To figure out net carbs, subtract fiber and sugar alcohols from the total carbohydrates listed on a food label. The basic formula is: Total Carbs − Fiber − Sugar Alcohols = Net Carbs. This gives you a rough estimate of the carbohydrates your body actually absorbs and converts to blood sugar. It’s a useful number, but the calculation has a few important wrinkles depending on the type of fiber and sweetener involved.
The Basic Formula
Start with the “Total Carbohydrate” line on a nutrition label. That number includes everything: starches, sugars, fiber, and sugar alcohols. Your body doesn’t absorb fiber the way it absorbs sugar or starch, and it only partially absorbs most sugar alcohols. Subtracting them gives you a closer picture of the carbs that will actually raise your blood sugar.
For a food with 25 grams of total carbs, 7 grams of fiber, and 0 grams of sugar alcohols, the net carbs would be 18 grams. If the label also lists 5 grams of sugar alcohols, you’d subtract those too, bringing the total to 13 grams. That’s the simplified version, and for most whole foods with naturally occurring fiber, it works well enough.
Why the Formula Isn’t Always Exact
The American Diabetes Association notes that the net carb equation assumes fiber and sugar alcohols aren’t absorbed or metabolized, but that isn’t always true. Some types are partially digested and still provide calories and affect blood sugar. The real impact depends on which specific fibers and sugar alcohols are in the food, and labels don’t always break that down.
This matters most for packaged foods marketed as “low net carb” or “keto-friendly.” These products often contain sugar alcohols or added fiber sources that behave differently in your body than the fiber in broccoli or black beans. If you’re tracking net carbs for blood sugar management or a low-carb diet, it’s worth understanding the differences.
How Fiber Affects Blood Sugar
Your body doesn’t break down fiber the way it breaks down starch or sugar, which is why fiber doesn’t cause blood sugar spikes. Both types of fiber, soluble and insoluble, are subtracted in the net carb formula, but they work differently in your digestive system.
Soluble fiber dissolves in water and forms a gel-like substance in your stomach that slows digestion. This slowing effect helps control both blood sugar and cholesterol. Insoluble fiber doesn’t dissolve. It passes through your digestive system largely intact and helps improve insulin sensitivity. Neither type delivers glucose to your bloodstream, so subtracting all fiber from total carbs is standard practice and well supported.
Some packaged foods contain added “functional fibers” like isomaltooligosaccharides (often listed as IMO on labels). These ingredients were once widely used in protein bars and keto snacks because they could be labeled as fiber, but some research suggests they may raise blood sugar more than natural fiber sources. Many manufacturers have since reformulated their products, but it’s worth checking ingredient lists if a packaged food’s fiber count looks unusually high.
Sugar Alcohols: Not All Are Equal
Sugar alcohols are sweeteners commonly found in sugar-free candies, protein bars, and low-carb baked goods. Common ones include erythritol, xylitol, sorbitol, maltitol, mannitol, and lactitol. They break down slowly in the gut, and your body only absorbs part of their carbohydrate content. This keeps blood sugar and insulin from spiking the way regular sugar does.
The simple version of the formula subtracts all sugar alcohols completely. For erythritol, that’s reasonable: it has virtually no caloric or blood sugar impact. But maltitol is a different story. Maltitol is partially absorbed and raises blood sugar more than other sugar alcohols. The standard recommendation for maltitol is to subtract only half of its grams from total carbs, not all of them.
Here’s what that looks like in practice. A food label reads 20 grams total carbohydrate and 10 grams sugar alcohol (maltitol). Instead of subtracting all 10, you subtract 5 grams (half). That gives you 15 net carbs, not 10. If the label just says “sugar alcohols” without specifying which one, check the ingredient list. If maltitol or maltitol syrup is listed, use the half-subtraction rule.
What About Allulose?
Allulose is a newer sweetener showing up in more products. It’s technically a sugar, but your body processes very little of it. The FDA has issued guidance allowing manufacturers to exclude allulose from the “Total Sugars” and “Added Sugars” lines on nutrition labels, and it’s assigned only 0.4 calories per gram (compared to 4 calories per gram for regular sugar). If allulose is listed separately on a label, you can subtract it from total carbs. Some labels already exclude it from the carb count entirely, so check whether it appears as a line item before subtracting it a second time.
Reading Labels Outside the US
If you’re looking at food labels from the UK or European Union, the calculation changes. On US labels, fiber is included within the total carbohydrate number, which is why you subtract it. On UK and EU labels, fiber is listed as a separate line item and is not included in the carbohydrate total. The “carbohydrate” number on a European label is already closer to what Americans would call net carbs.
If you’re using a European product and subtract fiber again, you’ll undercount your carbs. Check where the product was made and how the label is formatted before doing any math.
A Step-by-Step Example
Say you pick up a protein bar with this nutrition panel:
- Total Carbohydrate: 24 g
- Dietary Fiber: 9 g
- Sugar Alcohols: 6 g (erythritol, per the ingredient list)
You’d calculate: 24 − 9 − 6 = 9 net carbs. Since the sugar alcohol is erythritol, subtracting the full amount is appropriate.
Now imagine the same bar, but the ingredient list shows maltitol instead of erythritol. The calculation becomes: 24 − 9 − 3 (half of 6) = 12 net carbs. That’s a meaningful difference if you’re tracking closely.
For whole foods like vegetables, fruits, and legumes, the process is simpler. A cup of cooked black beans has roughly 41 grams of total carbs and 15 grams of fiber: 26 net carbs. No sugar alcohols to worry about, no ambiguity about fiber types.
When Net Carbs Matter Most
Net carb counting is most commonly used by people following ketogenic or other low-carb diets, and by people managing diabetes who want a more accurate estimate of how food will affect their blood sugar. For keto diets, most people aim for 20 to 50 net carbs per day, so the difference between subtracting all of a sugar alcohol versus half of it can determine whether a food fits your daily limit.
For diabetes management, net carbs offer a better prediction of blood sugar impact than total carbs alone, since fiber genuinely doesn’t raise glucose levels. That said, the American Diabetes Association cautions that the equation isn’t perfectly accurate for every food, particularly processed ones with added fibers or sugar alcohols. Whole foods with naturally occurring fiber give you the most reliable net carb numbers.