How to Calculate an Insulin-To-Carb Ratio


Every person responds differently to insulin. This means that any two people with diabetes can have quite different carb-to-insulin ratios. For some adults, one unit of fast-acting insulin can usually cover 15 grams of carbohydrate. However, for a very young child, as little as 1/10 unit of fast-acting insulin might cover 15 grams of carbohydrate.

Healthcare professionals typically estimate a starting carb-to-insulin ratio for a person. Then they fine-tune the ratio over time by analyzing the person’s blood glucose readings before and after meals. A starting carb-to-insulin ratio can be estimated using the 450/500 Rule.

IMPORTANT: The 450/500 Rule is useful only for people with type 1 diabetes.

  • People with type 2 diabetes usually produce some insulin naturally, in addition to the insulin they inject, so a total daily insulin dose cannot be calculated reliably.

  • If you have type 2 diabetes and you read the chart using only the insulin dose that you inject, this chart will not give you a useful carb-to-insulin ratio.

500 Rule for Users of Humalog and Novolog

Divide 500 by the total daily dose of insulin. The result is the grams of carbohydrate that are approximately covered by 1 unit of insulin.

Example: A man takes 40 units a day of Humalog. His carb-to-insulin ratio is 1 unit of insulin for every 12.5 grams of carbohydrate. The equation is:
500 divided by 40 (total dose) = 12.5 (grams of carb covered by 1 unit of insulin).

450 Rule for Users of Regular Insulin

Divide 450 by the total daily dose of insulin. The result is the grams of carbohydrate that are approximately covered by 1 unit of insulin.

Example: A woman takes 30 units a day of Regular. Her carb-to-insulin ratio is 1 unit of insulin for every 15 grams of carbohydrate. The equation is:
450 divided by 30 (total dose) = 15 (grams of carb covered by 1 unit of insulin).


Adapted from Pocket Pancreas, © 1994, Diabetes Services, Inc.

Carb-to-Insulin Ratio
 
500 Rule
450 Rule
Total Daily
Insulin Dose
Grams of Carb
Covered by
1 Unit of Humalog
Grams of Carb
Covered by
1 Unit of Regular
20
25
23
25
20
18
30
17
15
35
14
13
40
13
11
50
10
9
60
8
8

 


The BD Diabetes Learning Center describes the causes of diabetes, its symptoms, and diabetes complications such as retinopathy and neuropathy. This site contains detailed information about blood glucose monitoring, insulin injection and safe sharps disposal. Interactive quizzes, educational literature downloads and animated demonstrations help to teach diabetes care skills.

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