What Are Net Carbs, and How Do You Calculate Them?
"Net carbs" is a label you'll find on nearly every product marketed to a keto or low-carb audience, calculated by subtracting fiber — and often sugar alcohols — from total carbohydrate grams. The idea is that these subtracted grams have a minimal effect on blood sugar, so counting them toward a strict carb limit misrepresents how the food actually behaves in the body. The concept has real physiological grounding, but it is not a standardized or FDA-regulated term, and the way it is calculated varies enough between brands that the number on the package deserves a second look.
Where the Concept Comes From
Fiber is a carbohydrate the body cannot fully digest — it passes through the small intestine largely intact and is fermented by gut bacteria in the colon, contributing little to blood glucose. The Institute of Medicine's Dietary Reference Intakes treat fiber this way for calorie purposes, and it is the reason total carbohydrate counts on a nutrition label can overstate the glycemic impact of a food. Sugar alcohols — ingredients like erythritol, xylitol, and maltitol — are absorbed and metabolized differently from sugar as well, with a smaller and slower effect on blood glucose, though the effect is not zero and varies considerably by type.
The Formula, and Why It Isn't Standard
The typical net carb formula is: Total Carbohydrates minus Dietary Fiber minus Sugar Alcohols equals Net Carbs. The FDA does not require or define "net carbs" on a nutrition label, so this subtraction is a marketing calculation, not a regulated one. Brands are inconsistent about which sugar alcohols they subtract at full value: erythritol has a negligible glycemic effect and is reasonable to subtract close to fully, while maltitol has a meaningfully higher glycemic index than other sugar alcohols, and subtracting it at full value can understate a product's real effect on blood sugar.
Where the Math Overstates the Case
A 2003 review by Livesey on the health effects of polyols found that glycemic responses to sugar alcohols vary widely — erythritol has almost no measurable glycemic or insulin response, while maltitol raises blood glucose meaningfully more than the others. A product using maltitol as its primary sweetener but subtracting it in full from the net carb count can understate its real glycemic impact, which matters specifically for people managing blood sugar for medical reasons rather than simple carb-counting preference.
Fiber Counts Toward "Net" for a Reason
Unlike sugar alcohols, the case for subtracting fiber is well supported: fiber has minimal caloric availability and negligible direct effect on blood glucose in most people. The exception is resistant starch and certain fermentable fibers in very large amounts, which can still affect some individuals' digestion, though not their blood sugar in the way sugar does. The Fiber Gap: How to Fix It covers fiber's other benefits beyond the carb-counting math.
Should You Actually Track Net Carbs?
For someone following a ketogenic diet for the metabolic effect of ketosis, net carbs are a reasonable practical shortcut, since the goal is limiting the carbohydrate that actually reaches the bloodstream as glucose. For someone managing blood sugar directly, relying on a package's net carb number without checking which sugar alcohols it contains is riskier — the glycemic load of a food, or a direct look at the ingredient list, is more reliable than trusting the front-of-package math.
The Practical Takeaway
Net carbs are a useful shorthand, not a regulated guarantee. Subtracting fiber from total carbohydrate is well supported by the science of how fiber is digested; subtracting every gram of every sugar alcohol at full value is not always accurate, particularly for products built on maltitol rather than erythritol. Reading the ingredient list, not just the net carb line, is the more reliable habit.
