Before I describe how to calculate net carbs, I should mention that this is only necessary in the US. This is because food is labeled differently in other countries.
The US Food and Drug Administration (FDA) requires that every packaged food item have a "Nutrition Information" label. On this label, the "Total Carbohydrates" will be prominently displayed in grams per serving. Indented under this, if the food item contains fiber (some food items don't), the amount of fiber will be listed in grams per serving. To calculate the net carbs, simply subtract the number of fiber grams from the total carbohydrate.
You will not see "Net Carbs" or "Net Carbohydrates" listed on the food label. You will need to do the simple arithmetic described above to calculate the "net carbs." Be sure to pay attention to serving size and how many servings are in the entire package.
If the food item does not have any fiber, it will not say zero fiber. It just won't mention fiber. In this case, the net carbs are the same as the total carbs.
This method works for all US food labels but it does not work for European food labels and other food labels around the world such as those found in Japan. This is because it is not necessary to calculate "net carbs" on other food labels because other countries do not include the fiber carbs when they list total carbs on their food label. So, the total carbs or simply listed as "Carbohydrates" is the net carbs.
The difference in food labeling between countries becomes very important when dealing with imported foods and when looking up information online. You need to always be aware of the source when reading food labels to calculate net carbs. This became very apparent to me when comparing dark chocolate labels. However, keep in mind that some companies that are headquartered in one country will have a subsidiary in another country that uses that follows that country's food label laws. For example, the Swiss chocolate company Lindt has a subsidiary in the US with and a US version of their website that has nutrition information that follows US laws.
For more information on net carbs, click here.