Insulin to Carb Ratio (How to calculate it)

This might seem like a stupid question to some of you guys, but I have no clue how to calculate my insulin to carb ratio.

Is there a specific formula, or is something that only a doctor can calculate?



[quote user="Catie Miller"]

Is there a specific formula, or is something that only a doctor can calculate?



HI Catie, =) why is that dumb?...  a lot of people on insulin use a chart and NEVER figure out how to do their own I/C.   The formula I/C means"units of insulin per gram of carbohydrate".   There is no formula foy YOUR I/C - you are the formula =)   and you test by trial and error.

Anyway you start off with a good blood sugar 75-120 and 4 hours since your last shot of fast acting. 

Most folks use a starting I/C of 9 unless you are verrrrrry sensitive to insulin.  Eat a snack or small meal (not high fat) with a known amount of carbs.  Take 1 unit of insulin for each 9 grams of carbs.  Check your blood sugar at 1 hour (if you are low take sugar and stop the test)  and then test at about 2 hours.  it's helpful if you are not more or less active then normal when you do the test.

At 2 hours it should be within 50 points of your starting value.  Higher than 50? then you use a lower number (in this example your next I/C would be 8) next time you check your I/C.  check at 3 and 4 hours to make sure you are okay.  if you go low anytime you treat for hypo.  If your lower than 50+your starting BG or go hypo anytime, use a bigger I/C (in this case 10) next time.

you might notice you need a different I/C in the morning than at night.  I go from 6 in the AM to 9 in the PM.  My endo wanted me to re-check my I/C's after my last visit  Take care and good luck.

oh yea and Catie,  if you are real sensitive to insulin pls be careful.  don't start with and I/C of 9 unless you know for sure it's almost right.  a lot of people especially when they first get diabetes are really sensitive to insulin because they are still making some.    people who are honeymooning may have an I/C of 30.

and when I said above 50 I mean 50 plus your starting bg - NOT 50 mg/dl! 

Hey thanks for so much info! My only other question is I take novolog and NPH. This is just a calculation for the novolog correct? 

Catie be carefull! My daughter has an I/C ratio of 1 to 80!!

[quote user="Becky"]

Catie be carefull! My daughter has an I/C ratio of 1 to 80!![/quote]

I highly doubt your daughter's Insulin:Carb ratio is 1:80.  That would mean she could eat 10 oreos or 6 whole oranges with taking a single unit of insulin.  Is that the case?  With her age and having had diabetes for 6 years it doesn't really make sense.  I've seen a lot of kids at diabetes camp and the highest I:C ration I've seen is 1 u:40 carbs.

I think you might be thinking of her sensitivity factor, which would mean 1 unit of insulin drops her blood sugar 80 points...which makes a lot more sense.


Catie, the info Joe gave is just for your short-acting Novolog.  Your NPH (or any other long-acting) dose will usually stay pretty consistent regardless of what you're eating at meals. For meals and random snacks that require insulin you just count up the carbs and figure out how many units you would give according to your ratio and give that much Novolog.

My Insulin:Carb ratio is close to 1:10, which is pretty convenient because the math is super easy.  Most people will have a ratio between 1:6 to 1:15.  Unless you were recently diagnosed you're probably going to be within that range, but you just have to guess and check.  If you record your BG's and insulin dosage for the amount carbs you're eating for a couple days and take it to your Endo or to a dietitian they'd be able to figure it out pretty quickly.

my ratio is 1 to 15 and it took a lot of trial and error to figure it out.

Yes!  She is I/C 1 to 80!! Until about 1 yr ago she was 1 to 100!! She only has MAX 11 units of Novolog per day!! She is VERY sensitive to insulin.

[quote user="Becky"]

Yes!  She is I/C 1 to 80!! Until about 1 yr ago she was 1 to 100!! She only has MAX 11 units of Novolog per day!! She is VERY sensitive to insulin.


That's insane.  Does she eat snacks like it's nothing since he hardly has to worry about taking insulin if that many carbs doesn't raise her BG?  10-11 units of Novolog would mean 800 carbs/day, which is almost twice as much as what adults would eat...sticking to that 1:80 rule works out perfectly?  It kind of blows my mind. 

Regardless, I doubt Catie needs to worry about being anywhere near 1:80.  She's a lot older and probably nowhere near as sensitive.  Besides your daughter, the highest I'm used to seeing is 1:20, and that's even higher than normal.  Crazy.

How would 10-11 unites of Novolog mean 800 carbs a day? That does not seem right...


Yeah I am not as sensitive as 1:80. 


