Net carbs represent the portion of carbohydrates in a food that your body actually breaks down into glucose and uses for energy. The basic formula is simple: take the total carbohydrates listed on a nutrition label, then subtract the grams of fiber and sugar alcohols. What remains is the net carb count, the carbs that will meaningfully affect your blood sugar.
The concept exists because not all carbohydrates behave the same way once you eat them. Some pass through your digestive system largely intact, contributing little to no glucose to your bloodstream. Subtracting those gives you a more accurate picture of a food’s real metabolic impact.
The Formula and Why It Works
The calculation itself takes seconds. If a food has 25 grams of total carbohydrates, 8 grams of fiber, and 3 grams of sugar alcohols, the net carbs are 14 grams (25 minus 8 minus 3). That 14-gram figure reflects what your body will actually convert to blood sugar.
Fiber earns its subtraction because it isn’t broken down into glucose like other carbohydrates. Instead, it passes through your digestive tract largely undigested. Your small intestine simply can’t break the chemical bonds that hold fiber together, so it moves on to the large intestine where bacteria may partially ferment it. Even then, fiber contributes roughly 0 to 2 calories per gram, compared to 4 calories per gram for regular carbohydrates. Traditional nutrition labeling counts fiber under total carbohydrates, which overstates the actual usable energy in high-fiber foods.
Sugar alcohols (the ingredients ending in “-ol” like erythritol, xylitol, and sorbitol) get subtracted for a similar reason. Your body absorbs them poorly or incompletely, so they don’t trigger the same blood sugar response as regular sugar. They’re common in protein bars, sugar-free candy, and low-carb baked goods.
Not All Sugar Alcohols Are Equal
Here’s where net carb math gets less straightforward. Sugar alcohols vary widely in how much they actually affect your blood sugar. Using glucose as the baseline (glycemic index of 100), xylitol scores about 8 and lactitol around 2. Those are genuinely minimal. But maltitol, one of the most common sugar alcohols in processed “low-carb” foods, can score anywhere from 30 to as high as 89 depending on the specific form used. That’s comparable to some regular carbohydrates.
If you subtract all maltitol grams from total carbs as though they have zero impact, you’ll undercount the carbs your body actually processes. Many people who track net carbs closely, especially for blood sugar management, subtract only half the grams of maltitol and sorbitol rather than the full amount. Erythritol, by contrast, has essentially no glycemic impact and is safe to subtract entirely.
What About Allulose?
Allulose is a newer sweetener showing up in low-carb products that doesn’t fit neatly into either the sugar or sugar alcohol category. It’s technically a rare sugar, but your body barely metabolizes it. The FDA has recognized this by allowing manufacturers to exclude allulose from both “Total Sugars” and “Added Sugars” on nutrition labels, assigning it just 0.4 calories per gram instead of the standard 4. If you see allulose listed in the ingredients, it’s reasonable to subtract it from your net carb count, though it may still appear under total carbohydrates on some labels depending on how the manufacturer reports it.
Why Net Carbs Matter for Blood Sugar
When you eat carbohydrates that your body can fully digest, they break down into glucose and enter your bloodstream. Simple carbohydrates like white bread and pasta break down quickly, causing sharp spikes in blood sugar followed by steep drops. Complex carbohydrates with intact fiber break down more slowly, producing a gentler rise.
Fiber does more than just “not count” as a carb. It actively slows the absorption of the digestible carbohydrates you eat alongside it. When you pair carbs with fiber, protein, or healthy fats, your blood sugar rises less dramatically than if you ate those same carbs alone. This is why a whole apple (with its fiber) and a glass of apple juice (fiber removed) have very different effects on your body despite similar total carb counts. Net carbs capture this distinction in a single number.
For people with diabetes or insulin resistance, tracking net carbs can give a more useful estimate of how a meal will affect glucose levels than tracking total carbs alone. That said, individual responses vary. Two people eating the same 20 net-gram meal may see different blood sugar readings.
Net Carbs and Ketogenic Diets
The net carb concept is central to ketogenic and other low-carb diets. A standard ketogenic diet limits total carbohydrate intake to less than 50 grams per day, often as low as 20 grams. According to Harvard’s School of Public Health, the typical keto breakdown is 70 to 80 percent of calories from fat, 5 to 10 percent from carbohydrates, and 10 to 20 percent from protein. On a 2,000-calorie diet, that translates to roughly 40 grams of carbohydrate.
Most people following keto track net carbs rather than total carbs, which means high-fiber vegetables like broccoli, spinach, and cauliflower remain on the menu. A cup of broccoli might have 6 grams of total carbs but only about 3.5 net carbs after subtracting fiber. That distinction makes a big difference when your daily budget is 20 to 50 grams.
Where the Label Falls Short
“Net carbs” is not a term regulated by the FDA. You won’t find it on standard Nutrition Facts panels. When food companies print net carb counts on their packaging, they’re using their own calculations, and those calculations aren’t always conservative. Some products subtract every gram of fiber and every gram of sugar alcohols regardless of type, which can paint an overly optimistic picture.
Processed “low-carb” foods deserve extra scrutiny. Ingredients like polydextrose, a synthetic fiber used as a bulking agent, are classified as soluble fiber in many countries and get subtracted from total carbs. Polydextrose is about 90 percent soluble fiber with only 1 calorie per gram, and roughly 60 percent of it passes through you undigested. That’s a legitimate subtraction. But some functional fibers added to processed foods may not behave identically to the fiber naturally present in whole vegetables and legumes. The safest approach is to focus your net carb counting on whole foods where the fiber content is straightforward, and to be more skeptical of packaged products advertising very low net carb counts.
How to Calculate Net Carbs Yourself
For whole foods, the math is clean. Look at the Nutrition Facts label (or use a food database), find total carbohydrates, and subtract dietary fiber. A medium avocado with 17 grams of total carbs and 13 grams of fiber has 4 net carbs. A cup of black beans with 41 grams of total carbs and 15 grams of fiber has 26 net carbs.
For packaged foods with sugar alcohols, start with total carbohydrates, subtract all the fiber, then handle sugar alcohols based on type. Subtract erythritol fully. Subtract half the grams of maltitol, sorbitol, and isomalt. Subtract xylitol fully or nearly so, given its glycemic index of about 8. If the label just says “sugar alcohols” without specifying which kind, subtracting half is a reasonable middle ground.
For foods containing allulose, subtract the full amount. If you’re unsure whether a product’s allulose is already excluded from the total carb count on the label, check the ingredient list and compare the math. Some manufacturers have already adjusted the total carbohydrate number to exclude it, while others list it within total carbs and note the net carbs separately on the front of the package.