16 Tips for Calculating Carb Content accurately & efficiently
Becoming a pro at managing type 1 diabetes hinges on figuring out how many grams of carbs there are in what you eat, wherever you eat. There are many tips and tricks that go beyond the basic rules of carb-counting to help us guesstimate through tricky situations that lead to high post-meal spikes. What do you do about a cob or a rind? Dry or cooked? How can we teach our T1D kids to visually estimate carbs? What do most breads and buns have in common? What's the issue with "lite" products? Do chocolate pancakes have more carbs? Here we share with you the rules of thumb that have simplified our lives as D-parents, and made managing diabetes, well… more manageable!
1. Calculating carbs by weight is more reliable than by volume.
It’s habit to whip out the measuring cups to figure out the volume of those Saturday-morning Cheerios, but you may get more accurate results by turning to your nutritional scale. Think about what happens when you fill a measuring cup with large or irregularly shaped items – such as dry cereal, or whole strawberries, or chunks of potato – the volume of food can vary each time you measure, depending on how big the pieces are and how the material settles. The bigger the air spaces between the pieces of food, the less accurate your volume measure is.
The weight of a given sample of food, however, is not affected by how big the pieces are, how the material settles, nor whether there are pockets of air between the pieces. So weight is consistent, and thus is a more accurate and precise measure for carb counting.
2. What do we do with a cob, core or rind?
If you use a nutritional scale to calculate the carb content of a cob of corn, the weight of the cob is included in the calculation, even though it will not be eaten. That is to say, if you place a full cob on the scale and enter the Cob Corn function, the scale output may say that there are 40g of carbs in the sample, but this calculation assumes that the full weight of the food on the scale is corn kernels, when in fact about half of the weight is the cob. The same issue arises for apples on the core and watermelon on the rind.
If you don't want to have to cut the edible portion off of the core, cob or rind, one solution to this problem is to calculate the total carbs, including the cob, using the scale’s “Cob Corn” food category. Then after your child has eaten the corn, repeat the process with just the cob (using the same food category, “Cob Corn”), and then subtract the second result from the first. This calculates a carb count for just the amount that was actually consumed. In effect, you are calculating carbs for the total weight of the cob + kernels, acting as if the cob was corn kernels; then you are subtracting out the cob amount, leaving you with just the carb content of the kernels consumed.
The same process applies to whole apples and pears on the core, and watermelon on the rind. You may simply use the scale’s “Apple With Skin” category for both the pre- and post- measures of the apple, even though what you are measuring the second time is a core, not an apple with skin.
This process also works if your child eats less of a given food than expected; if they eat half, but not all, of a banana, or pizza slice, or bowl of cereal: subtracting the "after" weight from the "before" weight will leave you with the amount that was actually eaten.
Tips from the Trenches
When our family eats corn on the cob, we follow the above process. But in order to calculate the meal insulin dose so our son receives insulin before he eats, we estimate the corn to contain 20g of carbs. Then, when the meal is done, we calculate the actual amount of corn consumed and give him more insulin if needed.
3. Visual estimation
Visual estimation of carb content is a SUPER handy skill to develop. With a little practice and feedback, you can get close enough to the actual carb content of a food item by looking at a portion of food and using an educated guess about how many carbs that portion contains. This helps at restaurants, at a friend's house, at a party, or any other time when you are away from your carb-counting tools (scales, measuring cups, books and apps).
Learn to Guesstimate
Visual estimation is a learned skill. You can hone your visual estimation skills, or teach visual estimation of carbs to your T1D child/teen, by:
- Estimate the amount of carbs in that portion (before actually calculating the carbs).
- Measure the actual carb content, using a nutritional scale, app, carb book, website or other reliable source.
- Compare your guess to the actual carb content. Did you guess too high? Too low? Was your guess exactly right? This gives you valuable feedback for next time.
Over time, your guesses will be closer and closer to the actual carb content. Once you find that your guesses are often "close enough" for a certain food, your confidence in your visual estimation skills will allow you to save time and hassle by skipping the scale.
