Net carbs represent the carbohydrates in a food that your body actually digests and converts into blood sugar. The basic formula: take total carbohydrates and subtract fiber and sugar alcohols. A food with 30 grams of total carbs, 8 grams of fiber, and 5 grams of sugar alcohols would have 17 grams of net carbs. The idea is simple, but the math isn’t always as clean as it looks.
Why Some Carbs “Don’t Count”
Not all carbohydrates behave the same way once you eat them. Sugars and starches break down into glucose, enter your bloodstream, and trigger an insulin response. Fiber and sugar alcohols take a different path, which is why they get subtracted in the net carb formula.
Fiber passes through your small intestine largely undigested. Soluble fiber (found in oats, beans, and fruit) forms a gel-like substance that thickens the contents of your gut. This slows digestion, delays the absorption of glucose, and blunts the blood sugar spike you’d otherwise get from a meal. Insoluble fiber (found in whole grains, nuts, and vegetables) adds bulk and moves through your system mostly intact. Neither type delivers glucose into your bloodstream the way starch or sugar does, so neither meaningfully raises blood sugar.
Sugar alcohols, the sweeteners you’ll see listed as erythritol, xylitol, sorbitol, or maltitol on ingredient lists, are only partially absorbed in the small intestine. Whatever isn’t absorbed travels to the large intestine, where gut bacteria ferment it. The result is that sugar alcohols deliver fewer calories and cause a smaller blood sugar response than regular sugar.
Not All Sugar Alcohols Are Equal
Here’s where the simple subtraction formula starts to break down. Different sugar alcohols are absorbed at wildly different rates, which means some raise blood sugar more than others. For reference, table sugar has a glycemic index of 69. Compare that to common sugar alcohols:
- Erythritol: glycemic index of 0, virtually no blood sugar impact
- Mannitol: glycemic index of 0
- Sorbitol: glycemic index of 9, with roughly 80% absorbed in the small intestine
- Xylitol: glycemic index of 13
- Maltitol: glycemic index of 35, about half that of table sugar
A protein bar sweetened with erythritol and one sweetened with maltitol could show the same “net carbs” on the package, but the maltitol version will raise your blood sugar noticeably more. The nutrition label doesn’t tell you which sugar alcohol a product uses, so you’ll need to check the ingredient list to know what you’re actually getting. If maltitol is the sweetener, many people in low-carb communities count only half of those sugar alcohol grams as “free,” rather than subtracting them entirely.
How to Calculate Net Carbs From a Label
On a U.S. Nutrition Facts panel, you’ll find Total Carbohydrate as a main line, with Dietary Fiber and Total Sugars indented underneath. Sugar alcohols may also appear as a sub-line, though manufacturers aren’t always required to list them unless they make a related claim. The calculation is straightforward:
Net Carbs = Total Carbohydrates − Dietary Fiber − Sugar Alcohols
Some newer products also contain allulose, a rare sugar that shows up under total carbohydrates on the label but isn’t metabolized by your body. It causes no measurable increase in blood glucose or insulin. If a product contains allulose, you subtract those grams too. You’ll typically see this called out on the packaging since manufacturers want you to know about the lower effective carb count.
One important note for international readers: labels in many countries outside the U.S. already exclude fiber from the carbohydrate total. If you’re reading a European food label, the “carbohydrates” number may already reflect something close to net carbs, and subtracting fiber again would give you an inaccurately low number.
Why “Net Carbs” Isn’t an Official Term
The FDA has never defined “net carbs,” and the term has no legal meaning on food packaging. The American Diabetes Association doesn’t use or endorse it either. Both organizations recommend paying attention to total carbohydrates instead.
The core problem is that the formula assumes fiber and sugar alcohols contribute zero usable carbohydrate, which isn’t strictly true. Some fibers are partially fermented into short-chain fatty acids that your body absorbs for energy. Some sugar alcohols, as noted above, are significantly absorbed. The nutrition label doesn’t break down which types of fiber or which sugar alcohols a product contains, so the net carb number is always an approximation.
This matters most for people managing diabetes. If you’re adjusting insulin doses based on a net carb count that underestimates the actual blood sugar impact, you could end up with higher glucose levels than expected. For this reason, many diabetes educators suggest using total carbs as the more reliable number for dosing decisions.
When Tracking Net Carbs Is Useful
Net carbs became popular through ketogenic and other low-carb diets. The reasoning is practical: if you’re trying to keep carb intake low enough to stay in ketosis (typically under 20 to 50 grams per day), counting the fiber in a cup of broccoli against your limit would be unnecessarily restrictive. That fiber isn’t turning into glucose, so it shouldn’t eat into your carb budget.
For most people following a low-carb diet, tracking net carbs allows a wider variety of vegetables, nuts, seeds, and other high-fiber whole foods without blowing past their daily target. This tends to produce a more nutritious and sustainable eating pattern than one that avoids all carbohydrate-containing foods equally.
The approach works less well when applied to heavily processed “keto-friendly” products that advertise very low net carbs through large amounts of sugar alcohols or specialty fibers. A candy bar claiming 3 grams of net carbs but containing 25 grams of maltitol is not truly a 3-net-carb food for your body. The more a product relies on sugar alcohols to bring its net carb number down, the more skepticism that number deserves.
A Practical Approach
For whole foods like vegetables, fruits, legumes, and nuts, the net carb calculation is reliable. Fiber in these foods genuinely does not raise blood sugar, and subtracting it gives you a useful picture of how that food will affect your body. A medium avocado with 12 grams of total carbs and 9 grams of fiber really does deliver roughly 3 grams of digestible carbohydrate.
For packaged foods, treat the net carb number on the front of the package as a marketing figure rather than a lab result. Flip to the Nutrition Facts panel, check the total carbohydrates, note the fiber and sugar alcohols separately, and look at the ingredient list to see which sugar alcohol is used. If it’s erythritol, subtracting fully is reasonable. If it’s maltitol or sorbitol, expect a bigger blood sugar response than the “net carbs” number suggests. If you’re managing blood sugar carefully, testing with a glucose monitor after eating a new product will tell you more than any label math.