Net carbs, explained

Net carbs are total carbohydrate minus dietary fiber and (in some calculations) part of sugar alcohols. The idea: only count the carbs that meaningfully raise blood glucose. The reality is messier — here's how to do the math correctly, and when net carbs are the right tool.

The net carbs formula

Net carbs = Total Carbohydrate − Dietary Fiber − (Sugar Alcohol ÷ 2)

Common keto / low-carb formula in the United States

For most whole foods (vegetables, fruit, nuts, meat) sugar alcohols are zero, so the formula simplifies to:

Net carbs = Total Carbohydrate − Dietary Fiber

Why fiber gets subtracted

Dietary fiber is technically a carbohydrate, but humans don't have the enzymes to break it down for energy. Soluble fiber gets fermented by gut bacteria into short-chain fatty acids — a slow, modest energy source. Insoluble fiber passes through largely intact. Neither raises blood glucose the way sugar or starch does.

That's why low-carb and keto plans subtract fiber: a 30g serving of broccoli with 6g total carbs and 2g fiber has 4g of "actual" carbs that hit your bloodstream as glucose.

Sugar alcohols: subtract all, half, or none?

Sugar alcohols (polyols) like erythritol, xylitol, maltitol, and isomalt are sweeteners that taste like sugar but don't fully metabolize the same way. The rules vary by alcohol:

The "subtract half" rule is a reasonable default if you don't know which alcohol the product uses. Maltitol is the gotcha — it's common in cheap "sugar-free" candy and behaves more like real sugar than people expect.

Worked examples

Avocado, 1 medium

• Total Carbohydrate: 12g
• Dietary Fiber: 9g
• Sugar Alcohols: 0g
• Net carbs: 12 − 9 = 3g

Quest Protein Bar (cookies & cream)

• Total Carbohydrate: 22g
• Dietary Fiber: 14g
• Sugar Alcohols (erythritol): 7g — subtract 100%
• Net carbs: 22 − 14 − 7 = 1g

"Sugar-free" candy with maltitol, 25g

• Total Carbohydrate: 22g
• Dietary Fiber: 0g
• Sugar Alcohols (maltitol): 18g — subtract ~50%
• Net carbs: 22 − 0 − 9 = 13g

This is why a "sugar-free" candy isn't keto-friendly the way the box implies.

Diabetes: count net carbs or total carbs?

Most U.S. diabetes care teams recommend total carbs for insulin dosing. The reasoning is risk-management: fiber values on labels can be off by ±20% under FDA rounding rules; mixed-dish fiber is unpredictable; under-dosing insulin is more dangerous than over-dosing. See our diabetes carb-counting guide for the full discussion.

That said, many endocrinologists are comfortable with net-carb counting for high-fiber whole foods (beans, lentils, vegetables) where the fiber number is reliable and the difference is large.

Keto: how strict is <20g net?

The widely-cited "ketogenic threshold" is under 20–30g of net carbs per day. The exact number is individual — some people stay in measurable ketosis at 50g, others fall out at 30g. The way to know is a blood ketone meter, not a hunch.

If you're targeting therapeutic ketosis (epilepsy, certain neurological conditions) the ratio matters more than the gram count, and you should be working with a clinician who specializes in it.

Why "net carbs" doesn't appear on U.S. labels

The FDA does not recognize "net carbs" as a regulated nutrition claim. Nutrition Facts labels list Total Carbohydrate, Dietary Fiber, Total Sugars, Added Sugars, and (separately) Sugar Alcohols when present. Manufacturers can print "X g net carbs" elsewhere on the package, but the formula they use is up to them — which is why the same product can show different "net carb" numbers across countries.

References

• U.S. FDA. Guidance for Industry: A Food Labeling Guide. Chapter 7: Nutrition Facts.
• Livesey G. Health potential of polyols as sugar replacers. Nutr Res Rev 2003;16(2):163–191.
• Jenkins DJ, et al. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr 1981;34(3):362–366.