Net carbs are calculated by subtracting fiber and sugar alcohols from total carbohydrates on a nutrition label. For a food with 25 grams of total carbs, 7 grams of fiber, and 5 grams of sugar alcohols, the net carbs would be 13 grams. That basic formula is simple enough, but for people managing diabetes, the details matter: not all fibers and sugar alcohols affect blood sugar equally, and rounding errors can add up across a full day of eating.
The Basic Net Carb Formula
Start with the total carbohydrates listed on the nutrition facts panel. Subtract the grams of dietary fiber and the grams of sugar alcohols. What remains is the net carb count, which represents the portion of carbohydrate most likely to raise your blood sugar.
Net Carbs = Total Carbohydrates − Fiber − Sugar Alcohols
The FDA recommends that people focus on total carbohydrates, and the American Diabetes Association notes that this subtraction method isn’t entirely accurate because different types of fiber and sugar alcohols behave differently in the body. That said, net carb counting is widely used by people with diabetes because it gives a more realistic picture of how a food will affect blood glucose than the total carb number alone.
Why Fiber Gets Subtracted
Fiber is a carbohydrate your body can’t digest and absorb the way it absorbs starch or sugar. It passes through your digestive system largely intact, which means it doesn’t convert to glucose in your bloodstream. That’s why it gets subtracted from the total.
The two main types of fiber affect your body differently. Soluble fiber, found in oats, beans, and fruits, forms a gel-like substance during digestion that slows the absorption of other carbohydrates. This can blunt the post-meal blood sugar spike even beyond what the subtraction formula accounts for. Insoluble fiber, found mostly in whole grains and vegetables, doesn’t dissolve in water and adds bulk. Population studies have consistently linked higher insoluble fiber intake with lower type 2 diabetes risk, and clinical trials show it can improve insulin sensitivity and reduce inflammation over time.
Both types count as dietary fiber on nutrition labels, and both get subtracted in the net carb formula. For practical purposes, you can subtract all the fiber listed on the label. Some people prefer a more conservative approach and only subtract half the fiber when a food has fewer than 5 grams, reasoning that small amounts may still contribute trace glucose. This is a personal preference rather than a clinical guideline.
Sugar Alcohols Are Not All Equal
This is where net carb math gets tricky. Sugar alcohols are a category of sweeteners that includes erythritol, xylitol, sorbitol, maltitol, and several others. They’re partially absorbed in the small intestine, and the degree of absorption varies dramatically from one type to the next. Absorption ranges from essentially 0% for lactitol to about 80% for sorbitol.
Their impact on blood sugar, measured by glycemic index, varies just as much:
- Erythritol: glycemic index of 0, zero calories, virtually no blood sugar impact
- Xylitol: glycemic index of 13, modest impact
- Sorbitol: glycemic index of 9, low impact despite high absorption
- Maltitol: glycemic index of 35, significant impact
- Isomalt: glycemic index of 9, low impact
- Mannitol: glycemic index of 0, negligible impact
For comparison, table sugar has a glycemic index of 69. Erythritol and mannitol can be fully subtracted with confidence. Maltitol is the one to watch: with a glycemic index of 35, it raises blood sugar roughly half as much as regular sugar. If a product uses maltitol, subtracting all the sugar alcohol grams will overestimate how “free” those carbs really are. A more accurate approach is to subtract only half the maltitol grams. Many sugar-free chocolates, protein bars, and candies use maltitol because it’s cheap and tastes close to sugar, so check the ingredient list rather than trusting the sugar alcohol line alone.
When a label lists “sugar alcohols” without specifying the type, a reasonable middle ground is to subtract half the sugar alcohol grams instead of all of them. This accounts for the possibility that higher-impact sweeteners like maltitol are in the mix.
What About Allulose?
Allulose is a newer low-calorie sweetener showing up in more products. It’s technically a rare sugar, not a sugar alcohol, and it behaves differently from both. A meta-analysis of clinical trials in people with type 2 diabetes found that allulose significantly reduced post-meal blood sugar levels and improved time spent in healthy glucose ranges. It contributes minimal calories and has little to no effect on blood sugar for most people.
The FDA allows manufacturers to exclude allulose from both the “total sugars” and “added sugars” lines on nutrition labels, but it’s still included in total carbohydrates. If you see allulose listed in the ingredients, you can subtract those grams from the total carb count. Some products now list allulose separately on the label, making this easier.
A Worked Example
Say you’re looking at a protein bar with this nutrition panel: 30 grams total carbohydrates, 9 grams dietary fiber, 8 grams sugar alcohols (erythritol), and 3 grams allulose. The calculation would be: 30 − 9 − 8 − 3 = 10 grams net carbs. Because erythritol has a glycemic index of zero, subtracting the full amount is appropriate here.
Now imagine a different bar: 28 grams total carbohydrates, 4 grams fiber, 10 grams sugar alcohols (maltitol). The conservative calculation would be: 28 − 4 − 5 (half the maltitol) = 19 grams net carbs. If you’d subtracted all the sugar alcohols, you’d have gotten 14 grams, a number that underestimates the actual blood sugar impact by about 5 grams of effective carbohydrate.
Whole Foods Without Labels
Packaged foods make net carb counting relatively straightforward because the numbers are printed on the label. Whole foods require you to look up values in a database or app. A medium avocado has roughly 12 grams of total carbs and 9 grams of fiber, leaving about 3 grams of net carbs. A cup of broccoli has around 6 grams of total carbs and 2.4 grams of fiber, so roughly 3.6 grams net.
Whole foods rarely contain sugar alcohols, so the formula simplifies to total carbs minus fiber. The challenge is portion estimation. Research on carb counting accuracy has found that people typically make errors of about 20% per meal, averaging around 15 grams off in either direction. Underestimation is more common than overestimation, and larger, more complex meals produce bigger errors. Using a food scale for starchy foods like rice, potatoes, and pasta can significantly improve accuracy.
Net Carbs and Insulin Dosing
If you take mealtime insulin and dose based on carbohydrate ratios, the question of whether to use net carbs or total carbs directly affects your dose. Using total carbs when most of those carbs are fiber could lead to too much insulin and a low blood sugar episode. Using net carbs when the product contains maltitol could result in too little insulin and a post-meal spike.
There’s no single rule that works for every person or every food. A practical starting point: use net carbs for high-fiber whole foods where you’re confident the fiber is legitimate (vegetables, legumes, nuts). For processed “low carb” or “sugar free” products with sugar alcohols, use a modified net carb count that only subtracts half the sugar alcohols unless you know the specific type and its glycemic index. Then monitor your blood sugar response and adjust. Your continuous glucose monitor or post-meal fingerstick readings will tell you more than any formula about how a specific food affects you personally.
Common Mistakes That Throw Off Your Count
The most frequent error isn’t math but portion size. If you eat two servings of something but calculate net carbs for one serving, no formula will save you. Always confirm the serving size at the top of the nutrition panel before subtracting anything.
Another common pitfall is trusting “net carb” claims printed on the front of packages. These aren’t regulated by the FDA. Manufacturers can use whatever subtraction method they want, and some subtract all sugar alcohols regardless of type, or subtract fiber sources that may still partially contribute to blood sugar. Always calculate from the nutrition facts panel yourself rather than relying on marketing claims.
Finally, be cautious with imported foods. European labels often list carbohydrates with fiber already excluded, so subtracting fiber again would double-count the reduction and understate your true carb intake. If a product was made in the EU, check whether the label says “carbohydrates” (fiber likely already removed) or “total carbohydrates” (fiber still included).