Net carbs equal total carbohydrates minus fiber and sugar alcohols. For a food with 25 grams of total carbs, 7 grams of fiber, and 4 grams of sugar alcohols, the net carbs would be 16 grams. The idea behind this calculation is simple: not all carbohydrates raise your blood sugar the same way, so net carbs attempt to count only the ones that do.
The Basic Formula
For whole foods like vegetables, fruits, nuts, and legumes, the calculation is straightforward:
Net carbs = total carbohydrates − fiber
A medium avocado, for example, contains 17.1 grams of total carbs but 13.5 grams of that is fiber. Subtract the fiber and you get 3.6 grams of net carbs. The same logic applies to any whole food: look up its total carbohydrate and fiber content (the USDA food database is a free, reliable source), then subtract.
For packaged foods that contain sugar alcohols, the formula adds one more step:
Net carbs = total carbohydrates − fiber − sugar alcohols
Both numbers appear on U.S. nutrition labels, listed under total carbohydrates. Sugar alcohols include ingredients like sorbitol, xylitol, mannitol, maltitol, isomalt, and erythritol. You’ll find them most often in products marketed as “sugar-free” or “low-carb,” such as protein bars, sugar-free candy, and flavored drinks.
Why Fiber and Sugar Alcohols Are Subtracted
Your body can’t fully break down fiber. It passes through the digestive tract largely intact, so it contributes little to no glucose to your bloodstream. That’s why it gets subtracted entirely from the total carb count.
Sugar alcohols are partially absorbed. They break down slowly in the gut, and your body only takes in a portion of their carbohydrates. As Harvard Health nutrition researcher Dr. Frank Hu explains, this keeps blood sugar and insulin from spiking the way regular sugar does. Their reduced glycemic response is the reason they’re treated differently in net carb math.
The Sugar Alcohol Problem
Here’s where the formula gets less precise. The standard guideline from UCSF’s Diabetes Teaching Center is to subtract half the grams of sugar alcohols from total carbohydrates, not all of them. That’s because different sugar alcohols affect blood sugar to different degrees. Erythritol, for instance, has virtually no impact on blood glucose and is often subtracted completely. Maltitol, on the other hand, raises blood sugar more noticeably and probably shouldn’t be fully subtracted.
Many food companies subtract all sugar alcohols when printing a “net carbs” number on the front of their packaging. This can make a product look lower-carb than it functionally is. If you want a more conservative estimate, use the half-subtraction rule: take total carbs, subtract all the fiber, then subtract only half the sugar alcohols. If erythritol is the only sugar alcohol in the product (check the ingredient list), subtracting the full amount is reasonable.
What “Net Carbs” Actually Means, Legally
The term “net carbs” has no legal definition. The FDA does not use it, and the American Diabetes Association does not recognize it as an official measurement. The FDA recommends using total carbohydrates on nutrition labels for carb counting purposes. When you see “net carbs” on packaging, that number is calculated by the manufacturer using their own methodology, and there’s no regulation ensuring consistency from brand to brand.
This matters because the equation assumes fiber and sugar alcohols contribute zero usable carbohydrates, which isn’t always true. The American Diabetes Association notes that some fibers and sugar alcohols are partially digested, still provide calories, and still affect blood glucose. Since nutrition labels don’t specify which types of fiber or sugar alcohols a product contains, the net carb number is always an approximation.
Reading Labels Outside the U.S.
If you’re using food labels from the UK or EU, the math changes. European labels list carbohydrates and fiber as separate line items, and the carbohydrate figure already excludes fiber. So if you’re looking at a British product that says 12 grams of carbohydrates, fiber has already been removed. Subtracting fiber again would double-count it, giving you an artificially low number.
On U.S. labels, fiber is included within the total carbohydrate number, which is why you need to subtract it yourself. Always check which labeling system you’re working with before doing the math.
Allulose: A Newer Exception
Allulose is a low-calorie sugar that behaves differently from both regular sugar and sugar alcohols. The FDA currently exercises enforcement discretion allowing manufacturers to exclude allulose from “Total Sugars” and “Added Sugars” on the label, and to count it at only 0.4 calories per gram instead of the standard 4 calories per gram for sugar. Some manufacturers also exclude it from total carbohydrates, though formal rulemaking on this is still pending. If you see allulose in an ingredient list, it contributes minimal blood sugar impact and can generally be treated like erythritol for net carb purposes.
Net Carbs for Keto and Low-Carb Diets
Most people measuring net carbs are doing so to stay within a target range for a ketogenic or low-carb diet. A standard keto diet draws 5 to 10 percent of calories from carbohydrates, which typically works out to 20 to 50 grams per day. Many people following keto count net carbs rather than total carbs, since fiber doesn’t interfere with ketosis.
Whether you track net or total carbs is partly a matter of how strict you want to be. Using total carbs gives a more conservative estimate that accounts for any partial absorption of fiber or sugar alcohols. Using net carbs gives you more flexibility and a larger food selection, particularly with high-fiber vegetables. If you’re eating mostly whole foods, the difference between the two approaches is smaller than you might expect, since the fiber in vegetables and nuts is genuinely indigestible. The gap widens with processed “low-carb” products that rely heavily on sugar alcohols.
Practical Tips for Accuracy
For whole foods, use a food database rather than guessing. 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. A cup of blueberries has about 21 grams of total carbs and 3.6 grams of fiber, landing at around 17.4 net carbs. These differences matter when you’re working within a tight daily budget.
For packaged foods, read the nutrition facts panel rather than trusting the front-of-package “net carbs” claim. Find the total carbohydrates line, then look below it for dietary fiber and sugar alcohols. Subtract fiber fully. Subtract half the sugar alcohols unless the product specifies erythritol as the only one used. If a product lists multiple sugar alcohols or doesn’t specify which ones, the half-subtraction approach is your safest bet.
If you’re managing diabetes and adjusting insulin based on carb counts, talk with your care team about whether net carbs or total carbs is the better metric for your dosing. The imprecision of net carb calculations can be meaningful when medication is involved.