We never bolus for snacks. She eats 2-4 15 carb snacks per day and has 65 -80 carbs per meal. She has a very high metabolism also. I make her eat before soccer to get a BS of around 250 and then she eats again @ 1/2 time. We turn her basal to 50% during ANY exercise. She has baffled her Endo. staff as well.

my doctor tells me mine

Our doctor calculated ours for us, and my daughters is 1/200, so def be careful if you try it on your own.  I would check with your dr, as they may already have that info available for you and maybe you didn't even know it.  Good luck.

whoah yours is 1:200?

so for ever 200g of carbs you eat you only do 1 unit of insulin?

hey Catie...  don't forget a lot of newly diagnosed diabetics make their own insulin.  This can last for years and can really make injecting insulin unpredictable and make it seem like sensitivities are really high (carb ratios too).

the other thing is your NPH.   wow that makes the I/C thing tough.  NPH has a big, sometimes unpredictable peak.  It is not "flat" like Lantus or the other basal insulins.  So if you are trying to figure out a carb ratio - you have to allow for NPH that tends to peak at lunch time.  Your lunch I/C may be so high (Due to NPH on board) you may not even have to inject for lunch...... that doesn't mean you have a infinite carb ratio (1/a zillion) , it means your NPH is working instead of a bolus shot of Novolog/Humalog.   

People still take NPH because (among other things) if they have a perfect schedule they can skip the lunch bolus.  I took NPH for over 20 years.  Control was tougher for me because I HAD to eat at lunchtime, no fooling around, or I would pass out.  It's been better with a pump for me because the pump gives me a true basal, and I can eat lunch at 11 AM or 3 PM and it's no different for me.   You can get similar results with Lantus (or Levemir), but you need to take a shot of fast acting EVERY TIME YOU EAT.  So, you may go from 3 shots a day to 5+.  You will also have to take your insulin wioth you to school, work, etc.    it's a concession, there's more shots - but many people find their control is better and their freedom of schedule is better.  same thing as pump therapy. =) hope this wasn't too long or tmi


I have a hard time believing that you have never heard of i:c of 1:80 or higher.  My DD has been on a pump for a little over a month.  Was dx last July and currently has 3 differnt i:c ratios - 1:70. 1:80 and 1:90 depending onf the time of day,  She is on the pump for the reason that it was very hard to give small amts of insulin with a syringe.  the smallest we could give was 1/2 unit and most of the time that was TOOO much.  She is 8 and about 50# and if you look only at her weight and age you would automatically put her at about 1:30 ratio- but that would bottom her out. 

Read booksl like Think Like A Pancreas (cannot remember the author),  or Using Insulin by John Walsh, or Pumping Insulin by John walsh.  There are lots of formulas that help any person on different insulin regimens figure out correct insulin sensitivites, i:c ratios and basal needs. 


One poster said that lantus is a flat basal insulin and in theory that is true - but Lantus and I believe levimir too - there is a peak at about 5 hours post dose.  It is much harder to figure out i:c ratios on NPH that is true and a Humalog Lantus regimen or a pump offeres much more flexibility as far as eating and really precision tuning your dosing.


One basic rule for those people on humalog/lantus is the 500 rule.  (remembering that this can vary from time of day)

But knowing your Total Daily Dose take 500/TDD=i:c ratio - for example my dd's is about 8 units TDD (basal and bolus) so 500/8 = 62.5 or her i:c would be 1:62.5 and that is on target at about 9 am but my noon she is more like 1:80 and by dinner she is more like 1:70


Hope this helps!


I wish my ratio was as high as some of these...would make the prescription cost come way down. I'm 1:10 and would kill for 1:200... I <3 carbs

Hi Catie !

You can go either with trial and error method or by scientific one....

First you need to know your TDD (total daily dose), then you can go to work.


Here are rules ( I hope they will be displayed correctly):

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mg/dL mmol/L
Corr. Rule (Regular Insulin) 1500 84

Corr. Rule (Fastacting Insulin)

1800 100

I:Carb Rule (Regular Insulin) 450 450
I:Carb Rule (Fastacting Insulin) 500 500
I:Carb Rule (Children) 300 300


CH/Insulin ratio = I:Carb rule / TDD

BG/Insulin ratio (corrections) = Correction Rule / TDD

BG/CH ratio = CH Insulin Ratio / BG Insulin Ratio


But be carefull. This are average ratios. Sensitity of your body to insulin changes through the day. For example most people are very sensitive from 0-4 hour, and after that this changes drasticaly. At 4 am there is time for dawn phenomem, which means, that you will need lots of insulin to cover it (if you are succeseptible to it, not everyone is)...

Take care,


When I was diagnosed I was really sick with a cold or the flu or something and my endo thought that I would have to change my doses. But, the only thing that really changed was my breakfast dose. I do 1:20 in the morning at breakfast and 1:15 for every other meal.

Something that really interests me is that my father does the same ratio 1:15. I was told that as you get older your dose sometimes needs to change since you may still be able to make enough insulin when you are first diagnosed. 

I just want to know if anyone else's doses have changed since getting older?