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Insulin-to-carb ratios to calculate meal insulin doses with type 1 diabetes

Published Nov. 8, 2018
• Reviewed by a UI Health Care Provider

Some children and teens want or need options in meal planning. Using an insulin-to-carb ratio is a way for you to get the right amount of insulin for the carbohydrate you eat if you are not sticking to a carbohydrate pattern. Then you can eat different amounts of carbohydrate at each meal.

Use an insulin-to-carb ratio if you:

  • Are not sure your very young child will eat all the carbohydrate in the meal
  • Are not hungry
  • Do not like some foods served with a meal
  • Are eating a meal with a lot of carbohydrate
  • Are eating a low carbohydrate meal
  • Need or want a larger snack

The insulin-to-carb ratio means you will take 1 unit of insulin for a certain amount of carbohydrate.

For example, if your insulin-to-carb ratio is 1 unit of insulin for every 10 grams of carbohydrate (written 1:10), you will take 1 unit of insulin for every 10 grams of carbohydrate you eat.

To use an insulin-to-carb ratio, you need to:

  • Plan ahead and eat all of your meal.
  • Take your rapid-acting insulin 15 minutes before you eat.
    • The only time it is okay to take the rapid-acting insulin after eating is for very young children who may not eat everything. If a child is taking their insulin after they eat, they must take it as soon as they finish eating, within 30 minutes of their first bite of food.

Taking insulin after eating will always result in a high blood sugar a few hours later.

Taking insulin before eating and then not eating all of the planned carbohydrate will result in a low blood sugar when the rapid-acting insulin peaks.

If you will be using an insulin-to-carb ratio to calculate rapid-acting insulin doses, you will need to be accurate at counting carbohydrate and doing math to calculate your dose.

Practice using an insulin-to-carb ratio

When using your insulin-to-carb ratio, you divide the total grams of carbohydrate by the ratio amount.

Practice 1

  • Your breakfast dose of rapid-acting insulin is 1:10.
  • You plan to eat a total of 55 grams of carbohydrate for breakfast.
  1. Divide your total grams of carbohydrate by your ratio of 10. So 55 ÷ 10 = 5.5
  2. Your breakfast dose will be 5.5 units of rapid-acting insulin.

Practice 2

  • Your lunch dose of rapid-acting insulin is 1:20.
  • You plan to eat 55 grams of carbohydrate.
  1. Divide your total grams of carbohydrate by your ratio of 20. So 55 ÷ 20 = 2.75
  2. Round up to the nearest half unit. 2.75 rounded to the nearest half unit is 3.
  3. Your lunch dose will be 3 units of rapid-acting insulin.

Your ratio may be the same or different at each meal. Using an insulin-to-carb ratio of 1:10 gives you more insulin than if you use a ratio of 1:20.

When deciding whether to round up or down, think about:

  • Rounding up if your blood sugar is high
  • Rounding down if your blood sugar is low
  • What you will be doing in the next few hours, such as being active or sitting around

This can be confusing at first, but doing the math can help you understand this better.

Changing the insulin-to-carb ratio

You will have to do some math to figure out how to change your ratio. The diabetes nurses will teach you how to do this. It will not be perfect at first. It will take some time.

Things to remember in order to change insulin doses:

  • If the pattern happens at the blood sugar check before breakfast, change the long-acting insulin dose by 10 percent.
  • If the pattern happens at the blood sugar check before lunch, change the breakfast rapid-acting insulin dose by 10 percent.
  • If the pattern happens at the blood sugar check 2 to 3 hours after lunch, change the lunch rapid-acting insulin dose by 10 percent.
  • If the pattern happens at the blood sugar check 2 to 3 hours after supper (before the bedtime snack), change the supper rapid-acting insulin dose by 10 percent.
  • If your blood sugar is above 180 two to three hours after a meal, ask yourself what caused this.
    • Common reasons for high blood sugar 2 to 3 hours after eating are:
      1. Not taking insulin at least 15 minutes before eating
      2. Eating too much carbohydrate or too much quick-acting carbohydrate
      3. Not taking enough insulin to cover the carbohydrate
      4. Not eating protein or fat in your meal
      5. Eating a very high fat meal

If you rule out numbers 1 and 2, you may need to take more insulin next time.

  • If the pattern is high blood sugar, you will increase the insulin dose that affects that column of blood sugars.
  • If the pattern is low blood sugar, you will decrease the insulin dose that affects that column of blood sugars.