Total carbs are every carbohydrate in a food, including sugars, starches, and fiber. Net carbs are total carbs minus the portions your body can’t digest or doesn’t convert to blood sugar, primarily fiber and sugar alcohols. The idea is simple: not all carbohydrates affect your body the same way, so net carbs attempt to count only the ones that do.
How Total Carbs Are Counted
The number listed as “Total Carbohydrates” on a nutrition label includes everything classified as a carbohydrate: sugars, starches, dietary fiber, and sugar alcohols. Your body handles each of these differently. Starches and sugars break down into glucose and raise your blood sugar. Fiber passes through largely undigested. Sugar alcohols fall somewhere in between, with some raising blood sugar modestly and others having virtually no effect.
Total carbs treat all of these the same, which is why someone tracking carbs for blood sugar management or a low-carb diet might want a more nuanced number.
How to Calculate Net Carbs
The basic formula is straightforward: start with total carbohydrates and subtract fiber and sugar alcohols. But the subtraction isn’t always one-to-one.
For fiber, a common guideline is that if a food has more than 5 grams of fiber, you subtract half the fiber from total carbs rather than all of it. If a food has 5 grams or fewer, you can subtract the full amount. This accounts for the fact that some fiber (the soluble kind) does slow digestion and interact with your gut in meaningful ways, even though it doesn’t spike blood sugar the way sugar or starch does.
For sugar alcohols, the general rule is to subtract half the sugar alcohol grams from total carbs. The exception is erythritol, which has a glycemic index of zero and zero calories per gram, so it’s typically subtracted entirely. By contrast, maltitol has a glycemic index of 35 (compared to 69 for regular sugar), meaning it does raise blood sugar to a noticeable degree. Subtracting all of it would give you a misleadingly low net carb count.
So for a protein bar listing 25 grams total carbs, 8 grams of fiber, and 6 grams of sugar alcohols, you’d subtract 4 grams for fiber (half, since it’s over 5 grams) and 3 grams for the sugar alcohols. That gives you 18 grams of net carbs.
Why Fiber Gets Subtracted
Your body doesn’t absorb and break down fiber, so it doesn’t cause the blood sugar spike that other carbohydrates do. But the two types of fiber work differently in your digestive system.
Soluble fiber dissolves in water and forms a gel-like substance in your stomach. This slows digestion and helps control blood sugar and cholesterol. It’s the kind found in oats, beans, and apples. Insoluble fiber doesn’t dissolve. It passes through your digestive system mostly intact, helps keep your bowels regular, and may increase insulin sensitivity over time. Neither type delivers glucose to your bloodstream the way starch or sugar does, which is why both get subtracted in net carb calculations.
Sugar Alcohols Vary Widely
Sugar alcohols are a category of sweeteners found in many “sugar-free” and “low-carb” products. They taste sweet but contain fewer calories than regular sugar and generally have a smaller effect on blood sugar. The problem is that the category includes ingredients that behave very differently from one another.
- Erythritol has a glycemic index of 0 and provides 0 calories per gram. It’s essentially invisible to your metabolism.
- Xylitol has a glycemic index of 13 and provides 2.6 calories per gram, roughly two-thirds the calories of sugar.
- Sorbitol has a glycemic index of 9 and provides 2.7 calories per gram.
- Maltitol has a glycemic index of 35 and provides 2.1 calories per gram. Of the common sugar alcohols, maltitol raises blood sugar the most.
This is why the blanket “subtract half” rule for sugar alcohols is a rough estimate. If the sugar alcohol is erythritol, you could subtract the full amount. If it’s maltitol, subtracting half still might undercount its impact. Many product labels don’t specify which sugar alcohol they use, which makes precision difficult.
What About Allulose?
Allulose is a newer sweetener showing up in low-carb products. It’s technically a sugar with a chemical structure similar to fructose, but your body barely metabolizes it. It contributes minimal calories and has a negligible effect on blood sugar and insulin levels. It appears to work by reducing glucose absorption in the intestine and promoting glycogen storage in the liver rather than releasing glucose into the bloodstream.
Because of this, allulose is typically subtracted from total carbs entirely when calculating net carbs. Some nutrition labels now list it separately under total carbohydrates, making it easier to identify.
“Net Carbs” Isn’t an Official Term
One important thing to know: “net carbs” has no official definition from the FDA. The Nutrition Facts panel on packaged food is required to list total carbohydrates, dietary fiber, total sugars, and added sugars. Manufacturers are not required to list net carbs, and when they do (often on the front of the package), they’re using their own formula.
This means two companies selling similar products could calculate net carbs differently. One might subtract all fiber, another only half. One might subtract all sugar alcohols, another might use the half rule. There’s no regulatory standard requiring consistency. If a product’s net carb claim seems surprisingly low, check the nutrition facts panel and do the math yourself.
Digestive Side Effects of Low Net Carb Foods
Foods marketed as low in net carbs often achieve that number by loading up on fiber and sugar alcohols. Both can cause digestive issues if you eat enough of them.
Sugar alcohols are the more common culprit. Sorbitol and mannitol can cause bloating, gas, or diarrhea at doses as low as 10 to 20 grams per day. Maltitol is better tolerated at lower amounts (30 grams in chocolate caused no significant symptoms in one study), but 45 grams caused diarrhea in 85% of participants. Xylitol generally requires higher amounts to cause problems, with most people tolerating a single dose of 10 to 30 grams without diarrhea, though 50 grams commonly triggers nausea and bloating.
Erythritol stands apart here too. It typically causes no gastrointestinal changes at normal consumption levels. In one study, adults consumed a single dose of 0.7 grams per kilogram of body weight (about 49 grams for a 150-pound person) without any laxative effect. Your body adapts to sugar alcohols over time as well. After about three weeks of regular consumption, some people can tolerate significantly higher doses than they could initially.
When Each Number Matters
Total carbs are what matter for people using insulin or counting carbs for medical reasons. Insulin dosing is based on total carbohydrate intake because the calculation needs to account for the full picture, and overestimating is generally safer than underestimating. Medtronic, which makes insulin pumps, notes this distinction specifically for people with diabetes.
Net carbs are more useful for people following a ketogenic or low-carb diet where the goal is to limit carbohydrates that raise blood sugar and trigger an insulin response. If you’re eating a salad with 12 grams of total carbs but 5 of those grams are fiber from leafy greens, counting 7 net carbs gives you a more accurate picture of how that food will affect your blood sugar.
For whole, unprocessed foods like vegetables, fruits, and legumes, the difference between total and net carbs is straightforward because fiber is the only thing being subtracted. Where it gets complicated is with packaged “low-carb” products that use multiple sugar alcohols, added fibers, and sweeteners like allulose. In those cases, read the full nutrition panel rather than trusting the front-of-package net carb claim.