluate the accuracy of your I:C ratio, you need to be certain that you've been calculating the carbs for your meals correctly and that you've been timing your bolus delivery consistently. It's also important that your basal rates have been set correctly and are stable. The key to checking the accuracy of your I:C ratio is, of course, measuring your post-meal blood glucose and finding results in the expected range.

Insulin-to-carbohydrate ratios will rarely stay the same over a long period of time. Keeping accurate blood-glucose records—as well as food-intake records, when requested—will ensure that your clinician can help you adjust the I:C ratio when needed.