By using this "guess and check" approach, you will get better and better at visually estimating carb content.
Use Your Fist
Another approach to visual estimation involves using your body parts to estimate the size of a serving, and so the amount of carbs.
For example, a pancake the size of my palm (with my hand spread out flat) contains about 10g of carbs; one the size of my husband's palm contains about 15g of carbs. My closed fist is about the same size as a cup, so I can compare the mound of rice on my son's plate to my fist to judge whether it's the same size, half as big, or twice as big as my fist. I then calculate the carbs based on 45g of carbs per cup of rice. If my son's portion is twice as big as my closed fist, he would bolus for 90g of carbs. A half-fist of mashed potatoes contains about 15g of carbs.
You can figure out how your hand - or your child's smaller hand - compares to pancakes, rice and potatoes by creating a mound of mashed potatoes the size of your hand, and then measuring the carbs in it. You can then use that rule of thumb for future guesstimates.
Use a Picture
Apps like Figwee and Carbs & Cals can also help with visual estimation. Each food entry includes a picture of different size portions of that food, with the corresponding carb content given. You compare the portion on your plate with the closest portion in the pictures, and use the listed carbs to inform your educated guess for your own portion.
(Photo credit: Figwee)
4. Carb factoring simplifies things.
A Carb Factor is a number which describes what percentage of a given food is carbohydrate. For example, the carb factor of Kellogg’s Cornflakes is 0.83, which means that 83% of its weight comes from carbohydrates; the carb factor for fresh strawberries is 0.06 which means that carbohydrates make up 6% of its weight; the carb factor for our family’s recipe for pineapple fried rice is 0.16 so 16% of its weight is carbs.
Why is this handy to know? A carb factor tells us the carb content of any serving size, whether it matches the serving size given on the package or not. With carb factors, our kids have the flexibility to eat any portion size of a given food while still accurately determining the carb content; they don't have to eat exactly 1/8 of that banana bread, or servings of cereal in 1/4-cup increments – they can dish out whatever portion fits their appetite and calculate carbs from there.
Carb Factors also saves us time and hassle: we can calculate the carb factor once for a given food, and then write that number on the package (of cereal, or crackers, or yogurt, or anything). Then we don't have to do the math every time our kids eat that food. We just weigh a portion of that food and multiply by the carb factor to determine the carb content. We can also keep a list of known carb factors for easy reference.
Also, carb factors allow us to “reverse engineer” meals if we need to figure out how much of a given food is needed to make a certain goal number of carbs. For example, if my son gives himself insulin for 50g of carbs for a bowl of chips, but then can't finish them all, I don't have to know if he ate half of the chips, or two-thirds. I simply weigh the chips that are left, multiply by the carb factor, and then know that he needs to have, say, 11g of juice, to avoid a low BG.
5. Dry or Cooked???
For foods that you buy dry and eat cooked (pasta, rice, rice noodles, quinoa, oats, barley, dry beans and legumes) we need to carefully consider how the carb content is cited on the package. Usually, the weight of the portion size on the package’s nutritional information panel refers to a dry (uncooked) weight. You cannot use this weight to determine a carb factor, as the dry weight will be different from the final cooked weight (which includes the water absorbed through boiling).
For example, on the bag of Only Oats that I have in my cupboard right now, it gives the serving size as 1/3 cup (40g), which contains 22g of carbs. This is a dry carb factor of 22 carbs out of 40 gram weight, or 0.55. When I add 2/3 cup of water and cook it, I end up with 2/3 cup of cooked oats that weighs 144 g. This serving still has 22g of carbs, but the carb factor for the cooked product is now 22 out of 144, or 0.15. If by mistake we used the dry carb factor to figure out the carbs in a cooked portion, we would give too much insulin (for 79g of carbs rather than 22!), which would result in dangerously low post-meal blood sugar.
