The average lime contains 1.38 grams of citric acid. This varies in reality according to the size of the lime, its liquid content, its ripeness and the season.
According to a study led by Dr. Kristina L. Penniston, entitled "Quantitative Assessment of Citric Acid in Lemon Juice, Lime Juice and Commercially-Available Fruit Juice Products," lime juice contains an average of 1.38 grams of citrus acid per liquid ounce of juice. If, as according to the J.C. Tropicals website, the average lime contains 2 tablespoons of juice, we can convert tablespoons to liquid ounces and find out how many grams are to be found in that volume of juice. Since one tablespoon is equivalent to half an ounce, 2 tablespoons gives us one ounce of juice, and, as we know from the study, each ounce of lime juice contains 1.38 grams of citric acid.