How to Subtract Fiber from Carbs on Keto?

November 2, 2025 •

4 min read

While the FDA doesn't regulate the term 'net carbs,' subtracting fiber from total carbohydrates is a widely accepted practice for keto dieters. Mastering this calculation is a fundamental skill for anyone on a ketogenic diet, allowing for a more accurate carb count and helping to ensure your body remains in ketosis.

Quick Summary

Tracking net carbs is essential for the keto diet to maintain ketosis. The process involves subtracting dietary fiber and certain sugar alcohols from a food's total carbohydrates. This calculation helps determine the actual number of carbs that impact blood sugar levels, enabling more effective diet management and meal planning.

Key Points

Identify Net Carbs: For keto, focus on net carbs, not total carbs, by subtracting fiber and certain sugar alcohols from the total carbohydrate count on a food's nutrition label.
Read Labels Accurately: Always check the serving size first, as all nutritional values are relative to that portion.
Beware of Sugar Alcohols: While fiber can be fully subtracted, use caution with sugar alcohols. Some, like erythritol, have minimal impact, while others, like maltitol, may affect blood sugar more significantly.
Embrace Fiber-Rich Foods: Calculating net carbs allows you to include nutrient-dense, high-fiber foods like leafy greens, broccoli, and avocados without exceeding your daily carb limit.
Utilize Nutritional Databases: For whole foods without labels, use online resources like the USDA Food Composition Databases to find total carbs and fiber for your specific portion.
Track for Ketosis: Keeping an eye on your net carb intake is crucial for staying in ketosis, as it directly relates to the amount of carbohydrates your body digests and converts into glucose.

Understanding Net Carbs vs. Total Carbs

On a ketogenic diet, the primary goal is to limit your carbohydrate intake to a very low number, typically between 20 and 50 grams per day, to force your body into a metabolic state called ketosis. However, not all carbohydrates affect your body the same way. The distinction lies between total carbs and net carbs. Total carbohydrates represent the full amount of all carbs in a food, including sugars, starches, and dietary fiber. Net carbs, on the other hand, are the carbohydrates that your body can actually digest and convert into glucose. Fiber is a carbohydrate that your body cannot break down, so it has a minimal impact on your blood sugar levels. By subtracting fiber, you can focus on the carbs that truly matter for maintaining ketosis.

The Simple Formula for Calculating Net Carbs

Calculating net carbs is a straightforward process that anyone can master. You only need the nutrition facts label from a packaged food item or a reliable online food database for whole foods.

Here is the basic formula:

For packaged foods (in the US and Canada): Start with the total carbohydrates listed on the label. Then, subtract the dietary fiber. If the product also contains sugar alcohols (like erythritol), you can subtract them as well. Some sugar alcohols are only partially digested, so it's a best practice for some to subtract half of their amount, though erythritol can often be fully subtracted as most of it is not metabolized. The formula is: Total Carbs - Dietary Fiber - (Sugar Alcohols) = Net Carbs.
For whole foods: Since whole foods don't have a label, you'll need to use an online nutritional database. Find the total carbs and dietary fiber for your serving size and apply the same formula. For example, a medium avocado has about 17.1 grams of total carbs and 13.5 grams of fiber, resulting in only 3.6 grams of net carbs.

Practical Steps for Accurate Calculation

Read the Label Carefully: Always check the serving size first. All the nutritional information is based on that specific portion. If you eat more than one serving, you must multiply the values accordingly.
Locate the Numbers: Find the 'Total Carbohydrate' and 'Dietary Fiber' lines. The fiber is typically listed directly under total carbs.
Perform the Subtraction: Use the formula to calculate your final net carb count. This is the number you track towards your daily keto limit.
Be Cautious with Sugar Alcohols: Processed low-carb products often contain sugar alcohols. Pay attention to the type. While erythritol is generally fine to subtract completely, others like maltitol can have a greater impact on blood sugar, so many dieters only subtract half of its amount.