As another example, let's take a look at popcorn. The nutrition panel on my box of Blue Menu microwave popcorn states that there are 24g of carbs in each 1/2 bag (40g) serving; it also says that this is "about 5 1/2 cups popped". The fact that they specify "popped" means that we can be confident that they are citing the carb content (24g of carbs) based on the volume of 5 1/2 cups after cooking, so we can use this fact to calculate the carbs in our popped popcorn.
But if we want to use weight rather than volume for popcorn, we need to use the weight of the final (cooked) product, as some moisture weight steams off during cooking. That being said, the weight before and after "cooking" popcorn doesn't differ much: the 80g bag of kernels yielded 70g of popped corn. As a comparison, the un-popped kernels have a carb factor by weight of 0.6; the carb factor for the cooked popcorn is 0.69. That means that for a 100g serving size (about 16 cups of popped corn), the difference would be 69 carbs (using the correct CF) rather than 60g of carbs (incorrectly using the pre-popped carb factor).
If the package is not clear on whether the carbs given are for a dry or cooked weight, and you're unsure, it's wise to double-check your results against another method of carb calculation before using your carb count to dose insulin. For example, compare the result to one of your “rule of thumb” guidelines: if your calculation from a pasta package results in 80g of carbs per cup, but you know from the rule of thumb that your RD gave you that a cup of cooked pasta contains about 30g of carbs, and then something is wrong; likely the portion size on the pasta box refers to the dry weight, not the cooked weight.
If in doubt, it’s wise to monitor your child’s blood sugar closely (for resulting highs or lows) for a few hours after eating, in case the carb count is off.
6. White Bread has a carb factor of ~0.5.
That means that about half of its weight comes from carbohydrates. The same is true for buns and bagels. This is handy to know when you have home-made white bread, or store-bought white buns with no carb information on them. It's also helps you to double-check when the carb info that you do have seems suspect: if you think to yourself, “Hmmm, that seems like a lot of carbs for this one little slice of bread,” you can verify it by using this 0.5 carb factor. Weigh the slice (in grams), multiply this weight by 0.5. If your suspicious slice of bread weighs 30g, that means it will contain approximately 15 grams of carbs. So if the bag says 45g (like a bread package once told me) then you know that something is wrong, and you can avoid what would have been a dangerously low blood sugar.
7. The carb factor for potatoes is ~0.2.
In other words, 20% of the weight of a given serving of potatoes (baked, boiled, or roasted) is carbs. For example, if you have 100g of mashed potatoes, you can estimate that there is about 20g of carbs in that serving.
The exceptions to this “rule of thumb” are french fries and hashbrowns which vary depending on the way they are prepared, whether they have any additional coatings, and how much fat/oil contributes to the weight. As an illustration:
- McDonald's Canada's (deep-fried) french fries have a carb factor of 0.38.
- McCain's Showstring Superfries (baked) have a carb factor of 0.33.
- McCain's Straight Cut Fries (baked) have a carb factor of 0.24
8. “Lite” often means “higher carb”.
In terms of salad dressing and other condiments, the fat contained in the regular, full-fat version isn't enough to raise blood sugar. When they replace the fat with something else, the “something else” may be high-fructose corn syrup or some other carb-rich ingredient, which increases the carb content of the “lite” version over the regular. Whether you choose one version or the other depends on your goals for your family’s nutrition – that is, is it more important to you that your family eats low fat salad dressing, or low carb salad dressing? There’s no universal “right” answer, but with information, parents can make informed choices.
9. Pure cocoa is carb-free.
So adding cocoa/cacao powder to homemade pancake batter doesn’t increase the carb content. And chocolate pancakes are yummy!
10. The Glycemic Index of foods affects post-meal blood sugar.
While the carb content of a food determines how much it will raise blood sugar, the Glycemic Index (GI) of a food focuses on how quickly it will raise blood sugar. This is largely because a food’s GI describes how quickly (or slowly) that food will be digested. A food with a low GI value will be digested more slowly than a food with a high GI value; thus a low GI food (such as oatmeal) will raise blood sugar slowly, while a high GI food (such as Cheerios) will raise blood sugar quickly (and probably raise it higher, too, as the food snowballs and the insulin cannot match that fast rate of digestion).
