To calculate net carbs, subtract the grams of fiber from the grams of total carbohydrates on a nutrition label. If the product contains sugar alcohols, subtract half of those grams as well. The result is the number of carbs your body actually absorbs, which is what matters for staying in ketosis. Most people on keto aim for 20 to 50 grams of net carbs per day.
The Basic Formula
The standard calculation looks like this:
Net Carbs = Total Carbohydrates − Fiber − (Sugar Alcohols ÷ 2)
Fiber is subtracted because your body can’t digest it into glucose. It passes through your digestive tract largely intact, so it doesn’t raise blood sugar or interfere with ketosis. Sugar alcohols are partially absorbed, which is why you only subtract half. Both fiber and sugar alcohols are listed under “Total Carbohydrate” on U.S. nutrition labels, so you need to pull them out manually to get your net number.
Here’s a quick example: a food with 12 grams of total carbs, 5 grams of fiber, and 4 grams of sugar alcohols would give you 12 − 5 − 2 = 5 grams of net carbs.
Why Sugar Alcohols Need Special Attention
Not all sugar alcohols behave the same way in your body, and this is where the “divide by two” rule gets a little rough. Erythritol has a glycemic index of 0, meaning it has essentially no effect on blood sugar. Mannitol also scores 0. Xylitol lands at 13, which is still quite low. Maltitol, on the other hand, sits at 35, and its absorption in the small intestine ranges anywhere from 5% to 80%. That’s a wide range, and it means maltitol can meaningfully affect blood sugar in a way other sugar alcohols don’t.
For erythritol, many keto dieters subtract it entirely rather than just half, since it contributes virtually nothing to blood glucose. For maltitol, the half-subtraction rule is generous. If you’re strict about staying in ketosis, you may want to subtract less than half of maltitol, or simply avoid products that rely on it as a sweetener. Check the ingredients list, not just the sugar alcohol line, to see which one a product uses.
What About Allulose?
Allulose is a newer sweetener showing up in keto-friendly products. The FDA currently allows manufacturers to exclude allulose from the “Total Sugars” and “Added Sugars” lines on nutrition labels, and it’s counted at only 0.4 calories per gram instead of the usual 4. However, allulose may still be included in the total carbohydrate count on some labels. If you see allulose listed in the ingredients but the carb count seems higher than expected, check whether it’s been folded into total carbs. If it has, you can subtract it entirely, since your body absorbs very little of it.
The Label Rounding Problem
FDA rules allow manufacturers to round carbohydrate content to the nearest gram, and if a serving contains less than 0.5 grams of carbs, it can be listed as zero. This means a product labeled “0g carbs” could contain up to 0.49 grams per serving. That sounds negligible, but it adds up if you’re using multiple servings of things like cooking spray, heavy cream, or certain seasonings throughout the day. Five or six “zero carb” servings could quietly contribute 2 to 3 grams of carbs you didn’t account for.
When in doubt, check the ingredients list. If you see any form of starch, sugar, or dextrose in a “zero carb” product, assume there’s a fraction of a gram hiding in each serving.
If You’re Reading a UK or EU Label
This is a common source of confusion. On U.S. labels, fiber is included within the total carbohydrate number, which is why you subtract it. On UK and EU labels, fiber (“fibre”) is listed as a separate line item outside of carbohydrates. The carbohydrate figure on a European label already excludes fiber.
That means if you’re reading a UK or EU label, the “Carbohydrate” number is already close to your net carb count. Subtracting fiber again would give you an artificially low number. The same applies to polyols (the European term for sugar alcohols), which are listed as a subcategory under carbohydrate on EU labels. If polyols appear indented under carbohydrates, you can still subtract half of them.
Applying the Formula to Real Foods
Whole foods don’t come with nutrition labels, so you’ll rely on databases or apps. Here’s how the math works with a common keto vegetable: 100 grams of mature spinach contains 2.64 grams of total carbs and 1.6 grams of fiber, giving you roughly 1 gram of net carbs. That’s why leafy greens are a staple on keto. You can eat large volumes without making a dent in your daily carb limit.
An avocado is another good example. A whole avocado typically has around 12 to 13 grams of total carbs but nearly 10 grams of fiber, netting out to about 3 grams. Almonds run roughly 22 grams of total carbs per 100 grams with about 12.5 grams of fiber, landing around 9 to 10 net carbs per 100-gram serving. Since 100 grams of almonds is a substantial handful, a more typical one-ounce portion comes to about 2.5 to 3 net carbs.
For packaged foods, protein bars and keto snacks often list net carbs prominently on the front of the package. It’s worth doing the math yourself using the Nutrition Facts panel on the back, since marketing claims aren’t regulated in the same way. Subtract fiber from total carbs, subtract half the sugar alcohols (or all of them if the only sugar alcohol is erythritol), and subtract allulose if it’s been included in the total. That’s your real number.
Fiber Types and Whether They Matter
You’ll sometimes see advice suggesting that soluble fiber should count partially toward net carbs because it can be fermented into short-chain fatty acids in the gut. In practice, both soluble and insoluble fiber have minimal direct effects on blood glucose. Soluble fiber may slightly slow glucose absorption from other foods you eat at the same meal, which is actually beneficial for blood sugar stability. Insoluble fiber may support insulin sensitivity as well.
For the purposes of keto tracking, subtract all fiber from total carbs regardless of type. The caloric contribution from fiber fermentation is small enough that it won’t affect ketosis for most people. If you’re eating reasonable portions of vegetables and nuts, the distinction between soluble and insoluble fiber is not something you need to track.