Total carbs are everything counted as carbohydrate on a nutrition label, including sugars, starches, fiber, and sugar alcohols. Net carbs are what’s left after you subtract the carbohydrates your body can’t fully digest or absorb, primarily fiber and sugar alcohols. The idea is simple: if a carbohydrate passes through you without raising blood sugar or providing significant energy, it shouldn’t “count” the same way a spoonful of sugar does.
The distinction matters most to people managing diabetes or following low-carb diets, but the concept is useful for anyone trying to understand what a nutrition label actually tells you about a food.
How Total Carbs Are Calculated
The number you see on a nutrition facts panel isn’t measured directly. The FDA requires manufacturers to calculate total carbohydrate by subtracting protein, fat, moisture, and ash from the total weight of the food. Whatever is left gets labeled “carbohydrate.” This method, called “by difference,” lumps together compounds that behave very differently in your body. Table sugar and cellulose from a vegetable skin both count the same on the label, even though one spikes your blood sugar and the other passes through your digestive tract largely untouched.
Research from the Food and Agriculture Organization has shown that this approach overestimates the energy a food actually provides by 12 to 21 percent, depending on how much fiber the food contains. That gap is exactly what net carb counting tries to close.
The Net Carb Formula
The basic calculation is straightforward:
Net carbs = Total carbs − Fiber − Sugar alcohols (or half of sugar alcohols)
Fiber is subtracted entirely because humans lack the enzymes to break it down. Some fiber does get fermented by gut bacteria in the large intestine, producing small amounts of fatty acids your body can absorb. But even in a high-fiber diet, that contribution is negligible for practical purposes.
Sugar alcohols are trickier. UCSF Health recommends subtracting only half the grams of sugar alcohol from total carbs, because most sugar alcohols are partially absorbed. Their example: a food with 29 grams of total carbohydrate and 18 grams of sugar alcohol would count as 20 grams of net carbs (29 minus 9, since you halve the 18).
The one exception is erythritol, which has a glycemic index of just 1 (compared to 65 for table sugar) and is almost entirely excreted without being metabolized. Many people subtract erythritol completely. Maltitol, on the other hand, has a glycemic index of 35, meaning it raises blood sugar meaningfully. Subtracting all of it would give you an inaccurate picture.
Why Different Sugar Alcohols Matter
Not all sugar alcohols are created equal, and this is where net carb labels on packaged foods can mislead you. Here’s how the common ones compare:
- Erythritol (GI: 1) — Virtually no blood sugar impact. Reasonable to subtract fully.
- Sorbitol (GI: 4) — Very low blood sugar impact, though it can cause digestive discomfort in large amounts.
- Xylitol (GI: 12) — Mild blood sugar effect. Subtracting half is a reasonable approach.
- Maltitol (GI: 35) — Raises blood sugar about half as much as table sugar. Subtracting all of it from total carbs significantly undercounts the impact.
When a protein bar advertises “only 4 net carbs” but contains maltitol, the real metabolic impact is higher than that number suggests. Checking which sugar alcohol a product uses tells you more than the net carb claim on the front of the package.
Where Allulose Fits In
Allulose is a newer sweetener that occupies an unusual regulatory space. It’s technically a sugar (a rare form of fructose), so it shows up in total carbohydrates on nutrition labels. But your body handles it very differently from regular sugar. Clinical research shows that allulose lowers postprandial blood sugar and insulin levels compared to sucrose, and it dose-dependently reduces the glycemic response when consumed alongside other carbohydrates. A 12-week study found it even reduced BMI compared to sucralose.
The FDA agreed in 2019 that allulose does not need to be listed as an “added sugar,” and many manufacturers now exclude it from their net carb counts. If you see allulose on an ingredients list, it’s reasonable to treat it similarly to fiber when counting net carbs.
“Net Carbs” Has No Official Definition
This is the most important caveat. The term “net carbs” has no legal definition and is not recognized by the FDA or the American Diabetes Association. When a food company prints “5g net carbs” on its packaging, there’s no standardized formula they’re required to follow. One brand might subtract all sugar alcohols completely. Another might subtract only fiber. There’s no enforcement mechanism ensuring consistency.
The FDA recommends using total carbohydrates from the nutrition facts panel as your reference point. Sugar alcohol content is only required on the label when a product makes a claim about sugar or sweeteners; otherwise, listing it is voluntary. That means you sometimes can’t even verify a net carb claim because the underlying data isn’t on the label.
Which Number to Use for Diabetes
The American Diabetes Association does not endorse net carbs for managing blood sugar or calculating insulin doses. Their concern is practical: fiber and sugar alcohols are not completely unabsorbed in every person, and some sugar alcohols still provide calories and raise blood glucose. Relying on net carbs could lead to under-dosing insulin or underestimating the effect of a meal.
For people using insulin, total carbs is the safer starting point. If you eat a food that’s very high in fiber (say, 15 or more grams per serving), your doctor or diabetes educator may suggest subtracting some of that fiber. But this is an individualized adjustment, not a blanket rule. Blood glucose monitoring after meals remains the most reliable way to see how a specific food actually affects you.
Net Carbs on a Keto Diet
Net carbs became a mainstream concept largely because of the ketogenic diet. Most keto guidelines recommend keeping total carbohydrate intake below 50 grams per day, and often as low as 20 grams, to maintain ketosis. At those levels, the distinction between total and net carbs is significant. A cup of broccoli has about 6 grams of total carbs but only around 3.5 grams of net carbs after subtracting fiber. If your daily budget is 20 grams, that difference means an extra serving of vegetables.
Using net carbs makes keto more flexible and easier to sustain, and for most healthy people tracking carbs for weight management, it works well enough. The fiber in whole foods genuinely doesn’t contribute to blood sugar spikes, so subtracting it gives a more accurate picture of metabolic impact. Where it gets unreliable is with heavily processed “keto-friendly” products that use sugar alcohols and creative math to advertise impossibly low net carb counts.
A Practical Approach
For most people, subtracting fiber from total carbs is straightforward and well-supported by how your body actually processes food. Fiber doesn’t raise blood sugar, doesn’t contribute meaningful calories, and appears on every nutrition label. This alone gets you a useful net carb number.
Sugar alcohols require more caution. The half-subtraction rule is a reasonable default for most sugar alcohols, with erythritol being the one you can safely subtract in full. If a product doesn’t specify which sugar alcohol it uses, subtracting half keeps your estimate conservative. And if sugar alcohols aren’t listed on the label at all, you’re stuck with total carbs as your only reliable number.
The bottom line: net carbs are a useful tool for understanding how food affects your blood sugar, but they’re only as accurate as the information on the label and the formula you use to calculate them. When in doubt, the nutrition facts panel’s total carbohydrate line is the number with a legal definition behind it.