To calculate net carbs, subtract fiber and sugar alcohols from total carbohydrates on a nutrition label. For most whole foods, the formula is simple: total carbs minus fiber equals net carbs. A typical keto diet aims for 20 to 50 grams of net carbs per day, so getting this calculation right determines which foods fit your daily budget.
The Basic Formula
Net carbs represent the carbohydrates your body actually absorbs and uses for energy. Fiber passes through your digestive system largely undigested, so it doesn’t raise blood sugar or interfere with ketosis the way starches and sugars do. That’s why you subtract it.
For whole foods with no sugar alcohols or specialty sweeteners, the math is straightforward:
Net carbs = Total carbohydrates − Fiber
Take an avocado as a practical example. A whole avocado has roughly 17 grams of total carbs and 13 grams of fiber, leaving you with about 4 grams of net carbs. Compare that to a cup of raspberries: 15 grams of total carbs minus 8 grams of fiber gives you 7 grams of net carbs. Both are keto-friendly in reasonable portions. A peach with 15 grams of total carbs but only 2 grams of fiber lands at 13 grams net, which eats up a much bigger share of your daily allowance.
This is why fiber-rich foods tend to be the most keto-compatible. The gap between total and net carbs can be dramatic in high-fiber vegetables and berries, while starchy or sugary foods barely budge. Brown rice, for instance, has 77 grams of total carbs per serving but only 3 grams of fiber, netting 74 grams. Fiber barely makes a dent.
How Sugar Alcohols Change the Math
Packaged keto products often contain sugar alcohols as sweeteners, and these require an extra step. Sugar alcohols are partially absorbed by the body, so they contribute some calories and have a modest effect on blood sugar, but less than regular sugar. Common ones include sorbitol, xylitol, mannitol, maltitol, isomalt, and erythritol.
The general guideline from diabetes nutrition experts at UCSF is to subtract half the grams of sugar alcohol from total carbs. So the adjusted formula becomes:
Net carbs = Total carbohydrates − Fiber − (Sugar alcohols ÷ 2)
If a keto protein bar lists 20 grams of total carbs, 8 grams of fiber, and 6 grams of sugar alcohols, you’d calculate: 20 − 8 − 3 = 9 grams of net carbs.
Erythritol is a special case that many keto followers treat differently. It has almost zero calories, virtually no effect on blood sugar, and passes through the body largely unchanged. Many people subtract erythritol fully rather than halving it. If you take the more conservative approach and only subtract half, you’re unlikely to overcount, but you may be leaving room on the table. Maltitol, on the other hand, behaves more like sugar than other sugar alcohols and raises blood sugar more significantly. Some keto dieters choose not to subtract maltitol at all.
The Allulose Exception
Allulose is a newer sweetener showing up in keto-friendly ice creams, syrups, and baked goods. It’s technically a sugar, which means FDA rules require it to be counted under total carbohydrates on the nutrition label. But your body handles it very differently from regular sugar. It provides only about 0.4 calories per gram (compared to 4 calories per gram for table sugar), and the FDA allows manufacturers to exclude it from both “Total Sugars” and “Added Sugars” on the label.
This creates a confusing situation: allulose inflates the total carbohydrate number on the label even though it barely affects blood sugar or energy intake. When calculating net carbs, you can subtract allulose along with fiber and sugar alcohols. Check the ingredient list to confirm allulose is present, then look for it broken out on the label or in the product’s marketing materials, since not all labels make it easy to find the exact amount.
Reading a Nutrition Label Step by Step
Start at “Total Carbohydrate” on the Nutrition Facts panel. Listed beneath it, you’ll see dietary fiber, total sugars, added sugars, and sometimes sugar alcohols. These are all subcategories of total carbs, not additions to it.
A common mistake is adding fiber and sugar alcohols to total carbs instead of subtracting them. They’re already included in the total number. Your job is simply to remove the portions your body won’t fully absorb.
Here’s a real-world walkthrough for a hypothetical keto cookie label:
- Total Carbohydrate: 18 g
- Dietary Fiber: 5 g
- Sugar Alcohols: 8 g (erythritol)
Conservative approach (halving sugar alcohols): 18 − 5 − 4 = 9 g net carbs. Full-subtraction approach for erythritol: 18 − 5 − 8 = 5 g net carbs. The right answer depends on your personal approach and how strictly you’re tracking.
Why “Net Carbs” Isn’t an Official Term
The FDA does not define, regulate, or recognize “net carbs” as a labeling term. Federal nutrition labeling rules cover total carbohydrate, dietary fiber, total sugars, added sugars, and sugar alcohols, but nowhere in the regulations does “net carbs” appear. Harvard’s School of Public Health describes “net carbs” and “impact carbs” as interchangeable marketing terms invented by food manufacturers to make products appear lower in usable carbohydrate.
This matters because there’s no legal standard for how companies calculate the net carb claims on their packaging. One brand might fully subtract all sugar alcohols, another might subtract only half. Some include allulose in their subtraction, others don’t clearly disclose it. The American Diabetes Association notes that the equation isn’t entirely accurate because different types of fiber and sugar alcohols are absorbed to different degrees. You’re better off running the numbers yourself from the Nutrition Facts panel rather than trusting a front-of-package net carb claim.
Total Carbs vs. Net Carbs for Staying in Ketosis
There’s genuine disagreement within the keto community about whether tracking net carbs or total carbs is more reliable for maintaining ketosis. The 20-gram target that strict keto protocols use originally referred to total carbs, not net. As the diet became more popular, net carb counting became the norm because it allows more vegetables, nuts, and berries without blowing the daily limit.
For most people, tracking net carbs works well and keeps ketosis stable. But if you’re eating a lot of packaged “keto” products loaded with sugar alcohols and specialty fibers, the math gets less reliable. Some of those subtracted carbs are still partially digested and can nudge blood sugar up enough to slow ketone production. If you’re stalled or struggling to stay in ketosis despite keeping net carbs low, the sugar alcohols and processed fiber in packaged foods are the first place to look.
A practical middle ground: track net carbs for whole foods (vegetables, nuts, seeds, berries) where the fiber subtraction is well-established, and be more conservative with processed keto products by halving sugar alcohols rather than fully subtracting them. If a product’s net carb claim seems too good to be true for something that tastes like a brownie, it probably is.