A net carb is the portion of carbohydrate in a food that your body actually digests and converts into blood sugar. The basic formula is simple: take the total carbohydrates listed on a nutrition label, then subtract the fiber and all or part of the sugar alcohols. What remains is the “net” amount your body processes for energy.
The concept exists because not all carbohydrates behave the same way once you eat them. Some pass through your digestive system without being absorbed, so they don’t raise blood sugar or trigger an insulin response the way starches and sugars do. Net carbs attempt to capture only the carbs that matter metabolically.
How to Calculate Net Carbs
The calculation depends on what’s in the food. For whole foods that contain fiber but no sugar alcohols (vegetables, fruits, beans, whole grains), the math is straightforward:
- Net carbs = Total carbohydrates − Fiber
For processed foods that contain sugar alcohols (common in protein bars, sugar-free candy, and keto snacks), it gets slightly more involved. The UCSF Diabetes Teaching Center recommends subtracting only half the sugar alcohols from total carbs, because your body still partially digests them. So the formula becomes:
- Net carbs = Total carbohydrates − Fiber − (Sugar alcohols ÷ 2)
For example, a product with 29 grams of total carbohydrates and 18 grams of sugar alcohol would count as 20 grams of net carbs. You divide the 18 grams of sugar alcohol in half (9 grams), then subtract that from the total.
One exception worth knowing about: allulose. This rare sugar shows up in a growing number of low-carb products. The FDA has taken the position that allulose can be excluded from both “Total Sugars” and “Added Sugars” on nutrition labels because it contributes minimal calories (roughly 0.4 per gram instead of the usual 4) and barely affects blood sugar. Most people subtract allulose completely when calculating net carbs.
Why Fiber Gets Subtracted
Fiber is technically a carbohydrate, but your body lacks the enzymes to break it down. It passes through your digestive tract largely intact, which means it doesn’t cause a spike in blood sugar the way other carbohydrates can.
The two types of fiber work differently, though both earn their exclusion from net carbs. Soluble fiber dissolves in water and forms a gel-like substance in your stomach, slowing digestion. This helps blunt blood sugar and cholesterol responses after a meal. Insoluble fiber doesn’t dissolve. It adds bulk and helps increase insulin sensitivity. Neither type delivers the glucose hit that starches and sugars do, which is why subtracting fiber from the total makes physiological sense.
Why Sugar Alcohols Are Only Half-Subtracted
Sugar alcohols (names like erythritol, sorbitol, maltitol, and xylitol on ingredient lists) are hard for the body to digest, so their effect on blood sugar is less than regular sugar. But “less than regular sugar” isn’t the same as zero. Different sugar alcohols are absorbed at different rates. Erythritol has almost no blood sugar impact, while maltitol raises blood sugar nearly as much as table sugar. The “divide by two” rule is a rough average that works reasonably well across the category.
There’s a practical concern here too. Sugar alcohols in large amounts cause digestive distress: bloating, gas, and a laxative effect. Research on tolerance thresholds found that sorbitol triggers laxative effects at relatively low doses, around 0.17 to 0.24 grams per kilogram of body weight depending on sex. For a 150-pound person, that’s roughly 12 to 16 grams. Erythritol is better tolerated, with thresholds around 0.66 to 0.80 grams per kilogram, meaning you can handle significantly more before problems start. If a “low net carb” product relies heavily on sugar alcohols, the math on the label might look great while your stomach disagrees.
Net Carbs Are Not an Official Measurement
This is the most important thing to understand: “net carbs” has no legal definition. The FDA does not use the term, and the American Diabetes Association does not recognize it. The FDA recommends using total carbohydrates on the nutrition facts label for tracking purposes.
The ADA’s concern is practical. The net carb formula assumes that fiber and sugar alcohols are not absorbed or metabolized, but this isn’t always true. Some fibers are partially digested, some sugar alcohols still affect blood sugar meaningfully, and the degree varies from product to product. For someone dosing insulin based on carbohydrate intake, relying on net carbs instead of total carbs could lead to underdosing and dangerous blood sugar swings.
Food manufacturers, meanwhile, have an incentive to make the net carb number as low as possible. There’s no regulatory body checking their math or their assumptions. A product labeled “4g net carbs” might use a generous subtraction method that doesn’t reflect how your body actually handles those ingredients.
How Net Carbs Are Used in Keto and Low-Carb Diets
The net carb concept is most popular among people following ketogenic or other low-carb diets. The ketogenic diet typically reduces total carbohydrate intake to less than 50 grams a day, and sometimes as low as 20 grams. Counting net carbs instead of total carbs allows people on these diets to eat more vegetables, nuts, and high-fiber foods without feeling like they’ve used up their daily carb budget on nutrition that barely affects blood sugar.
This is where net carbs genuinely help. A cup of broccoli has about 6 grams of total carbs but roughly 2.4 grams of fiber, making it around 3.6 net carbs. Without the net carb framework, a strict keto dieter might avoid vegetables that are clearly beneficial. The fiber in those foods isn’t going to knock anyone out of ketosis.
The approach gets shakier with packaged “keto-friendly” products. When a protein bar advertises 3 net carbs but contains 25 grams of total carbohydrates from a mix of fiber additives and sugar alcohols, the real metabolic impact is harder to predict. Some people find that these products stall their weight loss or raise their blood sugar more than expected. If you’re tracking net carbs for a specific health goal, whole foods with naturally occurring fiber give you the most reliable numbers. Processed products with engineered carb profiles are where the math starts to break down.