What are net carbs?
Net carbs are the carbohydrates in food that are actually digested and absorbed by your body. Not all carbohydrates behave the same way metabolically - dietary fiber passes through the digestive tract largely intact, fermenting in the colon without significantly raising blood glucose. Because fiber does not contribute to blood sugar or insulin response in the way that sugars and starch do, it is subtracted from total carbohydrates to arrive at the net carb count.
On a ketogenic diet, net carbs are the only number that matters for maintaining ketosis. Your body enters and stays in ketosis when blood glucose remains low enough for the liver to begin converting fatty acids into ketones. By limiting net carbs - typically to 20-50g per day - you keep blood glucose in a range that promotes ketone production. Total carbs, which include non-digestible fiber, are less relevant because fiber does not break this requirement.
How to calculate net carbs from a nutrition label
On a US nutrition facts label, Total Carbohydrates appears as a bolded line item. Directly beneath it, indented, are Dietary Fiber and Total Sugars. The formula is simple: Net Carbs = Total Carbohydrates − Dietary Fiber. If the product also lists sugar alcohols (also indented under Total Carbohydrates), you subtract some or all of those as well, depending on the type.
Note that serving size matters. Everything on the nutrition label - including the net carb calculation - applies to one serving as defined on that label. If you eat two servings, double the result. The servings field in this calculator handles that multiplication for you.
Net carbs and sugar alcohols - what you need to know
Sugar alcohols are a class of sweeteners that are partially or poorly absorbed by the digestive tract, giving them a lower glycemic impact than regular sugar. They are found in many keto-labeled products, protein bars, sugar-free gum, and specialty foods. The key issue is that not all sugar alcohols are equal - their glycemic impact varies significantly by type.
Erythritol - the keto-safe sugar alcohol
Erythritol has a glycemic index of zero and is absorbed in the small intestine and excreted unchanged in urine - it is not metabolized in any meaningful quantity. Clinical studies consistently show no significant blood glucose or insulin response from erythritol at typical serving sizes. For this reason, erythritol grams are subtracted in full from total carbohydrates when calculating net carbs.
Maltitol - why it is different (and why it matters)
Maltitol has a glycemic index of approximately 35 - significantly higher than erythritol and enough to raise blood glucose noticeably in many people. It is widely used in "sugar-free" chocolate, candy, and keto-labeled snacks because it mimics the taste and texture of sugar well. Many products label themselves as zero net carbs by subtracting maltitol entirely, which can be misleading for strict keto dieters. MeKeto subtracts only 50% of maltitol grams to account for this glycemic impact.
Xylitol, sorbitol, and other sugar alcohols
Xylitol, sorbitol, isomalt, and other sugar alcohols fall between erythritol and maltitol in their glycemic impact. They are partially fermented in the gut and partially absorbed, with a moderate glycemic index ranging from 7 (xylitol) to around 65 (sorbitol, depending on source). The standard approach - used here - is to subtract 50% of these sugar alcohols from total carbs. If you have CGM data or ketone meter readings suggesting they affect your ketosis, adjust accordingly.
Net carb limits on keto
The standard net carb limit for nutritional ketosis is 20-50g per day. The lower end (20g) is appropriate for beginners, for people with insulin resistance or type 2 diabetes, and for those who want the fastest possible ketosis onset. Starting at 20g leaves a margin for hidden carbs - amounts that slip in from condiments, sauces, and foods assumed to be zero-carb but are not.
After 4-6 weeks of fat adaptation, many people find they can tolerate 30-50g net carbs per day and still maintain ketosis. The exact threshold is individual and varies with activity level - athletes who exercise intensively often have higher carb tolerance because their muscles are better at clearing blood glucose. The only reliable way to find your personal limit is to use a blood ketone meter and test at different carb intakes.
High-carb foods that surprise keto dieters
Some foods appear keto-friendly but contain more net carbs than expected. Low-carb tortillas often list 3-5g net carbs per serving because of added fiber, but at 2-3 tortillas per meal the total adds up quickly. Protein bars marketed as keto frequently contain maltitol or other high-glycemic sugar alcohols that contribute more net carbs than the label implies. Nuts are also commonly underestimated - cashews have about 8g net carbs per ounce, which can consume half a day's budget in a single snack.
Other common surprises: flavored sparkling waters that list "0g sugar" but contain fruit juice concentrates; dairy products where carbs from lactose add up across multiple servings; and certain vegetables like cherry tomatoes, onions, and bell peppers that are often eaten in quantities that push net carbs above expectations. Tracking with a food scale and verified nutrition data - rather than estimating - is the most reliable way to stay within your target.