Net carbs equal total carbohydrates minus fiber and sugar alcohols. That’s the core formula, and for most foods, it takes about five seconds with a nutrition label in hand. The concept exists because not all carbohydrates raise your blood sugar the same way. Fiber and sugar alcohols pass through your body without being fully absorbed, so they’re subtracted from the total to give you a more accurate picture of the carbs that actually affect you.
The Basic Formula
Start with the total carbohydrates listed on a nutrition label. Subtract the grams of dietary fiber. Then subtract the grams of sugar alcohols, if any are listed. The number you’re left with is your net carbs.
For a whole food like an avocado, you’re usually only subtracting fiber. A medium avocado has about 12 grams of total carbs and 9 grams of fiber, giving you roughly 3 net carbs. For packaged “low-carb” products, sugar alcohols often make the bigger difference. A protein bar with 24 grams of total carbs, 3 grams of fiber, and 15 grams of sugar alcohols would come out to 6 net carbs.
Why Fiber Gets Subtracted
Fiber is technically a carbohydrate, but your body can’t break it down into glucose. Soluble fiber dissolves in water and forms a gel in your digestive tract, slowing the absorption of other nutrients. Insoluble fiber passes through largely intact, adding bulk. Neither type gets converted into blood sugar, which is why fiber doesn’t count toward net carbs regardless of the type.
This is also why vegetables like broccoli, spinach, and cauliflower have very low net carb counts despite containing a fair number of total carbs. Most of those carbs are fiber.
Not All Sugar Alcohols Are Equal
Sugar alcohols are sweeteners commonly found in “sugar-free” candy, protein bars, and low-carb baked goods. They’re poorly absorbed by your small intestine, so they don’t spike blood sugar the way regular sugar does. But how much they affect blood sugar varies quite a bit depending on the specific type.
Erythritol has a glycemic index of just 1 (compared to 65 for table sugar). It’s absorbed in the small intestine and then excreted in urine without being metabolized, making it essentially zero-impact on blood sugar. Most people subtract 100% of erythritol from their net carb count. Xylitol has a glycemic index of 12, which is still very low. Maltitol, on the other hand, sits at 35. That’s far below table sugar but high enough that it can raise blood sugar noticeably, especially in larger amounts.
If a product uses erythritol, subtracting the full amount is reasonable. If it uses maltitol, some people subtract only half the sugar alcohol grams instead of all of them to account for its higher blood sugar impact. The nutrition label doesn’t always specify which sugar alcohol is used, but the ingredients list will. It’s worth checking, particularly if you’re managing diabetes or tracking carbs closely for ketosis.
Where Allulose Fits In
Allulose is a newer low-calorie sweetener showing up in more products. It’s technically a sugar, not a sugar alcohol, but your body handles it differently than regular sugar. Most of it passes through without being metabolized. The FDA has issued guidance allowing manufacturers to exclude allulose from the “Total Sugars” and “Added Sugars” lines on nutrition labels, and to count it at only 0.4 calories per gram instead of the usual 4.
Some brands have already removed allulose from their total carbohydrate count on labels, while others still include it. If allulose appears in the total carbs but is also listed separately, you can subtract it just like you would fiber or sugar alcohols. Check the ingredients list if the label isn’t clear.
Reading a Nutrition Label Step by Step
Nutrition labels in the U.S. list total carbohydrates as a main line item, with dietary fiber, total sugars, and added sugars indented underneath. Sugar alcohols may or may not appear as a separate indented line. If the product contains sugar alcohols but doesn’t break them out on the label, look at the ingredients list to confirm they’re present and identify which type.
Here’s what the math looks like in practice for a low-carb tortilla:
- Total carbohydrates: 19 g
- Dietary fiber: 11 g
- Sugar alcohols: 0 g
- Net carbs: 19 – 11 = 8 g
And for a sugar-free chocolate bar:
- Total carbohydrates: 23 g
- Dietary fiber: 2 g
- Sugar alcohols: 18 g (erythritol)
- Net carbs: 23 – 2 – 18 = 3 g
For whole foods without a label (fruits, vegetables, nuts), you can look up the total carb and fiber values in any nutrition database and do the same subtraction. Sugar alcohols are only relevant for processed or packaged products.
Net Carbs vs. Total Carbs for Blood Sugar
The reason net carbs matter is blood sugar. When you eat carbohydrates that your body fully digests and absorbs, they’re broken down into glucose, which enters your bloodstream. Fiber and most sugar alcohols skip that process. For someone following a ketogenic diet, counting net carbs instead of total carbs is standard practice because it gives a more accurate read on whether you’ll stay in ketosis. Most keto guidelines set the target at 20 to 50 net carbs per day.
For people with diabetes, the picture is slightly more nuanced. The fiber subtraction is straightforward since fiber genuinely has no blood sugar impact. But certain sugar alcohols, especially maltitol, can still cause a measurable glucose response. If you notice blood sugar spikes after eating products sweetened with maltitol or sorbitol, counting only half of those sugar alcohols in your subtraction gives a more conservative and often more accurate estimate.
Tracking total carbs is the more cautious approach and the one many diabetes educators recommend as a starting point. Tracking net carbs gives you more flexibility with high-fiber foods and products sweetened with erythritol or allulose, but it requires paying closer attention to which specific ingredients you’re subtracting.