Dr Araz Rawshani

Carbohydrate counting: rule of 500 rule, rule of 300 and rule of 100, for insulin dosing


Below you will get information about carbohydrate counting. We’ll go through what carbohydrate count means and why you count carbohydrates. To help you count on carbohydrates and insulin, there is also a carbohydrate counter.

Carbohydrate count: dose insulin with the 500 rule, 300 rule and 100 rule

Carbohydrate count is a simple tool for those who have diabetes and use insulin. Dosage of insulin is important, but time-taking. It is important that the dosage is correct in order to best balance your blood sugar. You should not have too high blood sugar or risk blood sugar falling. In the vast majority of cases, it is recommended to keep blood sugar in the range of 4.0 mmol/l to 7.0 mmol/l. This is because we are different old, have different body build-up, differences in insulin sensitivity, etc.

Carbohydrate count is used to the following:

  • Calculate how many units of insulin are needed for breakfast (300 rule).
  • Calculate how many units of insulin are needed for other meals (500 rule).
  • Calculate how much blood sugar drops by 1 unit of insulin, in order to correct its blood sugar during the day (100 rule)

The mathematics itself is simple and the vast majority of people quickly get good at carbohydrate counting. Remember that carbohydrate count is used to dose direct-acting insulin. The aim is thus to estimate how many units of insulin you need to take care of carbohydrates in food and drink.

You use three simple mathematical rules to do carbohydrate counting. These rules are called the 500 rule, 300 rule and 100 rule.

  • The 300 rule is used to dose insulin for breakfast.
  • 500 rule is used to dose insulin for other meals.
  • 100 rule is used to estimate the effect of 1 unit of insulin, which can be used to correct blood sugar (for example, before a meal).

Carbohydrate counting with 300 rule

In the morning, the body is less sensitive to insulin, which is due to the fact that hormones released during the night make the body less susceptible to insulin. Therefore, as a rule, a larger dose of insulin is needed for breakfast, compared to other meals during the day.

Here’s how to do it:

Divide the figure 300 by the total amount of insulin you use in 24 hours. The total amount of insulin should include both long-acting and fast-acting insulin. When you divide 300 by the number of units of insulin, you will get a figure showing how many grams of carbohydrates 1 unit of insulin will suffice.


Johan uses 25E Lantus and 25E Humalog daily. In total, Johan uses 50E insulin.300/50 = 6This means that 1E insulin is enough for 6 grams of carbohydrates.For breakfast, Johan intends to drink milk and eat sandwiches, which in total contains 60 grams of carbohydrates. Then Johan needs the following insulin dose: 60 grams/6 = 10E insulin.

Carbohydrate count with 500 rule

The sensitivity of the body to insulin increases during the day, which makes it necessary to use less insulin compared to the need for breakfast. Therefore, the 500 rule is used for other meals. The 500 rule means dividing 500 by your total daily insulin. The figure you get corresponds to the number of grams of carbohydrates that 1 unit of insulin is enough.


Johanna needs a total of 40 units of insulin for one day. This includes both fast-acting and long-acting insulin.500/40=12.5.That is, 1 unit of insulin takes care of 12.5 grams of carbohydrates. Johanna decides to eat a meal with a total of 25 grams of carbohydrates and then he calculates that he needs 2 units of insulin.

100 rule is used to correct blood sugar

Sometimes you need to correct (lower) your blood sugar. For example, it is common to have a slight decrease in blood sugar before a meal. Then you need to know how great the effect 1E insulin has on blood sugar. To calculate this, use the 100 rule. This rule indicates how sensitive your body is to insulin (read more about Insulin sensitivity). The calculation is simple. You divide 100 by the number of units of insulin you use in 24 hours and the number you receive indicates how much blood sugar (in mmol/l) drops of 1E insulin.


Zara uses a total of 50E insulin for one day. Now her blood sugar is 8 mmol/l and she wants to correct this before a meal. She wants blood sugar to be 6 mmol/l before she starts eating.100/50 = 2This means that 1E insulin corrects blood sugar by 2 mmol/l. For Zara to lower her blood sugar from 8 to 6 mmol/l, she needs to take 1E insulin.

Does Zara have to wait for her blood sugar to be corrected before she starts eating? No, she can add the corrective dose to the meal dose (as calculated by the 500 rule or 300 rule) and start eating.

Important to consider with carbohydrate counting

The 500 rule, the 300 rule and the 100 rule are rough rules of thumb for how much direct acting insulin you need for a certain amount of carbohydrates. These rules are used both in Sweden and abroad and they are very good for learning how to dose insulin. In practice, however, you need to weigh more things in addition to the amount of carbohydrates, and you sometimes need to adjust the figures 500, 300 and 100 to better suit you. For example, the figure 450 may be suitable than 500 and this is something you learn over time (of course you should discuss this with your doctor and nurse).

Remember that the dose of insulin is affected by many factors, some of which are:

  • How much to move before and after the meal.
  • Blood sugar before the meal.
  • What kind of carbohydrates you eat. If you eat fast carbohydrates, your blood sugar can rise fast even if you dose correctly. If you eat slow carbohydrates, you can actually get a blood sugar drop because slow carbohydrates take longer to take up.
  • Insulin uptake may vary.

Also consider the following

If a child who has received his calculated dose of insulin does not eat the meal for which the dose is calculated, it is important to compensate for the missed carbohydrates with sandwich or fruit (equivalent to the amount of carbohydrates not eaten) in order to avoid too low blood sugar. results in a digit with decimal places and you should round to the nearest half of the unit.

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