To calculate net carbs, subtract fiber from total carbohydrates. If the food contains sugar alcohols, subtract those too. The result tells you how many carbs your body actually digests and converts to blood sugar, which is the number that matters for low-carb and keto diets.
The basic formula looks like this: Total Carbs − Fiber − Sugar Alcohols = Net Carbs. But the details matter, because not all sugar alcohols are equal, nutrition labels differ by country, and newer sweeteners like allulose add another wrinkle.
The Basic Net Carb Formula
Start with the “Total Carbohydrate” number on a U.S. or Canadian nutrition label. That figure includes everything: fiber, sugars, starches, and sugar alcohols. Since fiber passes through your digestive system without being absorbed, you subtract it entirely. If the label lists sugar alcohols, subtract those as well.
For example, a food with 25 grams of total carbohydrates, 8 grams of fiber, and 5 grams of sugar alcohols would have 12 grams of net carbs. If there are no sugar alcohols listed, simply subtract fiber alone: 25 − 8 = 17 net carbs.
This number is sometimes called “available carbohydrate,” and it represents the portion that your body breaks down into glucose. It’s the figure most people track when counting carbs for blood sugar management or ketosis.
Why Labels Work Differently in the UK and EU
If you’re reading a nutrition label from the UK or Europe, the carbohydrate number already excludes fiber. Under UK and EU labeling rules, fiber is classified separately from carbohydrate and listed on its own line (if it’s listed at all, since it’s voluntary). That means the “carbohydrate” figure on a European label is already your net carb number for most purposes.
This catches people off guard when they switch between American and European products. A protein bar made in the UK showing 10 grams of carbohydrate is not the same as a U.S. bar showing 10 grams of total carbohydrate. The U.S. number still has fiber baked in. If you subtract fiber from a European label, you’ll undercount your actual carb intake.
Quick rule: on U.S. and Canadian labels, always subtract fiber. On UK and EU labels, the work is already done.
Not All Sugar Alcohols Are Created Equal
Sugar alcohols are sweeteners found in many “sugar-free” and “keto-friendly” products. The standard advice is to subtract them from total carbs because your body absorbs them poorly compared to regular sugar. That’s true for most of them, but the degree of absorption varies widely.
Erythritol has a glycemic index of just 1 (compared to 65 for table sugar), meaning it has virtually no effect on blood sugar. You can subtract it completely. Xylitol, sorbitol, mannitol, isomalt, and lactitol all land in the low single digits on the glycemic index, so subtracting them fully is reasonable for most people.
Maltitol is the outlier. With a glycemic index of 35, it raises blood sugar about half as much as table sugar. Many low-carb practitioners subtract only half the maltitol grams rather than all of them. So if a food has 10 grams of maltitol, you’d subtract 5 instead of 10. Check your ingredient list: if a product advertises very low net carbs but uses maltitol as its primary sweetener, the real impact on your blood sugar is higher than the label math suggests.
Where Allulose Fits In
Allulose is a newer sweetener showing up in low-carb breads, ice creams, and protein bars. It’s technically a sugar (a rare one found naturally in small amounts in figs and raisins), 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 labels and to count it at only 0.4 calories per gram instead of the usual 4.
Here’s the tricky part: allulose is still included in the “Total Carbohydrate” line on U.S. labels. So when you calculate net carbs, you should subtract allulose along with fiber and sugar alcohols. Some brands list allulose separately on the label or in a footnote showing their net carb math. If they don’t, check the ingredient list and look for the specific gram amount, which some brands provide voluntarily.
A Step-by-Step Example
Say you pick up a keto cookie with this nutrition panel per serving:
- Total Carbohydrate: 18 g
- Dietary Fiber: 4 g
- Sugar Alcohols (erythritol): 8 g
- Allulose: 3 g
Your calculation: 18 − 4 − 8 − 3 = 3 grams of net carbs. If that cookie used maltitol instead of erythritol, a more conservative calculation would only subtract half the sugar alcohols: 18 − 4 − 4 − 3 = 7 grams of net carbs.
Common Mistakes That Throw Off the Count
The most frequent error is subtracting fiber from a European label, which double-counts the reduction and gives you an artificially low number. Always check where the product was made and what labeling standard it follows.
Another common mistake is trusting the “net carb” claim on the front of a package without checking which sweeteners are inside. Manufacturers subtract all sugar alcohols at full value, even maltitol. If blood sugar control matters to you, do the math yourself and adjust for maltitol.
Serving size trips people up too. The net carbs you calculate are per serving, not per package. A bag of low-carb tortillas might list 4 net carbs per tortilla, but if you eat three, that’s 12 net carbs, which can matter on a strict 20-gram daily target.
Finally, whole foods without nutrition labels (fruits, vegetables, nuts) still have net carbs. You can look up their total carbohydrate and fiber content per serving in any food database and apply the same subtraction. A medium avocado, for instance, has roughly 17 grams of total carbs and 13 grams of fiber, leaving about 4 grams of net carbs.