# Formula for determining carb count from blood sugar changes?

Prior to eating I count carbs and then use my insulin carb ratios to determine how much insulin I should take. This is straight forward when I'm cooking my food, going to a chain restaurant, or have clear carb numbers.

Sometimes however I end up in a restaurant where no carb information is available. I order my food and then guess how many carbs there are. I try to err on the side of being too low so that if anything my blood sugar will go high and I can take a correction dose.

What I do today is something like this.

3:00pm - Took 5 units of insulin for Dinner #3 at X restaurant. Pre meal BGL was 85. 5:00pm - BGL is 200

Then I say my BGL went up 115, next time take more insulin. Next time I will add a couple of units. I keep doing this until I get 'close'. What would be ideal is if there is some formula I can use to calculate how many carbs that was, so the next time I eat the same thing I can dose correctly.

If you know your insulin-to-carb ratio and your correction factor (how much your blood sugar drops per unit of insulin), you should be able to calculate how much you should have had fairly simply – at least in theory.

Assuming that your blood sugar would have remained level had you not eaten (which will not usually be the case in real life), you just need to subtract your expected blood sugar from your actual blood sugar.

``````85 - 200 = 115 [mg/dL]

115 [mg/dL] * (correction factor) [units / mg/dL] = (insulin deficit) [units]
``````

Suppose your correction factor is 1:50, e.g. 1 unit brings you down 50 mg/dL.

``````115 [mg/dL] * (1/50) [units / mg/dL] = 2.3 [units]
``````

You can then estimate about how many extra carbs were in the meal, using your insulin-to-carb ratio:

``````(insulin deficit) [units] * (carb ratio) [g carb/unit] = (excess carbs) [g carb]
``````

Suppose your ratio is 1:10, e.g. 1 unit covers 10g of carb:

``````2.3 [units extra needed] * 10 [g carb/unit] = 23 [g carb more than estimated]
``````

If you use 15g carb = 1 exchange/choice:

``````23 g / 15 g = 1.5 exch
``````

So, you now know that the meal had 23g more carb in it than you expected, and that in the future you would take 2.3 more units.

Of course, in reality it's not quite so simple. Different foods take longer to digest, convert to glucose, and enter the bloodstream. Activity level, injection site, and other factors affect how quickly insulin kicks in. The above calculation is the most straightforward way to calculate the food/insulin/blood sugar balance, but there will always be other factors at play.

• Hi, nhinkle, welcome to Health! Here, references are strongly encouraged in answers to back up points made therein. Unsourced material may be downvoted or deleted - and is certainly frowned upon. Can you add citations to support what you've written here? Besides that, this looks like a good answer. Commented Feb 4, 2016 at 0:30
• @HDE226868 I understand your point, but I'm not sure how I can "reference" basic math and unit conversion. What part of my answer would you say needs citing? Commented Feb 4, 2016 at 15:55