To get net carbs, subtract fiber and sugar alcohols from the total carbohydrates listed on a nutrition label. The basic formula is: total carbohydrates minus dietary fiber minus sugar alcohols equals net carbs. A food with 30 grams of total carbs, 5 grams of fiber, and 10 grams of sugar alcohols would have 15 net carbs. But the calculation has some nuances worth understanding, especially when it comes to different types of fiber and sugar alcohols.
The Basic Formula
Every nutrition label lists total carbohydrates, which includes starches, sugars, fiber, and sugar alcohols all bundled together. Fiber and sugar alcohols are listed underneath total carbohydrates because they’re subcategories of it. Since your body can’t fully digest fiber, and it processes sugar alcohols very differently from regular sugar, these are subtracted to estimate the carbs that actually raise your blood sugar.
Here’s the math in its simplest form:
- Net carbs = Total carbohydrates − Dietary fiber − Sugar alcohols
If a food has no sugar alcohols listed (most whole foods won’t), you simply subtract fiber from total carbs. A medium avocado with 17 grams of total carbs and 13 grams of fiber has roughly 4 net carbs.
Why Fiber Gets Subtracted
Fiber is technically a carbohydrate, but your body can’t break it down into glucose the way it handles starch or sugar. That’s why the UCSF Diabetes Teaching Center advises subtracting fiber grams from total carbohydrates: fiber doesn’t increase blood glucose levels.
Not all fiber behaves identically, though. Soluble fiber dissolves in water and forms a gel in your gut that slows digestion. This is the type found in oats (as beta-glucans), beans, and fruits (as pectins). It can actually help prevent blood sugar spikes after meals by slowing the rate at which other carbs are absorbed. Insoluble fiber, found in whole grains and vegetables, passes through your system largely intact, adding bulk to stool. Neither type delivers meaningful glucose to your bloodstream, so both are subtracted in the net carb formula.
One exception worth noting: resistant starch, found in cooked-and-cooled potatoes and green bananas, is a fermentable fiber that also helps normalize blood sugar. It’s sometimes listed separately on specialty food labels, and it counts the same as other fiber for net carb purposes.
Sugar Alcohols Are Trickier
Sugar alcohols show up in sugar-free candies, protein bars, and low-carb baked goods. Common ones include erythritol, xylitol, maltitol, and sorbitol. They taste sweet but contain fewer calories than sugar, typically between 0 and 2 calories per gram compared to sugar’s 4 calories per gram. They also don’t cause the same sharp blood sugar spikes that regular sugar does.
Here’s the catch: not all sugar alcohols are created equal. Your body absorbs some of them more than others. Erythritol has virtually no effect on blood sugar, so subtracting it entirely from your carb count makes sense. Maltitol, on the other hand, raises blood sugar more noticeably, so many low-carb guides recommend subtracting only half the maltitol grams from total carbs. The UCSF Diabetes Teaching Center uses this same rule of thumb: count about half of sugar alcohol grams as carbohydrate, depending on the type.
If a protein bar lists 25 grams of total carbs, 3 grams of fiber, and 10 grams of erythritol, you’d subtract all of them: 25 − 3 − 10 = 12 net carbs. But if that same bar used maltitol instead, a more conservative approach would be: 25 − 3 − 5 (half the maltitol) = 17 net carbs. When a label just says “sugar alcohols” without specifying the type, subtracting half is a reasonable middle ground.
What About Allulose?
Allulose is a newer low-calorie sweetener showing up in more products. It’s chemically classified as a sugar, but your body barely absorbs it, contributing only about 0.4 calories per gram. The FDA currently allows manufacturers to exclude allulose from “Total Sugars” and “Added Sugars” on nutrition labels, though it may still appear under total carbohydrates.
Some brands subtract allulose from their own “net carb” count printed on the front of the package, and this is generally reasonable given how little it affects blood sugar. If you see allulose listed on the nutrition panel under total carbohydrates, you can subtract it the same way you would fiber or erythritol.
Reading a Label Step by Step
Start by checking the serving size. Everything on the label, including total carbohydrates, is based on that single serving. If the label says one serving is 3 pieces at 30 grams of total carbs, eating 6 pieces means you’re consuming 60 grams of total carbs. This is the most common mistake people make: doing the net carb math correctly on numbers that don’t reflect how much they actually ate.
Next, find total carbohydrates. Directly below it, you’ll see dietary fiber and sugars (both total and added) indented underneath. Sugar alcohols may also appear here if the product contains them. Remember that fiber and sugars are already included in the total carb number. You don’t need to add them up. You only subtract fiber and sugar alcohols from total carbs.
A quick example with a sugar-free chocolate bar:
- Total carbohydrates: 24 g
- Dietary fiber: 2 g
- Sugar alcohols: 14 g (erythritol)
- Net carbs: 24 − 2 − 14 = 8 g
Why “Net Carbs” Isn’t an Official Term
The FDA does not regulate or define the phrase “net carbs.” When you see it on a product’s front label, that’s the manufacturer’s own calculation. The American Diabetes Association notes that the net carb equation isn’t entirely accurate because the contribution of fiber and sugar alcohols to total carbohydrates depends on which specific types are present. A bar sweetened with erythritol and a bar sweetened with maltitol could show the same net carb count on their packaging while having genuinely different effects on your blood sugar.
This doesn’t mean the concept is useless. For most people tracking carbs for weight management or a ketogenic diet, the basic formula works well enough to guide food choices. But if you’re managing diabetes and dosing insulin based on carb counts, relying on manufacturer-printed net carb numbers introduces some unpredictability. In that case, paying attention to which specific sugar alcohols and fibers a product contains gives you a more accurate picture.
Whole Foods vs. Packaged Foods
Net carb counting is straightforward with whole foods because there are no sugar alcohols to worry about. You subtract fiber from total carbs, and that’s it. A cup of broccoli with about 6 grams of total carbs and 2.4 grams of fiber has roughly 3.6 net carbs. A cup of raspberries with 15 grams of total carbs and 8 grams of fiber comes to about 7 net carbs.
Where it gets complicated is with packaged “low-carb” or “keto” products that engineer their carb counts down using combinations of fiber additives, sugar alcohols, and allulose. These foods can show impressively low net carb numbers on the label while still causing a noticeable blood sugar response in some people. If you find that certain low-carb products stall your progress or spike your glucose despite their net carb claims, the type of sugar alcohol or added fiber in the product is likely the reason. Trying a different product or sticking with whole food sources of carbs can help you troubleshoot.