Net carbs are the carbohydrates in a food that your body actually digests and converts to blood sugar. The basic formula is simple: total carbohydrates minus fiber minus sugar alcohols equals net carbs. A protein bar with 24 grams of total carbs, 10 grams of fiber, and 8 grams of sugar alcohols would have 6 grams of net carbs.
Why Some Carbs Don’t Count
Not all carbohydrates behave the same way once you eat them. Your body breaks down starches and sugars into glucose, which enters the bloodstream and raises blood sugar. Fiber and sugar alcohols take a different path.
Fiber is a carbohydrate that your body can’t digest. It passes through your digestive system largely intact, so it doesn’t cause the blood sugar spike that other carbs do. High-fiber foods slow the overall rate of digestion, producing a more gradual and smaller rise in blood sugar compared to low-fiber foods with the same total carb count. That’s why fiber gets subtracted from the total.
Sugar alcohols (ingredients like erythritol, xylitol, and maltitol commonly found in “sugar-free” products) are a similar story. Your body can’t fully absorb or digest them. Regular sugar provides about 4 calories per gram, while sugar alcohols provide between 0 and 2 calories per gram depending on the type. Because they have a lower glycemic index and a much smaller effect on blood sugar, they also get subtracted.
Not All Sugar Alcohols Are Equal
This is where net carb math gets less precise. Different sugar alcohols affect blood sugar to different degrees. Erythritol has essentially zero glycemic impact and zero calories per gram. Maltitol, on the other hand, still raises blood sugar noticeably, though less than regular sugar. Subtracting 100% of every sugar alcohol from the carb count treats them all as if they have no effect, which isn’t accurate.
Some people following low-carb diets use a more conservative approach: they subtract all the erythritol but only half the maltitol or other higher-impact sugar alcohols. There’s no single official method, which is part of the problem with the whole concept of net carbs.
“Net Carbs” Has No Legal Definition
The FDA does not define, regulate, or recognize “net carbs” as a labeling term. Federal nutrition labeling rules specify how to declare total carbohydrates, dietary fiber, total sugars, added sugars, and sugar alcohols, but the phrase “net carbs” appears nowhere in those regulations. When a food package says “only 4g net carbs” on the front, that number is the manufacturer’s own calculation using whatever formula they chose.
This matters because companies have an incentive to make that number as low as possible. A product could subtract every gram of maltitol even though maltitol still has a meaningful blood sugar effect. Without a standard definition, two products with identical ingredients could claim different net carb counts.
Newer Sweeteners Add Complexity
Allulose is a relatively new sweetener showing up in low-carb products. It’s technically a sugar (a rare sugar found naturally in small amounts in figs and raisins), but your body metabolizes almost none of it. The FDA has issued guidance allowing manufacturers to exclude allulose from “Total Sugars” and “Added Sugars” on the Nutrition Facts label and to count it at just 0.4 calories per gram instead of the standard 4 calories per gram for sugar.
However, allulose still appears under “Total Carbohydrate” on most labels. This means a product sweetened with allulose might show a surprisingly high total carb number on the Nutrition Facts panel while advertising a low net carb number on the front of the package. If you’re counting net carbs, you’d subtract allulose along with fiber and sugar alcohols, but you need to read the ingredient list to know it’s there.
How Net Carbs Are Used in Practice
Net carbs are most commonly tracked by people following ketogenic or other low-carb diets. A standard ketogenic diet typically limits total carbohydrate intake to less than 50 grams per day, and often as low as 20 grams. Whether those thresholds refer to total or net carbs varies by source, but most keto practitioners count net carbs, which gives them more room for vegetables and other high-fiber foods.
People with diabetes also sometimes use net carbs for carb counting, since the goal is to predict the blood sugar impact of a meal. Subtracting fiber from total carbs can give a more accurate estimate of how much insulin a meal requires, though individual responses vary.
How to Calculate Net Carbs From a Label
Pull up the Nutrition Facts panel and find three lines:
- Total Carbohydrate: your starting number
- Dietary Fiber: subtract all of it
- Sugar Alcohols: subtract all or half, depending on the type
If the product contains allulose, check the ingredient list. The label won’t break it out separately, but some brands list it in the carb breakdown voluntarily. For whole foods like vegetables, fruits, and legumes, the calculation is straightforward: total carbs minus fiber. A cup of broccoli has about 6 grams of total carbs and 2.4 grams of fiber, putting it at roughly 3.6 grams of net carbs.
For packaged “low-carb” products loaded with sugar alcohols, fiber supplements, and newer sweeteners, the math is messier and the real blood sugar impact harder to predict. The safest approach is to treat net carb claims on packaging as estimates rather than guarantees, especially if you’re managing blood sugar closely.