Net carbs are calculated by subtracting fiber from total carbohydrates. For whole foods, that’s the entire formula: total carbs minus fiber equals net carbs. For packaged foods containing sugar alcohols, the math gets slightly more complicated, and the rules differ depending on which sugar alcohol is used.
The Basic Formula for Whole Foods
For any unprocessed food, you subtract the grams of fiber from the grams of total carbohydrates. Fiber is technically a carbohydrate, but your body can’t break it down into glucose. It passes through your digestive system undigested, so it doesn’t raise blood sugar the way starches and sugars do.
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. That small number reflects the carbohydrates your body actually absorbs and converts to energy. The same logic applies to vegetables, nuts, seeds, and legumes, all of which carry significant fiber that offsets their total carb count.
How Sugar Alcohols Change the Math
Packaged foods, especially protein bars and sugar-free snacks, often contain sugar alcohols as sweeteners. These compounds are partially absorbed by your body, which means you can’t ignore them entirely, but you also shouldn’t count them at full value. The general rule: subtract half the grams of sugar alcohols from total carbs, along with all the fiber.
The formula for processed foods looks like this:
Total carbs – fiber – (sugar alcohols × 0.5) = net carbs
Erythritol is the one exception. It has a glycemic index of zero, meaning it produces virtually no blood sugar response. If erythritol is the only sugar alcohol in a product, you can subtract its full carb value rather than just half.
Not all sugar alcohols behave the same way. Maltitol, commonly found in “sugar-free” candy and bars, has a glycemic index between 35 and 52, which is substantially higher than other sugar alcohols like xylitol (7 to 13) or sorbitol (9). That means maltitol affects your blood sugar more than the label might suggest. Some product labels advertise very low net carb counts by subtracting sugar alcohols entirely, which overstates the benefit. An Atkins bar listing 3 grams of net carbs, for instance, actually works out to about 8.5 grams when you only subtract half the maltitol: 23 grams total carbs minus 9 grams fiber minus 5.5 grams (half of the 11 grams of sugar alcohols).
Why Labels Can Be Misleading
The FDA has no official definition of “net carbs.” The term doesn’t appear anywhere in federal nutrition labeling regulations. Total carbohydrate is the only carb-related value the FDA requires on a Nutrition Facts panel, and it includes fiber and sugar alcohols within that number. When a food company prints “net carbs” on a package, they’re using their own interpretation of the math, and different brands may calculate it differently.
Neither the FDA nor the American Diabetes Association endorses the net carb concept. Both recommend that people tracking carbohydrates focus on total carbs listed on the label. That said, the underlying logic is sound for whole foods: fiber genuinely doesn’t convert to glucose, so subtracting it gives you a more accurate picture of a food’s blood sugar impact.
Where things get less reliable is with heavily processed products that use engineered fibers or sugar alcohols to drive the net carb number down. Some of these ingredients are more absorbable than manufacturers claim, which means the “net carb” figure on the package may undercount what your body actually processes.
What About Allulose?
Allulose is a newer sweetener showing up in low-carb products. It’s technically a sugar, but your body absorbs very little of it. The FDA currently allows manufacturers to exclude allulose from both “Total Sugars” and “Added Sugars” on the label, and to count it at only 0.4 calories per gram instead of the usual 4. This means allulose may already be excluded from the carb count you see on a label, so there’s nothing additional to subtract. Check the ingredients list to confirm whether a product contains it.
Reading Labels Outside the US
If you’re looking at a food label from the UK or the European Union, the calculation works differently because the label itself is set up differently. In the US, “Total Carbohydrate” includes fiber, which is why you need to subtract it. In the UK and EU, the “Carbohydrate” line on the label already excludes fiber. Fiber is listed separately, as a voluntary addition to the nutrition panel. So the carbohydrate number on a European label is essentially already a net carb figure. No subtraction needed.
This catches a lot of people off guard when comparing products across countries. A food that shows 20 grams of carbohydrate on a UK label and 28 grams of total carbohydrate on a US label may actually have the same net carb content once you subtract the 8 grams of fiber from the US number.
Putting It Into Practice
For fruits, vegetables, nuts, and other whole foods, the calculation is straightforward and reliable. Subtract fiber from total carbs and trust the result. These foods contain naturally occurring fiber that your body genuinely cannot digest.
For packaged foods, be more cautious. Subtract all the fiber, subtract half the sugar alcohols (or all of them if the only sugar alcohol is erythritol), and treat the result as an estimate rather than a precise number. Pay attention to which sugar alcohol a product uses. A bar sweetened with erythritol will have a meaningfully different blood sugar impact than one sweetened with maltitol, even if the label shows the same net carb count.
If a packaged food is low in both protein and fiber but claims very low net carbs, it’s likely relying on sugar alcohols or engineered ingredients to reach that number. The actual metabolic impact may be higher than advertised.