11. Pizza is tricky.
Pizza is a tricky food to carb count and to bolus insulin for. There are a few reasons for this.
But we have a solution for pizza!
The Pizza Solution:
12. Think critically.
There are times when the carb count you calculate may seem too high or too low – trust your gut and do some more research before giving your child insulin based on this result. Double-check the carb count using a different method of carb calculation (see Introduction to Counting Carbs and Carb Factoring). If still in doubt, it’s wise to monitor your child’s blood sugar closely (for resulting highs or lows) for a few hours after eating, in case the carb count is off.
13. Check the ACTUAL serving size.
The standard serving size in the nutritional panel may not reflect the actual contents of a given package. For example, in single serving packages (such as chips or snack crackers), the amount of product contained in the package can, within the law, vary by a small percentage. This percentage of error may be enough to affect blood sugar. Further, some companies concern themselves with making sure the package contains at least the stated amount of product, but if there is extra product in the bag they see that as a bonus for the consumer. This may mean that your child is not getting enough insulin for the amount of product she has eaten, resulting in post-snack highs. If you see out-of-target blood sugars often in relation to a certain packaged food, next time you may want to check the actual weight of the serving size (using a kitchen scale or nutritional scale) to make sure the resulting carb count for the portion is correct.
14. Which lasagna???
There is great variety in mixed recipes such as lasagna, cinnamon buns, or baked beans: some lasagna recipes are heavy on the noodles, some heavy on the ground beef; some baked bean recipes go for the sweet effect with lots of molasses or sugar, others go for the savoury effect, focusing on lower-carb tomato sauce. Some pizza slices have thick crusts with few toppings, others weigh the same but have thin crusts with lots of toppings. Some cinnamon buns have tons of icing. So when you use the built-in database in a nutritional scale to find the carb content of mixed dishes, the results may - or may not - be accurate.
When you access the “lasagna” function in your scale’s database (as opposed to using the scale to calculate the weight of the lasagna only, and then using a recipe-specific carb factor to calculate the carb content of a serving), the scale results are based on a reference “recipe” or composition, which may or may not match the carb content of the actual recipe used for the food your child is eating. So take the scale results with a grain of salt. Where a carb factor is not known and cannot be determined, the scale’s database for mixed dishes will at least get you into the ballpark in terms of accuracy, but is not likely to hit a home run.
Note that the results for individual foods, such as fruits and vegetables, do not present this problem. Unlike lasagna, a strawberry is a strawberry is a strawberry.
15. In-Store Baked Goods
Be wary of the stated carbs in products purchased at grocery store bakeries. The carb info listed on the package may not be as precise as you need it to be for dosing insulin. Carb content may be stated based on a certain portion size, but this may not be consistent with the portion size contained in a given package. For example, once when buying cheesy bread sticks, I noticed that some were about 8” long, while some were shorter, about 6” long (all were about the same width). However, the carb content on all packages was listed according to “1 cheese stick”. But which cheese stick? The 6-inch one or the 8-inch one? A package that gives the carb amount in terms of weight will be more reliable.
In addition, a bakery label may reflect an average carb content across several different variations on a product, like cookies for example. In this case, it’s impossible to determine if the chocolate chip cookies you hold in your hand are higher than, lower than or the same as the average carb content of all the cookies produced in that bakery.
In all cases, it is wise to double check your carb calculation using an alternate method and monitor your child’s blood sugar for a few hours afterward.
16. Mananaging the effects of food is not an exact science.
It's more of an art, honed over time, through lots of trial and error.
But you can get closer and closer with practice. When in doubt, start with as accurate of a carb count as you can get, then document what you did, as well as the resulting blood glucose. Over time you will learn how carbs operate in your child's individual body; you will see trends and you will learn what works well for your individual child (and what doesn’t) in terms of the amount and timing of insulin. With that, you're well on your way to taming the diabetes dragon!
The above information was reviewed for content accuracy by clinical staff of the Alberta Children’s Hospital Diabetes Clinic.
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