Net carbs represent the carbohydrates in a food that your body actually absorbs and converts to blood sugar. You calculate them by taking the total carbohydrates on a nutrition label and subtracting fiber and sugar alcohols. A protein bar with 25 grams of total carbs, 10 grams of fiber, and 5 grams of sugar alcohols would have 10 grams of net carbs. The idea is simple, but the science behind it has some important nuances.
Why Fiber and Sugar Alcohols Get Subtracted
The logic behind net carbs comes down to what happens in your digestive system. Not all carbohydrates behave the same way once you eat them. Regular starches and sugars get broken down, absorbed into your bloodstream, and raise your blood sugar. Fiber and sugar alcohols don’t follow that same path, or at least not completely.
Your body can’t digest fiber. It passes through your stomach and intestines largely intact, which means it doesn’t cause a spike in blood sugar the way other carbohydrates do. Soluble fiber dissolves in water and forms a gel-like substance in your stomach, slowing down digestion and helping control blood sugar. Insoluble fiber doesn’t dissolve at all and moves through your system whole. Neither type delivers glucose to your bloodstream, so subtracting fiber from your carb count makes physiological sense.
Sugar alcohols are a different story. These are sweeteners like erythritol, xylitol, and maltitol that food manufacturers use in “sugar-free” or “low-carb” products. They contain between 0 and 2 calories per gram, compared to 4 calories per gram for regular sugar. They’re considered low glycemic index foods and cause only a slight rise in blood sugar levels. But here’s the catch: “slight” isn’t zero, and different sugar alcohols are absorbed at different rates. Some raise blood sugar more than others, which makes the simple act of subtracting all of them from total carbs a bit imprecise.
The Basic Formula
For whole foods with no sugar alcohols, the math is straightforward:
Net carbs = Total carbohydrates − Fiber
For packaged foods that contain sugar alcohols:
Net carbs = Total carbohydrates − Fiber − Sugar alcohols
A practical example from UCSF’s diabetes education program: if a food label lists 10 grams of total carbohydrate and 5 grams of dietary fiber, you’d count it as 5 grams of net carbs. That’s the number that matters for blood sugar impact.
Why the Formula Isn’t Exact
The American Diabetes Association puts it bluntly: the equation used to calculate net carbs is not entirely accurate because the contribution of fiber and sugar alcohols to total carbohydrates depends on the specific types present. A product sweetened with erythritol (which your body absorbs very little of) behaves differently than one sweetened with maltitol (which raises blood sugar noticeably more). Yet the net carbs label treats them identically, subtracting both completely.
This imprecision matters most for people managing diabetes or carefully tracking carbs for ketosis. Two products with the same “net carbs” number on the label can produce meaningfully different blood sugar responses depending on which sugar alcohols or fiber sources they contain. UCLA Health has noted that net carbs “isn’t an exact formula” and recommends that carb-conscious people fill their plates with whole foods naturally high in fiber and low in sugar rather than relying heavily on the number.
Net Carbs and Keto Diets
Most people searching for net carbs are following or considering a ketogenic diet. The threshold to achieve and maintain ketosis is roughly 50 or fewer grams of carbohydrates per day, though everyone’s metabolism behaves differently and some people need to restrict further. Tracking net carbs instead of total carbs gives you more room in that daily budget, because the fiber in vegetables, nuts, and seeds doesn’t count against your limit.
This is where the concept gets genuinely useful. Broccoli, avocados, and almonds are all relatively high in total carbs but much of that is fiber. Counting net carbs means you can eat these nutrient-dense foods without worrying that you’ve blown your carb budget. Without net carb counting, a strict keto diet would be far more restrictive and harder to sustain.
The risk, though, is on the processed food side. Counting net carbs can expand your food choices, but it can also become an excuse to add sweets and snacks to the diet. A “4 net carb” candy bar might technically fit your macro targets, but the sugar alcohols in it may still produce a blood sugar response, and the whole approach can pull you away from the whole-food eating pattern that makes low-carb diets effective in the first place.
How Labels Work Differently Around the World
If you’ve ever looked at a nutrition label from Europe and thought the carb numbers seemed low, you weren’t imagining it. In the United States and Canada, total carbohydrate on the label includes fiber. Labs calculate it by subtracting protein, fat, water, and ash from the total weight of a food, and whatever is left over counts as carbohydrate, fiber included. That’s why you need to subtract fiber yourself to get net carbs.
In the European Union and Mexico, labels show “available carbohydrate,” which already excludes fiber. The number you see on a European label is essentially the net carb value (minus any sugar alcohol adjustment). So if you’re buying imported foods or using international nutrition databases, keep in mind that the carb numbers may not be directly comparable to American labels.
The FDA Doesn’t Recognize “Net Carbs”
One important detail: “net carbs” is not an FDA-regulated term. The FDA has no official definition for it, no required calculation method, and no rules about how companies can use it on packaging. When you see “only 3g net carbs!” on a protein bar wrapper, that’s a marketing claim, not a standardized nutritional measurement. Different manufacturers may calculate it differently, and there’s no regulatory body checking their math.
This doesn’t mean net carbs are useless. The underlying biology is sound: fiber genuinely doesn’t raise blood sugar, and sugar alcohols raise it less than regular sugar. But the lack of standardization means you should treat bold net carb claims on packaged foods with some skepticism, especially when a product is loaded with sugar alcohols you’ve never heard of.
Newer Sweeteners Like Allulose
Allulose is a newer sweetener showing up in low-carb products that adds another layer of complexity. It’s a rare sugar with about 70% of the sweetness of table sugar but only 0.2 calories per gram, which is a 95% calorie reduction. Your body absorbs it through the small intestine but doesn’t convert it into energy. Studies in both rats and humans have found no increase in carbohydrate energy expenditure from allulose, even at high concentrations.
The FDA agreed in 2019 that allulose doesn’t need to be counted as a sugar or added sugar on nutrition labels, though it still shows up in total carbohydrates on U.S. labels. Many people subtract it when calculating net carbs, and the metabolic evidence supports doing so. But again, there’s no standardized rule, so whether a given product subtracts allulose from its “net carb” count varies by brand.
Digestive Side Effects to Watch For
Products engineered to be low in net carbs often achieve that by packing in fiber supplements and sugar alcohols. Your digestive system has limits for both. Sugar alcohols in particular can cause bloating, gas, cramping, and diarrhea when consumed in larger amounts, because they aren’t fully absorbed and end up fermenting in your large intestine. The threshold varies by person and by the specific sugar alcohol, but if a “low net carb” protein bar or ice cream leaves you uncomfortable, the sugar alcohols are the most likely culprit.
High doses of added fiber (like chicory root or inulin, common in low-carb products) can cause similar issues. If you’re new to high-fiber or sugar-alcohol-heavy foods, increasing your intake gradually gives your gut time to adjust.