Net carbs are the carbohydrates in food that your body actually digests and converts to blood sugar. You calculate them by taking the total carbohydrates on a nutrition label and subtracting fiber and sugar alcohols. If a protein bar has 25 grams of total carbs, 10 grams of fiber, and 5 grams of sugar alcohols, the net carbs are 10 grams.
Why Fiber and Sugar Alcohols Get Subtracted
Your body handles different carbohydrates in very different ways. Regular carbs like starch and sugar break down into glucose, which enters your bloodstream and triggers an insulin response. Fiber, on the other hand, passes through your digestive system without being broken down or absorbed. As the CDC puts it, fiber doesn’t cause a spike in blood sugar the way other carbohydrates can. It moves slowly through the stomach, which is why high-fiber foods keep you feeling full longer.
Sugar alcohols (the ingredients ending in “-ol” on labels, like erythritol, xylitol, and sorbitol) are partially absorbed but have a much smaller effect on blood sugar than regular sugar. Erythritol has a glycemic index of zero, meaning it has essentially no impact on blood glucose at all. Xylitol and sorbitol have glycemic indexes of 13 and 9, respectively, compared to table sugar’s glycemic index of 65. Because these sweeteners contribute little to no blood sugar response, net carb formulas subtract them from the total.
The Basic Formula
For whole foods like vegetables, fruits, and legumes, the calculation is straightforward:
Net carbs = Total carbs − Fiber
For packaged foods that contain sugar alcohols, the formula expands:
Net carbs = Total carbs − Fiber − Sugar alcohols
A medium avocado with 17 grams of total carbs and 13 grams of fiber has just 4 net carbs. A cup of broccoli with about 6 grams of total carbs and 2.4 grams of fiber comes to roughly 3.6 net carbs. This is why low-carb diets encourage vegetables that seem carb-heavy on the label but are actually mostly fiber.
Not All Sugar Alcohols Are Equal
Here’s where net carb counting gets tricky. The standard formula treats all sugar alcohols the same, but they aren’t. Erythritol has zero calories and zero glycemic impact, so subtracting it entirely makes sense. Maltitol, however, has a glycemic index of 35, which is roughly half that of table sugar. It still raises blood glucose, just less dramatically. Subtracting all of the maltitol grams from total carbs overstates how “free” those carbs really are.
Some people who track net carbs carefully will subtract only half the grams of maltitol, xylitol, and sorbitol to account for their partial absorption. They subtract erythritol fully. This more conservative approach gives a more accurate picture of blood sugar impact, especially if you’re managing diabetes or trying to stay in ketosis.
If you pick up a “low carb” candy bar and the label shows 2 net carbs but the ingredient list is heavy on maltitol, the real blood sugar effect is likely higher than 2 grams of regular carbs would suggest.
Allulose: A Newer Wrinkle
Allulose is a rare sugar that’s showing up in more products. It tastes about 70% as sweet as table sugar but has almost zero calories. Unlike sugar alcohols, allulose is technically a sugar, and it used to be counted under “total sugars” on nutrition labels. The FDA changed this in 2019, allowing manufacturers to exclude allulose from both total and added sugar counts. Your body absorbs allulose but doesn’t metabolize it for energy, so it passes through without meaningfully affecting blood glucose. When you see allulose on an ingredient list, it typically doesn’t need to be counted toward net carbs.
The FDA Doesn’t Regulate “Net Carbs”
This is the single most important thing to understand about net carbs: the term has no legal or regulatory definition. The FDA’s nutrition labeling rules (21 CFR 101.9) define “total carbohydrate” and its components, including dietary fiber, total sugars, added sugars, and sugar alcohol. The phrase “net carbs” does not appear anywhere in those regulations.
This means food manufacturers can calculate and display net carbs however they choose. Some subtract all fiber and all sugar alcohols. Others use more nuanced formulas. There is no standard they’re required to follow, and no agency is checking their math. When a product front-label says “4g net carbs,” that number might be perfectly accurate, or it might be optimistic depending on which sugar alcohols are used and how the company chose to count them.
The total carbohydrate, fiber, and sugar alcohol lines on the Nutrition Facts panel are regulated. You can always do the subtraction yourself using those numbers rather than trusting a marketing claim on the front of the package.
Who Benefits From Counting Net Carbs
Net carb counting is most useful for people following ketogenic or other low-carb diets, where the goal is keeping daily carb intake low enough to change how the body fuels itself. Standard keto guidelines typically aim for 20 to 50 net carbs per day. Counting net carbs rather than total carbs lets you eat generous amounts of non-starchy vegetables and high-fiber foods without worrying about exceeding your limit.
People with diabetes also find net carbs helpful for estimating how much a food will affect their blood sugar, though anyone using insulin to manage blood glucose should work with their care team on carb-counting methods, since the stakes of miscounting are higher.
If you’re not following a specific low-carb protocol, net carbs are less important as a daily metric. Total fiber intake, overall diet quality, and the types of carbs you eat (whole grains versus refined sugars, for instance) matter more for general health than a single net carb number.
Practical Tips for Accuracy
- Check the sugar alcohol type. If a product uses erythritol, subtract it fully. For maltitol, sorbitol, or xylitol, subtracting only half gives you a more realistic estimate.
- Use the Nutrition Facts panel, not the front label. Do the math yourself from the regulated numbers rather than trusting a marketing claim.
- Watch for “fiber” additives. Some processed foods add isolated fibers like inulin or soluble corn fiber to lower the net carb count on the label. These fibers are real, but products engineered to look low-carb through added fiber aren’t the same as naturally high-fiber whole foods.
- Remember that net carbs aren’t the whole story. A food with 3 net carbs can still be highly processed and nutritionally empty. Net carb count tells you about blood sugar impact, not overall nutritional value.