To calculate net carbs, subtract the grams of fiber from the total grams of carbohydrates listed on a nutrition label. If the product contains sugar alcohols, subtract half of those grams as well. The result is your net carb count, representing the carbohydrates your body actually absorbs and converts to blood sugar.
The Basic Formula
For most foods, the math is simple:
Net Carbs = Total Carbohydrates − Fiber
If a food has 30 grams of total carbohydrates and 7 grams of fiber, it has 23 grams of net carbs. Fiber is a carbohydrate by chemical structure, 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.
When sugar alcohols appear on the label, the formula adds one step:
Net Carbs = Total Carbohydrates − Fiber − (Sugar Alcohols ÷ 2)
The UCSF Diabetes Teaching Center recommends subtracting only half the sugar alcohol grams because your body does absorb a portion of them. For example, a protein bar with 29 grams of total carbohydrates, 3 grams of fiber, and 18 grams of sugar alcohols would work out like this: 29 − 3 − 9 (half of 18) = 17 grams of net carbs.
Why Fiber Gets Subtracted Entirely
Both types of fiber, soluble and insoluble, get subtracted from the total. Soluble fiber dissolves in water and forms a gel in your stomach that slows digestion, which actually helps stabilize blood sugar rather than spiking it. Insoluble fiber doesn’t dissolve at all. It passes through your system whole and can even improve insulin sensitivity. Neither type delivers usable glucose to your bloodstream, so both are excluded from net carb counts.
Sugar Alcohols Are Not All Equal
The “subtract half” rule is a practical shortcut, but sugar alcohols vary widely in how much they affect blood sugar. Erythritol has a glycemic index of just 1 (compared to about 65 for table sugar). Your small intestine absorbs most of it, but it passes through your body and is excreted unchanged in urine without being metabolized for energy. Many people on keto subtract erythritol fully rather than by half.
Maltitol is the opposite end of the spectrum, with a glycemic index of 35. That’s low compared to sugar but dramatically higher than other sugar alcohols like xylitol (12), sorbitol (4), or mannitol (2). If you’re closely managing blood sugar or tracking ketosis, a product sweetened with maltitol will have more impact than the label math suggests. Subtracting only half is already the right approach for maltitol, and some people subtract even less.
Here’s a quick reference for common sugar alcohols and their glycemic indexes:
- Erythritol: GI of 1 (minimal blood sugar impact, often fully subtracted)
- Mannitol: GI of 2
- Isomalt: GI of 2
- Lactitol: GI of 3
- Sorbitol: GI of 4
- Xylitol: GI of 12
- Maltitol: GI of 35 (subtract half or less)
What About Allulose?
Allulose is a newer sweetener showing up in low-carb products, and it’s handled differently from both sugar and sugar alcohols. The FDA has issued guidance allowing manufacturers to exclude allulose from the “Total Sugars” and “Added Sugars” lines on nutrition labels, and to count it at only 0.4 calories per gram instead of the 4 calories per gram that regular sugar provides. Some labels still include allulose in the total carbohydrate number, so if you see it listed in the ingredients but the sugar count seems low, this is why. Like erythritol, allulose has virtually no effect on blood sugar and can be subtracted from your net carb count.
Reading Labels in Different Countries
This is where people frequently make mistakes. In the United States, the “Total Carbohydrates” line on a nutrition label includes fiber. You need to subtract it yourself. In the UK and most of Europe, food labels list carbohydrates with fiber already excluded. The number you see is effectively already a net carb figure.
If you’re using a product from the UK or EU, subtracting fiber again would double-count it and give you an artificially low number. Check whether fiber appears indented under the total carbohydrate line (US style, meaning it’s included) or listed separately elsewhere on the label (UK/EU style, meaning it’s already removed).
Calculating Net Carbs in Whole Foods
Packaged foods have labels, but fruits, vegetables, nuts, and seeds require you to look up the values. Some whole foods are surprisingly low in net carbs because of their high fiber content.
Raspberries are a good example. One cup contains about 15 grams of total carbohydrates and 8 grams of fiber, leaving only 7 grams of net carbs. That makes them one of the most keto-friendly fruits available. Avocado is even more dramatic: a cup of sliced avocado has roughly 13 grams of total carbs but 10 grams of fiber (about 5 grams per half avocado), bringing net carbs down to just 3 grams. Chia seeds pack 4.1 grams of fiber into a single tablespoon, with only about 5 grams of total carbs, so one tablespoon has under 1 gram of net carbs.
For whole foods without labels, the USDA’s FoodData Central database (available free online) provides total carbohydrate and fiber values for nearly any food you can think of. Search the food, find both numbers, and subtract.
How Net Carbs Fit Into Common Diets
The concept of net carbs matters most to people following low-carb or ketogenic diets. A standard ketogenic diet limits total carbohydrate intake to fewer than 50 grams per day, and many people aim for 20 to 25 grams of net carbs to reliably maintain ketosis. Using net carbs instead of total carbs gives you more room to include fiber-rich vegetables and small portions of berries without exceeding your limit.
People managing diabetes also benefit from net carb counting. Since fiber doesn’t raise blood sugar, tracking net carbs gives a more accurate picture of how a meal will affect glucose levels. This is especially useful when eating foods like legumes or high-fiber tortillas that look carb-heavy on the label but have a much smaller glycemic impact once fiber is accounted for.
A Quick Example Start to Finish
Say you’re eating a low-carb protein bar. The label reads: 35 grams total carbohydrates, 12 grams fiber, 10 grams sugar alcohols (erythritol), and allulose is listed in the ingredients but not counted in sugars. Start with 35 grams. Subtract 12 grams of fiber to get 23. Since the sugar alcohol is erythritol (GI of 1), you can reasonably subtract all 10 grams rather than just half. That brings you to 13 grams of net carbs. If the label also includes allulose in the total carb count, you’d subtract that too.
If the same bar used maltitol instead of erythritol, you’d only subtract 5 grams (half of 10), giving you 18 grams of net carbs. The type of sugar alcohol can shift your count by several grams, which adds up across a full day of eating.