Net carbs are calculated by taking the total carbohydrates in a food and subtracting fiber and sugar alcohols. The basic formula is: net carbs = total carbs − fiber − sugar alcohols. Most people following a keto diet aim for 20 to 50 grams of net carbs per day, and getting this number right determines whether you stay in ketosis.
The Basic Formula
Every nutrition label in the U.S. lists total carbohydrates, which includes three components: starches, sugars, and fiber. Since fiber passes through your body without being digested into glucose, it doesn’t raise blood sugar the way starch and sugar do. That’s why you subtract it. The same logic applies to most sugar alcohols, which are only partially absorbed.
For whole foods with no sugar alcohols, the math is simple. A medium avocado has about 17 grams of total carbs and 13 grams of fiber, giving you 4 grams of net carbs. A cup of broccoli has roughly 6 grams of total carbs and 2.4 grams of fiber, so about 3.6 net carbs.
For packaged foods, check whether the label lists sugar alcohols. If it does:
Net carbs = total carbs − fiber − sugar alcohols
That’s the version most keto trackers and apps use. But the real picture is slightly more complicated, because not all sugar alcohols behave the same way in your body.
Why Sugar Alcohols Need a Closer Look
The formula above assumes sugar alcohols pass through without affecting blood sugar. That’s true for some and misleading for others. Erythritol, the sugar alcohol found in many keto-friendly sweeteners, is almost entirely excreted unchanged. You can subtract 100% of it from your count without concern.
Maltitol is a different story. It’s absorbed significantly and raises blood sugar about 75% as much as regular sugar. If you subtract all of it, you’re undercounting your actual carb impact by a wide margin. Maltitol shows up frequently in sugar-free candy, chocolate bars, and protein bars marketed as low-carb. If a product relies heavily on maltitol, a more conservative approach is to subtract only half of the listed sugar alcohol grams.
Here’s a practical guide for common sugar alcohols:
- Erythritol: Subtract fully. Minimal blood sugar impact.
- Xylitol: Subtract about half. Partially absorbed.
- Sorbitol: Subtract about half. Partially absorbed.
- Maltitol: Subtract only about 25 to 50%. Raises blood sugar substantially.
- Isomalt: Subtract about half. Moderate absorption.
Most nutrition labels group all sugar alcohols into one line, so you’ll need to check the ingredients list to see which specific sugar alcohol the product contains. If the label just says “sugar alcohols” and you can’t identify the type, subtracting half is a reasonable middle ground.
What About Allulose?
Allulose is a rare sugar that’s become popular in keto products. The FDA issued guidance in 2020 allowing manufacturers to exclude allulose from “Total Sugars” and “Added Sugars” on labels, and it carries only 0.4 calories per gram (compared to 4 calories per gram for regular sugar). It has virtually no effect on blood sugar or insulin.
The catch is that allulose still gets counted under total carbohydrates on U.S. nutrition labels in some products, depending on how the manufacturer handles labeling. If you see allulose listed in the ingredients and the total carb count seems high for what should be a keto-friendly product, you can subtract the allulose grams. Some brands now list allulose separately on the label to make this easier.
Reading a Nutrition Label Step by Step
Start at “Total Carbohydrates.” Directly beneath it, you’ll see indented lines for dietary fiber, total sugars, and sometimes sugar alcohols. These are subcategories of total carbs, not additions to it. This trips people up: the fiber listed on the label is already included in the total carb number, which is exactly why you subtract it.
Say a keto protein bar label reads: total carbohydrates 24g, dietary fiber 9g, sugar alcohols 10g (erythritol, per the ingredients list), total sugars 1g. Your net carbs would be 24 − 9 − 10 = 5 grams. If that sugar alcohol were maltitol instead, you’d be smarter to calculate 24 − 9 − 5 = 10 grams, subtracting only half the sugar alcohols.
One important note for people using international labels: in the European Union, Australia, and several other countries, the “carbohydrates” line on the label already excludes fiber. It shows only digestible carbs. If you’re reading a European label, subtracting fiber again would double-count the deduction and give you an artificially low number. Only subtract fiber from U.S. and Canadian labels, where fiber is included under total carbohydrates.
Not All Fiber Is Truly Zero-Impact
The net carb formula treats all fiber as if it contributes zero usable carbohydrates. That’s mostly accurate, but some types of soluble fiber are fermented by gut bacteria into short-chain fatty acids, which your body can absorb for a small amount of energy. Inulin and chicory root fiber, commonly added to keto bars and breads to boost the fiber count, fall into this category. They don’t spike blood sugar the way starch does, but they aren’t completely inert either.
Insoluble fiber (the kind in leafy greens, seeds, and whole vegetable skins) passes through with the least impact. Psyllium husk, despite being soluble, is nonfermentable and also has minimal blood sugar effect. The practical takeaway: if a packaged food has an unusually high fiber count that seems designed to push the net carbs down to an appealing number, check whether the fiber comes from added sources like inulin or soluble corn fiber. These products aren’t necessarily bad, but their true net carb count may be slightly higher than the label math suggests.
Why “Net Carbs” Isn’t an Official Term
The term “net carbs” has no legal definition. The FDA doesn’t use it, and the American Diabetes Association doesn’t formally recognize it. The ADA notes that the equation used to calculate net carbs “is not entirely accurate because the contribution of fiber and sugar alcohols to total carbohydrates depends on the types present.” This doesn’t mean the concept is useless. It means you should treat it as a useful estimate rather than a precise measurement.
For most people on keto, the standard formula works well enough with whole foods. Where it gets unreliable is with highly processed low-carb products that use large amounts of sugar alcohols or added fibers to engineer a low net carb count on the package. If a product claims 2 net carbs but contains 30 grams of total carbs, that gap is being filled by ingredients whose real-world impact varies from person to person.
Practical Tips for Tracking
If you’re counting net carbs to stay in ketosis, a few habits make the process more reliable. First, prioritize whole foods where the fiber is naturally occurring: vegetables, nuts, seeds, avocados. The net carb calculation is most trustworthy with these foods because the fiber is genuinely indigestible.
Second, when evaluating packaged keto products, identify the specific sugar alcohol in the ingredients list before subtracting the full amount. A bar sweetened with erythritol and one sweetened with maltitol can have identical labels but very different effects on your blood sugar.
Third, if you use a tracking app, be aware that most apps subtract all fiber and all sugar alcohols by default. Some allow you to customize the formula. If yours doesn’t, mentally add back half the sugar alcohols for any product containing maltitol, sorbitol, or xylitol.
Finally, your own blood glucose response is the most accurate test. If you have access to a glucometer, testing before and 30 to 60 minutes after eating a questionable product will tell you more than any formula about how your body actually handles it.