Net carbs equal total carbohydrates minus fiber and sugar alcohols. That’s the basic formula, and for most foods, it takes about ten seconds with a nutrition label. But the details matter, especially when sugar alcohols are involved or when you’re managing blood sugar. Here’s how to do the math correctly.
The Basic Formula
Start with the “Total Carbohydrate” line on a nutrition facts label. Subtract the grams of dietary fiber. If the product contains sugar alcohols, subtract those too (with a caveat covered below). The number you’re left with is the net carb count.
Net Carbs = Total Carbohydrates − Fiber − Sugar Alcohols
For a simple example: a food with 25 grams of total carbohydrates and 7 grams of fiber has 18 grams of net carbs. The logic is that fiber passes through your digestive system largely undigested, so it doesn’t raise blood sugar the way starches and sugars do. Whole foods like vegetables, nuts, and seeds often have a big gap between total and net carbs because of their fiber content. An avocado, for instance, has about 12 grams of total carbs per fruit but nearly 10 grams of fiber, bringing the net carbs down to roughly 2.
Why Sugar Alcohols Need Different Math
Sugar alcohols (listed on labels as erythritol, maltitol, xylitol, sorbitol, and others) are partially digested. They have a smaller effect on blood sugar than regular sugar, but they aren’t zero-impact. The UCSF Diabetes Teaching Center recommends subtracting only half the grams of sugar alcohol from total carbohydrates, not the full amount.
Here’s their example: a product lists 29 grams of total carbohydrate and 18 grams of sugar alcohol. You divide 18 by 2 to get 9, then subtract that from 29. The effective carb count is 20 grams, not the 11 you’d get if you subtracted all the sugar alcohols.
Many keto-branded products subtract 100% of their sugar alcohols when advertising net carbs on the front of the package. That can understate the real impact on your blood sugar. Using the half-subtraction rule gives you a more conservative and realistic number.
One exception worth noting: erythritol is absorbed and excreted almost entirely without being metabolized, so some people subtract it fully. If a label lists erythritol specifically (rather than a generic “sugar alcohols” line), full subtraction is more defensible than it is for maltitol or sorbitol.
Where Allulose Fits In
Allulose is a newer sweetener showing up in low-carb products. It’s technically a sugar, but the body absorbs very little of it. The FDA has issued guidance allowing manufacturers to exclude allulose from “Total Sugars” and “Added Sugars” on the label and to count it at only 0.4 calories per gram instead of the usual 4. This means allulose often won’t inflate your total carbohydrate number the way regular sugar would, though labeling practices vary. Check whether allulose is already excluded from the total carbs on a given product before subtracting it again.
Whole Foods vs. Packaged Products
Calculating net carbs for whole foods is straightforward: look up the total carbs and fiber in a database like the USDA’s FoodData Central, then subtract. A cup of broccoli has about 6 grams of total carbs and 2.4 grams of fiber, giving you roughly 3.6 net carbs. No sugar alcohols to worry about.
Packaged “keto” or “low-carb” products are where things get tricky. These often combine multiple fiber sources, sugar alcohols, and sweeteners like allulose. The “net carb” number printed on the front of the package is calculated by the manufacturer using their own methodology, and there’s no standard they’re required to follow. Your safest bet is to flip to the Nutrition Facts panel, find the total carbohydrate line, and do the subtraction yourself using the rules above.
Net Carbs Are Not an Official Term
“Net carbs” has no legal definition. The FDA does not use it, and the American Diabetes Association does not recognize it. The FDA recommends that consumers use total carbohydrates on the nutrition facts label as their reference point. The assumption behind net carbs, that fiber and sugar alcohols are not absorbed or metabolized, is not always true. Some fibers are partially fermented in the gut and can produce small amounts of energy. Some sugar alcohols raise blood sugar more than others.
For people managing diabetes, this distinction matters. The ADA’s position is to work with total carbohydrates rather than net carbs, because total carbs give you a more predictable picture of how food will affect your blood sugar. If you’re using net carbs for a keto diet and you don’t have diabetes, the margin of error is less consequential. But if you’re dosing insulin or tightly managing glucose levels, relying on net carbs can introduce enough variability to throw off your calculations.
A Quick Reference for the Math
- Simple foods (no sugar alcohols): Total Carbs − Fiber = Net Carbs
- Foods with sugar alcohols: Total Carbs − Fiber − (Sugar Alcohols ÷ 2) = Net Carbs
- Foods with erythritol specifically: Total Carbs − Fiber − Erythritol = Net Carbs (full subtraction is reasonable)
- Foods with allulose: Check whether allulose is already excluded from Total Carbs on the label before subtracting
The half-subtraction rule for sugar alcohols is the most commonly overlooked step. If a protein bar claims 3 net carbs but has 20 grams of maltitol, your body is likely processing more carbohydrate than that number suggests. Running the math yourself keeps you closer to reality.