High-Fiber Foods That Are Keto-Friendly

One of the benefits of counting net carbs is that it encourages the consumption of fiber-rich, whole foods. Many vegetables, nuts, and seeds are low in net carbs and provide essential nutrients often lacking in restrictive diets. Here is a list of excellent sources:

Leafy Greens: Spinach, kale, and collard greens are extremely low in net carbs and packed with fiber and vitamins.
Cruciferous Vegetables: Broccoli, cauliflower, and Brussels sprouts are staples on a keto diet for their fiber content and versatility.
Nuts and Seeds: Chia seeds, flax seeds, and almonds are high in fiber and healthy fats, but be mindful of portion sizes.
Avocados: This is a classic keto food for a reason, providing healthy fats and a significant amount of dietary fiber.
Berries: Small amounts of berries like raspberries and blackberries can add flavor and fiber to your diet with a low net carb impact.

Comparison: Total Carbs vs. Net Carbs


Feature	Total Carbs	Net Carbs	Relevance on Keto
Definition	All carbohydrates (sugars, starches, fiber, sugar alcohols).	Carbohydrates that are digested and absorbed by the body.	Focus: Net carbs are the relevant metric for controlling blood sugar and maintaining ketosis.
Calculation	Read directly from the nutrition label.	Calculated by subtracting fiber and certain sugar alcohols from total carbs.	Method: Calculating net carbs allows for more flexible meal planning with fibrous vegetables.
Impact on Ketosis	Not a direct indicator. High fiber within total carbs doesn't necessarily halt ketosis.	A high net carb count can raise blood sugar and potentially knock you out of ketosis.	Outcome: Staying within a net carb limit is the primary strategy for achieving and maintaining ketosis.
Food Variety	Counting total carbs can be very restrictive, limiting whole foods.	Counting net carbs allows for greater inclusion of high-fiber vegetables, nuts, and seeds.	Flexibility: Using net carbs enables a more diverse and nutrient-dense diet.

Conclusion: Making the Most of Net Carbs

Understanding how to subtract fiber from carbs is a foundational skill for anyone serious about following a keto diet successfully. By focusing on net carbs, you can enjoy a wider variety of nutrient-dense, high-fiber foods without jeopardizing your state of ketosis. This simple calculation empowers you to make informed food choices, prevent common keto side effects like constipation, and ensure a more sustainable and satisfying dietary plan. Always remember to read your labels, understand the difference between digestible and indigestible carbohydrates, and prioritize nutrient-rich whole foods to maximize your success on the keto journey. For further information on the composition of various foods, the USDA Food Composition Databases is a valuable tool.

Frequently Asked Questions

Total carbs include all carbohydrates in a food, such as starches, sugars, and fiber. Net carbs are the total carbs minus fiber and certain sugar alcohols, representing only the carbohydrates that impact your blood sugar.

To calculate net carbs, find the 'Total Carbohydrate' on the nutrition label and subtract the 'Dietary Fiber.' If sugar alcohols are also listed, you can subtract half or all of them, depending on the type.

Fiber is subtracted because it is an indigestible carbohydrate that does not affect blood sugar levels or interfere with ketosis. This allows you to consume fiber-rich foods while keeping your net carb count low.

Yes, many people on keto subtract sugar alcohols as they have a minimal impact on blood sugar. Erythritol is typically fully subtracted, while other types like maltitol may be only partially subtracted.

The term 'net carbs' is not regulated by the FDA, so you will often need to do the calculation yourself. Find the total carbs, fiber, and sugar alcohols and perform the simple subtraction.

For whole foods like vegetables or nuts, you can use an online nutritional database, such as the USDA Food Composition Databases. Look up the food and serving size to find the total carbs and fiber content.

While fiber doesn't count towards net carbs, you should still consume it in moderation as part of a balanced diet. It's beneficial for digestive health and helps with satiety, but excessive amounts can sometimes cause digestive discomfort.