Whey protein powder is generally low-carb or zero-carb, but there are some exceptions such as whey protein concentrates which retain some carbohydrates. Whey powders that use whey concentrate as the main ingredient can include fairly high levels of carbohydrate content meaning they are definably not low-carb. We generally recommend avoiding this less processed form of whey, in part because of its carb content. However, whey isolates have negligible levels of carbs or fats and whey hydrolysates have none. These make great low-carb sources of whey protein.
To check you can review the nutritional facts label which should show the carbohydrate content as well as list the primary whey protein ingredient as the first listed ingredient. We recommend choosing a whey protein powder that lists whey isolate as the main ingredient.