Calculating your starting ICR

An insulin to carb ratio of 1:10 is a good start. This means taking 1 unit of insulin for every 10g of carbohydrate in your meal. But if you want to calculate a starting ICR more accurately, then follow the steps below:

• Calculate your average total units of injected insulin over 24 hours. You can do this by combining your total bolus insulin injections and your basal insulin injections over a 24- hour period (if this differs each day, take an average of a few days). Then add these two values together for your total daily dose (TDD).

• The 500 Rule: Divide 500 by your total daily dose i.e. 500 ÷ TDD. The number that this gives you ‘X’ is the number of grams of carbohydrate that will be ‘covered’ by 1 unit of insulin. i.e. 1 unit of insulin will be needed for X grams of carbohydrate you eat.

Example: See below if the total daily insulin dose (TDD) is 35 units:

This means that 1 unit of insulin will be needed for every 14.28g of carbohydrate eaten (you can round this down to 14g or up to 15g for ease of calculations).

Always check your ICR calculations with a health care professional before using them. As you start to implement an ICR, look at the results on blood glucose levels 2 and 4 hours after a meal. Does your blood glucose stay relatively stable after eating? If not, you may need to make further small changes to your ICR. You may also need a slightly different ICR for different mealtimes and your ICR may change slightly over the years.

By regular glucose monitoring, you can work out if your ICR is correct and you should make small adjustments to it if needed. We have a handy ICR/carb counting summary sheet that you can download and print out.

Insulin stacking

It’s important to remember that bolus insulin works over 3 – 4 hours so you need to allow this time for the previous dose to work before you give a correction dose. Giving extra insulin before the previous dose has lapsed can lead to hypos.

Insulin pumps

If you use an insulin pump then you should consult your healthcare team and/or review advice on the insulin pump eLearning course, as the calculation for initial insulin to carbohydrate ratios (ICR) may be slightly different e.g. 400 ÷ TDD (rather than 500 ÷ TDD) is often used to calculate a starting ICR.