A net carbohydrate is the amount of carbohydrate in a food that your body actually digests and converts to blood sugar. You calculate it by taking the total carbohydrates on a nutrition label and subtracting fiber and sugar alcohols. If a protein bar lists 25 grams of total carbohydrates, 10 grams of fiber, and 5 grams of sugar alcohols, the net carbs would be 10 grams.
Why Some Carbs Don’t Count
Not all carbohydrates behave the same way once you eat them. Simple sugars and starches break down into glucose, enter your bloodstream, and trigger an insulin response. Fiber and sugar alcohols, on the other hand, pass through your digestive system largely intact or are only partially absorbed. The net carb concept tries to capture this difference by counting only the carbohydrates that meaningfully raise your blood sugar.
Fiber is the clearest case. Your body doesn’t break it down, so it doesn’t cause a blood sugar spike the way other carbohydrates do. Soluble fiber actually dissolves in water and forms a gel in your stomach, slowing digestion and helping stabilize blood sugar. Insoluble fiber passes through mostly unchanged. Either way, fiber contributes little to no usable glucose, which is why it gets subtracted from the total.
Sugar alcohols (like sorbitol, xylitol, and erythritol) fall somewhere in between. They contain 0 to 2 calories per gram, compared to 4 calories per gram for regular sugar, and they don’t cause sudden blood sugar spikes. But the standard net carb formula subtracts them entirely, which can be misleading. Some sugar alcohols are partially digested and do raise blood sugar slightly. Maltitol, for example, has a noticeably higher glycemic effect than erythritol.
The Formula
The basic calculation is straightforward:
Net Carbs = Total Carbohydrates − Fiber − Sugar Alcohols
You’ll find total carbohydrates, fiber, and sugar alcohols listed on U.S. nutrition labels, though sugar alcohols only appear when a product contains them. For whole foods without a label (a sweet potato, an avocado), you can look up the values in any nutrition database and do the same subtraction.
Net Carbs Are Not an Official Term
Despite appearing on plenty of food packaging, “net carbs” has no legal definition. The FDA doesn’t recognize it, and neither does the American Diabetes Association. Federal labeling regulations define “total carbohydrate” and require listing fiber and sugars underneath it, but they say nothing about net carbs. That means when a food company prints “only 4g net carbs” on the front of a package, there’s no regulatory body verifying that number or standardizing how it was calculated.
This lack of oversight matters because companies can be selective about what they subtract. Some subtract all sugar alcohols equally, even though certain types still raise blood sugar. Others subtract newer ingredients like allulose, a rare sugar that the body barely metabolizes. About 70% of consumed allulose is eliminated intact through urine and feces within 48 hours, and it provides no more than 0.4 calories per gram. The FDA still requires allulose to be counted under total carbohydrates on the label, but many brands subtract it when calculating net carbs on the front of the package.
US Labels vs. European Labels
If you’ve ever looked at a nutrition label from the UK or EU, you may have noticed that the carbohydrate number already looks lower than on a comparable American product. That’s because European labels list carbohydrates with fiber already excluded. Fiber often appears as a separate, voluntary line item. In the US, fiber is included within total carbohydrates, which is why Americans need to subtract it manually to get net carbs. A European label essentially gives you something close to net carbs by default.
Why Net Carbs Matter for Low-Carb Diets
The concept became popular alongside ketogenic and other low-carb diets. A standard ketogenic diet limits total carbohydrate intake to less than 50 grams per day, and sometimes as low as 20 grams. When your daily budget is that tight, it makes a real difference whether a cup of broccoli “costs” you 6 grams (total carbs) or 3.5 grams (net carbs after subtracting fiber). Counting net carbs lets people on these diets eat more vegetables, nuts, and seeds without worrying about exceeding their limit, since the fiber in those foods won’t knock them out of ketosis.
For people managing diabetes, the picture is more nuanced. While fiber genuinely doesn’t spike blood sugar, the assumption that all sugar alcohols have zero impact isn’t always accurate. Some sugar alcohols are partially digested and still provide calories and a modest blood sugar rise. If you’re using net carbs to dose insulin or manage blood glucose, treating all subtracted ingredients as though they’re invisible can lead to unexpected readings.
Resistant Starch Complicates Things Further
Some starches behave more like fiber than like typical carbohydrates. Resistant starch, found in foods like cooled potatoes, green bananas, and certain whole grains, travels mostly intact to the colon instead of being digested in the small intestine. Once there, gut bacteria ferment it into short-chain fatty acids that nourish the gut lining and may help regulate blood sugar. The glucose that does get absorbed tends to enter the bloodstream more gradually, producing a smaller spike.
Resistant starch doesn’t appear as a separate line on nutrition labels, so there’s no way to subtract it from a net carb calculation using standard packaging. It’s a reminder that the net carb formula is a rough estimate, not a precise measurement of how a food will affect your blood sugar.
How Reliable Is the Number?
Net carbs work well as a general guide for choosing higher-fiber, lower-sugar foods. The core logic is sound: fiber doesn’t raise blood sugar, and most sugar alcohols raise it far less than regular sugar does. But the number on a package isn’t as precise as it looks. Different sugar alcohols have different metabolic effects, resistant starch isn’t accounted for, and the FDA doesn’t regulate how companies arrive at their net carb claims.
The most reliable approach is to use net carbs as a rough filter, not a precise instrument. Pay attention to which ingredients are being subtracted, and be slightly skeptical of products with very low net carb claims built on large amounts of sugar alcohols or novel sweeteners. A food with 30 grams of total carbs and 26 grams of sugar alcohols listed as “4g net carbs” deserves a closer look than one that gets its low net carb count primarily from fiber.