If you use fast-acting insulin and count carbohydrates, you want to know your insulin-to-carbohydrate ratio. An insulin-to-carb ratio allows you to easily figure out how much of your fast-acting insulin is needed for the amount of carbohydrate you consume.
You can talk to your health care provider or certified diabetes educator (CDE) to help you know where to start and then through trial and error you can figure out the right ratio for you.
An example of an insulin-to-carb ratio is 1:15 meaning one unit of insulin is needed for every 15 grams of carbohydrate. Someone needing more insulin may have a ratio of 1:10 and someone needing less insulin may have a ratio of 1:20.
When you are trying to figure out your correct ratio, it is helpful to write down your blood sugar levels and check more frequently to see how the insulin-to-carb ratio you used affected your blood sugar levels two hours after eating.
How to Figure Out Your Insulin-to-Carb Ratio:
Calculating the best insulin-to-carb ratio is a process of trial and error:
- Check your blood sugar before eating and write down your result.
- Count your carbs and give your insulin and write down the insulin-to-carb ratio you are using.
- Check your blood sugar 2 hours after eating and write down your result.
Try this process out a for a few days before adjusting your ratio. When you do adjust your ratio, do so in small, safe increments. When an insulin-to-carb ratio works consistently well, keep it! But remember that factors like illness, skipping a meal, extra exercise, stress and other changes in routine may cause your insulin-to-carb ratio to change.
The 450/500 Rule for People with Type 1 Diabetes
In the book Using Insulin, the authors share the Rule of 500 to figure out a good starting ratio. This applies to people with type 1 diabetes who are taking multiple daily injections of insulin. They write:
The 500 Rule:
- estimates grams of carb per unit of Humalog or Novolog insulins (the 450 Rule is used with Regular insulin)
- 500 divided by your TDD (Total Daily Dose of insulin) = grams of carb covered by one unit of Humalog or Novolog
- let’s you keep your post meal readings normal!
Someone’s TDD = 50 units (i.e., the total amount of say Humalog and Lente insulins they used per day).
500/50 = 10 grams of carbohydrate covered by each unit of Humalog insulin
TDD = all fast insulin taken before meals, plus all long-acting insulin used in a day. If Humalog is used everyday to correct high readings, this may also need to be factored into the TDD. For instance, if someone’s TDD is “30 units” (5 H before each meal, plus 15 Lantus at bedtime), but they need 8 to 12 units more almost every day to bring down highs, at least some of this 8 to 12 units will need to be factored into a new TDD.
Caution: The 500 Rule will be most accurate for those who make no insulin of their own and receive 50% to 60% of their TDD as basal insulin. It works best for those who are using a basal/bolus approach. For others, such as those who use two injections a day with the morning basal insulin covering carbs at lunch, the 500 Rule works only as a rough guide for matching carbohydrate.
Insulin-to-Carb Ratios May Vary
Your body is often more resistant to insulin during different parts of the day or month. Therefore, it is common to have different insulin-to-carb ratios that are set in your routine.
For example, an insulin-to-carb ratio may be greater for breakfast versus lunch to help manage the dawn phenomenon they may experience each morning.
For women in the week before menstruation, insulin-to-carb ratios may be adjusted each month to cope with these hormone changes and their effects on blood sugar levels.
Different people need different insulin-to-carb ratios, so don’t worry if your ratio is different from another person with diabetes. The key is to find a ratio that works well for you.
Calculate Insulin Doses Using Your Insulin-to-Carb Ratio
The Academy of Nutrition and Dietetics clearly explains how to calculate an insulin dose for food:
- Add up the grams of carbohydrate in the foods you will eat.
- Divide the total grams of carb by your insulin-to-carb ratio.
“Let’s say you plan to eat 45 grams of carbohydrate and your insulin-to-carb ratio is 1 unit of insulin for every 15 grams of carbohydrate eaten. To figure out how much insulin to give, divide 45 by 15.”
This equals 3 units of insulin needed for this amount of carbohydrate.
The Diabetes Care of Nova Scotia provides a more complicated example:
If you count 72 grams of carbohydrate and your insulin-to-carb ratio is 1:15 then you need to divide 72 grams of carbohydrate by 15. (Don’t hesitate to use a calculator.) This give you 4.8 units of insulin. If you take injections you can round the 4.8 to 5 units.