Net carbs equal total carbohydrates minus fiber (and, for packaged foods, minus some or all sugar alcohols). This simple subtraction isolates the carbohydrates your body actually digests and converts to glucose, which is why people on low-carb and ketogenic diets use it as their primary tracking number.
The Basic Formula for Whole Foods
For any whole, unprocessed food, the calculation is straightforward:
Net carbs = Total carbohydrates − Fiber
Fiber is technically a carbohydrate, but your body can’t break it down into glucose. It passes through your digestive system largely intact, so it doesn’t raise blood sugar the way starches and sugars do. That’s why you subtract it.
A medium avocado, for example, contains 17.1 grams of total carbs. Of that, 13.5 grams is fiber. Subtract the fiber and you get 3.6 grams of net carbs. A cup of raw broccoli has about 6 grams of total carbs and 2.4 grams of fiber, leaving 3.6 grams of net carbs. The math is always the same: find the total carbohydrate number, find the fiber number, subtract.
You can pull both values from any nutrition label or a food database like the USDA FoodData Central. On a U.S. Nutrition Facts panel, fiber is listed directly below Total Carbohydrate because it’s considered a subcategory of carbs.
How Sugar Alcohols Change the Math
Packaged foods, especially protein bars, sugar-free candy, and keto snacks, often contain sugar alcohols as sweeteners. These show up on the label under names like sorbitol, xylitol, mannitol, isomalt, maltitol, lactitol, and erythritol. Because sugar alcohols are only partially absorbed, they contribute fewer calories and have a smaller effect on blood sugar than regular sugar.
The standard guidance from the UCSF Diabetes Teaching Center is to subtract half the grams of sugar alcohol from total carbohydrates. So if a protein bar lists 25 grams of total carbs, 8 grams of fiber, and 10 grams of sugar alcohols, you’d calculate:
25 − 8 − (10 ÷ 2) = 12 grams of net carbs
The exception is erythritol. It’s absorbed and then excreted in urine almost entirely without being metabolized, so most low-carb communities subtract erythritol fully rather than by half. If that same bar used 10 grams of erythritol specifically, you’d subtract all 10 instead of 5, dropping the net carb count to 7 grams.
This is where things get less precise. Different sugar alcohols are absorbed at different rates, and individual responses vary. Maltitol, for instance, raises blood sugar noticeably more than erythritol does. The “subtract half” rule is a useful approximation, not an exact science.
What About Allulose?
Allulose is a newer sweetener showing up in keto-friendly products. It’s chemically a sugar, but your body absorbs very little of it. The FDA has issued guidance 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 (compared to 4 calories per gram for regular sugar). A citizen petition has also requested its exclusion from “Total Carbohydrate” entirely.
In practice, this means some products already exclude allulose from their carb totals on the label, while others still include it. If allulose appears in the ingredient list but isn’t broken out on the nutrition panel, you may need to check the manufacturer’s website to know how they counted it. When allulose is listed separately, most people following a low-carb diet subtract it completely, similar to erythritol.
A Label Outside the U.S. Works Differently
If you’re reading a food label from the U.K., EU, or Australia, the “Carbohydrate” line on the nutrition panel typically already excludes fiber. These labels list fiber as a separate nutrient rather than a subcategory of carbohydrates. That means the carbohydrate number you see is already closer to a net carb figure, and subtracting fiber again would undercount your intake.
Check where the product was made. If fiber is indented under carbohydrates (the U.S. format), subtract it. If fiber is listed on its own line at the same level as carbohydrates, the subtraction has already been done for you.
Putting It Together Step by Step
Here’s a quick reference for reading any U.S. nutrition label:
- Whole foods (no sugar alcohols): Total Carbohydrate minus Fiber equals net carbs.
- Packaged foods with sugar alcohols: Total Carbohydrate minus Fiber minus half the Sugar Alcohols equals net carbs. Subtract erythritol fully instead of by half.
- Products with allulose: If allulose is included in Total Carbohydrate, subtract it fully. If the label already excludes it, no extra math needed.
Why Net Carbs Aren’t an Official Number
It’s worth knowing that “net carbs” is not a term regulated by the FDA. Harvard’s School of Public Health notes that “net carbs” and “impact carbs” are interchangeable terms created by food manufacturers as a marketing strategy. The FDA requires Total Carbohydrate on the label but has no official definition of net carbs.
This matters because manufacturers sometimes calculate net carbs generously, subtracting every gram of fiber, sugar alcohol, and allulose to print a low number on the front of the package. The actual effect on your blood sugar can be higher than that front-of-package number suggests, particularly with sugar alcohols like maltitol. There’s ongoing debate even within the ketogenic diet community about whether tracking net carbs or total carbs is more reliable for staying in ketosis.
For most people, the standard formula works well enough as a daily tracking tool. If you notice that certain “low net carb” products stall your progress or spike your blood sugar, the sugar alcohol content is the most likely culprit. Switching to products sweetened with erythritol or allulose, or simply eating whole foods where the math is clean and reliable, tends to solve the problem.