Net carbs represent the carbohydrates in a food that your body actually digests and converts to glucose. The basic formula is simple: take the total carbohydrates listed on a nutrition label, then subtract fiber and all or part of the sugar alcohols. The result is the number that matters for blood sugar management, ketogenic diets, or general low-carb eating.
The Basic Formula
For whole foods like vegetables, fruits, nuts, and legumes, the calculation is straightforward:
Net Carbs = Total Carbohydrates − Dietary Fiber
You subtract fiber because 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. A cup of broccoli with 6 grams of total carbs and 2.4 grams of fiber, for example, has roughly 3.6 net carbs.
For packaged foods that contain sugar alcohols, the formula adds one more step:
Net Carbs = Total Carbohydrates − Dietary Fiber − (Sugar Alcohols ÷ 2)
You only subtract half the sugar alcohols because most of them are partially digested. The UCSF Diabetes Teaching Center recommends this approach: if a protein bar lists 29 grams of total carbohydrate and 18 grams of sugar alcohol, you’d divide 18 by 2 to get 9, then subtract that from 29 for a net carb count of 20 grams.
Why Only Half of Sugar Alcohols Count
Sugar alcohols are a category of sweeteners that includes maltitol, sorbitol, xylitol, and several others. They taste sweet but are only partially absorbed in your small intestine, which means they raise blood sugar less than regular sugar. The “divide by two” rule is a practical average across the category, since some sugar alcohols have a bigger glucose impact than others.
Erythritol is the exception. It’s absorbed and then excreted almost entirely unchanged, contributing essentially zero calories and no meaningful blood sugar response. Many people subtract erythritol completely rather than halving it. If a label lists sugar alcohols but doesn’t specify which type, the safer approach is to subtract only half.
Maltitol sits at the other end of the spectrum. It raises blood sugar more than other sugar alcohols, closer to about 75% of what table sugar does. Halving it on a label may actually undercount its impact. Products sweetened primarily with maltitol, common in “sugar-free” candy and chocolate, tend to have a bigger blood sugar effect than their net carb count suggests.
Where to Find the Numbers on a Label
On a U.S. nutrition facts panel, “Total Carbohydrate” is the starting number. Indented beneath it you’ll find “Dietary Fiber,” “Total Sugars,” and sometimes “Sugar Alcohol.” Fiber and sugar alcohols are already included in the total carbohydrate number, so you’re subtracting them out, not adding anything.
One quirk worth knowing: the FDA requires manufacturers to round each nutrient to the nearest whole gram. A food with 2.4 grams of fiber rounds down to 2 on the label, while one with 2.5 grams rounds up to 3. These small rounding differences on each line can stack up, so your net carb math might be off by a gram or two compared to the actual content. That’s normal and not worth worrying about for a single serving, but it can compound if you’re eating multiple servings of the same product.
Also worth noting: total carbohydrate on U.S. labels isn’t measured directly. It’s calculated by subtracting protein, fat, moisture, and ash from the total weight of the food. This “by difference” method means the number can include non-digestible components beyond what’s listed as fiber, adding another small source of imprecision.
Allulose: A Special Case
Allulose is a low-calorie sweetener showing up in more products. Here’s what makes it confusing: the FDA requires it to be included in the “Total Carbohydrate” line on the label, even though it contributes negligible calories and doesn’t raise blood sugar in any meaningful way. At the same time, the FDA allows manufacturers to exclude it from “Total Sugars” and “Added Sugars.”
This creates a situation where the total carb number on a product sweetened with allulose looks higher than its actual digestible carb content. Some manufacturers print a separate “Allulose” line on the label; others note it in the ingredients without breaking it out numerically. If you can identify the allulose content, you can subtract it entirely when calculating net carbs.
Not All “Fiber” on Labels Is Equal
The standard advice to subtract all fiber assumes the fiber listed on the label actually behaves like fiber in your body. That’s true for naturally occurring fiber in vegetables, fruits, and whole grains. But some packaged foods use manufactured fiber ingredients that don’t behave the same way.
Isomaltooligosaccharides (often listed as IMO or “tapioca fiber” on ingredient lists) are a good example. Research published in the Journal of Metabolic Health found that IMO consumption raised blood glucose by nearly 50 mg/dL with a five-fold spike in insulin at 30 minutes, while soluble corn fiber produced no significant change in either. The researchers concluded that IMO doesn’t function as a dietary fiber and should instead be classified as a slow-digesting carbohydrate. Products made before this became widely known, particularly some older protein bar formulations, may list IMO as fiber on the label even though it acts more like a regular carb in your body.
If you notice blood sugar rising more than expected after eating a low-carb product, check the ingredients for IMO or tapioca fiber. The FDA has since tightened which fibers can be listed as dietary fiber on labels, but older or smaller-brand products may still use these ingredients.
Net Carbs for Keto and Low-Carb Diets
Most ketogenic diet protocols aim for under 50 grams of total carbohydrates per day, and many people target 20 to 30 grams of net carbs to reliably stay in ketosis. Tracking net carbs rather than total carbs gives you more room for high-fiber vegetables and foods sweetened with erythritol or allulose, since those don’t interfere with the metabolic state you’re trying to maintain.
For practical purposes, if you’re eating mostly whole foods, your net carb count is just total carbs minus fiber. The sugar alcohol math only becomes relevant when you’re eating packaged products like protein bars, sugar-free chocolate, or low-carb tortillas.
Net Carbs and Diabetes
If you use insulin, the question of whether to dose based on total carbs or net carbs matters. The American Diabetes Association frames carb counting around total carbohydrates, noting that the total carb line on the label “includes all carbs: sugar, starch and fiber.” This is the conservative approach, and it reduces the risk of underdosing insulin.
That said, many diabetes educators teach a modified approach: if a food has more than 5 grams of fiber per serving, subtract half the fiber from total carbs before calculating your insulin dose. This is different from the keto approach of subtracting all fiber. The logic is that some soluble fiber does slow glucose absorption enough to matter for dosing, but subtracting all of it could leave you short on insulin. If you’re adjusting insulin based on net carbs, work with your care team to find the method that matches your individual glucose response.
Labels Outside the U.S.
If you’re reading a nutrition label from the UK, EU, or Australia, the carbohydrate number already excludes fiber. These labels list fiber as a separate line item that isn’t nested under carbohydrates. That means the carb number you see is effectively already “net carbs,” and subtracting fiber again would double-count the deduction. This catches people off guard when they buy imported foods or use international recipe databases. Check whether fiber is indented under carbohydrates (U.S. style) or listed separately (international style) before doing any math.