Nutrition Diet: How do they calculate carbs?

November 2, 2025 •

3 min read

Did you know that food manufacturers in the United States and Canada use a 'carbohydrate by subtraction' method to determine the total carbohydrate count on a food label? Understanding how do they calculate carbs? is a crucial skill for managing your diet, especially for health conditions like diabetes or for following a low-carb eating plan.

Quick Summary

This article explains the various methods for calculating carbohydrate content in foods, covering the manufacturer's laboratory process and consumer-focused calculations like net carbs based on nutrition labels.

Key Points

Manufacturer's Method: Food manufacturers in the US and Canada calculate total carbs using a "by subtraction" method, measuring everything else and attributing the rest to carbohydrates.
International Labels: In contrast, countries in the EU and Mexico list "available carbohydrates" that exclude fiber, which is detailed separately on the label.
Total Carbs: This is the value regulated by the FDA and includes all sugars, starches, and fiber in a food. The ADA recommends tracking total carbs.
Net Carbs: This consumer-driven term represents digestible carbs. The calculation involves subtracting dietary fiber and certain sugar alcohols from the total carbohydrate count.
Sugar Alcohol Considerations: When calculating net carbs, typically half the grams of most sugar alcohols are subtracted, while the entire amount of erythritol can be removed.
Whole Food Calculation: For items without a label, like fruits or vegetables, reliable food databases, such as the USDA's FoodData Central, can provide the necessary nutritional information.
Personalized Approach: The best method for counting carbs depends on individual dietary goals and health conditions, so consulting a healthcare professional is advisable.

The Manufacturer's Laboratory Method

Food manufacturers determine a product's total carbohydrate value through laboratory analysis, although the specific method can differ by country. For example, the United States and Canada use a "carbohydrate by subtraction" method. This indirect process involves measuring protein, fat, water, and ash content and subtracting these values from the total weight of a food sample. The remaining weight is considered the total carbohydrate value. This figure includes all sugars, starches, and dietary fiber.

In contrast, countries within the European Union (EU) and Mexico primarily use an "available carbohydrate" calculation, which does not include fiber, and this is listed separately. This difference in calculation is a primary reason for variations in total carbohydrate values on labels for the same product across different regions.

Deciphering the Nutrition Facts Label

For consumers, the Nutrition Facts label is the main tool for calculating carbohydrates. It provides a detailed breakdown of the total carbohydrate content. It is essential to start by checking the serving size, as all nutritional information is based on this amount. The label lists total carbohydrates, followed by dietary fiber, total sugars (including natural and added sugars), added sugars, and sometimes sugar alcohols. Dietary fiber and sugar alcohols are not fully digested and have minimal impact on blood sugar levels.

Total Carbs vs. Net Carbs: The Consumer's Calculation

Net carbs is a term frequently used by individuals following low-carb diets or managing diabetes. It represents the digestible carbohydrates that affect blood sugar. However, it's important to note that regulatory bodies like the FDA do not regulate the term "net carbs".

How to Calculate Net Carbs

The basic method involves subtracting dietary fiber from the total carbohydrates listed on the label. Many people also subtract some or all of the sugar alcohols, as they are not fully absorbed. A common practice is to subtract half the grams of most sugar alcohols, with the exception of erythritol, which is almost completely undigested and can be fully subtracted.

For example, if a food label shows 25g total carbs, 5g dietary fiber, and 8g sugar alcohols, the net carb calculation would be: $Net Carbs = 25 - 5 - (8/2) = 16g$

Calculating Carbs in Whole Foods

When dealing with whole foods like fresh produce that lack a nutrition label, online food databases can provide the necessary carbohydrate information. Resources like the U.S. Department of Agriculture's FoodData Central offer nutritional data for numerous food items. You can search for the food, find the total carb and fiber content, and perform your calculation based on your preferred method.

Total Carbs vs. Net Carbs Comparison


Feature	Total Carbohydrates	Net Carbohydrates
Calculation Method	Calculated by manufacturers using the 'by subtraction' method.	Calculated by consumers; subtracting indigestible carbs from the total.
Components Included	Includes sugars, starches, and all dietary fiber.	Includes sugars and starches; excludes fiber and most sugar alcohols.
Regulatory Status	A legally recognized and regulated term on all nutrition labels.	Not a legally regulated term; defined and used by consumers and some manufacturers.
Impact on Blood Sugar	Less precise for blood sugar response, as it includes indigestible fiber.	A more accurate estimation of carbohydrates that impact blood sugar levels.
Best For	General diet tracking and following official nutritional guidelines (recommended by ADA).	Low-carb diets (like keto) where tracking digestible carbs is key to staying within limits.

Conclusion

Understanding the various methods for calculating carbohydrate content, used by both manufacturers and consumers, is crucial for making informed dietary choices. While the FDA and organizations like the American Diabetes Association recommend using the total carbohydrate value, those on specific diets often find net carb calculations useful for monitoring digestible carbohydrates. The most suitable method depends on individual health goals and dietary needs. If you have a medical condition, consulting a registered dietitian or healthcare professional is advisable to determine the best carb-counting approach for you.

The American Diabetes Association provides extensive resources on carb counting for diabetes management.

Frequently Asked Questions

Total carbs is the official, regulated value on a nutrition label that includes all sugars, starches, and fiber. Net carbs is an unregulated, consumer-defined term for the digestible carbs, calculated by subtracting fiber and most sugar alcohols from the total carbs.

In the United States and Canada, manufacturers use the "carbohydrate by subtraction" method. They measure the moisture, protein, fat, and ash in a food sample and subtract that total from the sample's total weight. The remainder is the total carbohydrate value.

The term "net carbs" is not regulated by the FDA, and its calculation can vary by manufacturer. For this reason, it's often more accurate to calculate net carbs yourself based on the dietary fiber and sugar alcohol information on the label.

For whole foods without a label, you can use a nutrition database, such as the USDA FoodData Central. Search for the food item to find its total carbohydrate and dietary fiber content, then make your calculation based on your preferred counting method.

Dietary fiber is not fully digested and has a minimal impact on blood sugar. Sugar alcohols are also only partially digested and can have a smaller effect than sugar, but their impact varies depending on the type and amount consumed.

Nutritional labels vary by country due to different regulations. For example, in the EU and Mexico, the labeled carbohydrate amount is for "available carbohydrates," which already excludes fiber, unlike the total carb value in the US and Canada.

The best method depends on your health goals. Total carb counting is the standard recommended by major health organizations like the ADA. Net carb counting is popular for low-carb diets, but requires careful attention to a food's specific components. Consulting a healthcare provider can help you decide.