According to the Center for Food Safety and Applied Nutrition at the U.S. Food and Drug Administration, fresh tomatoes fall into the 4.3-4.9 range when it comes to acidity. (pH is the most widely used scientific method for ranking acidity, and it goes from 0-14, with low numbers being the most acidic and high numbers being the least acidic.) For canned tomatoes and tomato paste, the pH range is 3.5-4.7. For tomato juice, the range is 4.1-4.6. Vegetables like celery, broccoli, carrots, romaine lettuce, and spinach typically have a higher (less acidic) pH range of 5-7, and fruits like apples, oranges, grapes, grapefruit, and blueberries typically have a range that include this 4-5 level but also dips down lower into the pH 3-4 range. (For a helpful list of foods and their acidity, you can visit the CFSAN website.)
The acidity of fresh tomatoes can be closely associated with their degree of ripeness. The more mature and ripe, the lower the acidity, with pH approaching the 4.9 end of the range described earlier. For this reason, if a person is looking for ways to decrease the acidity of his or her tomatoes, I would suggest buying only the ripest ones, and steering clear of anything less than fully ripe.
Canned tomatoes are typically more acidic than fresh tomatoes due to the impact of the canning process. The pH of canned tomatoes can dip down into the 3.5 pH range. For this reason, avoiding canned tomatoes would be another recommendation if a person were trying to consume foods with less acidity.
Is the acidity of tomatoes a bad thing? For individuals with healthy digestive function and no allergy to tomatoes, I don't consider their degree of acidity to be a bad thing in any way. All of the World's Healthiest Foods have their own unique pH, and the pH of tomatoes is fully in keeping with the pH range that covers most fruits and vegetables.
There's another perspective, however, for addressing the issue of acidity in tomatoes. This perspective does not look at the question of acidity from a Western science standpoint but from the standpoint of other traditions involving whole body acid-base balance and its relationship to diet. In the case of these other traditions, including that of traditional Chinese medicine and the macrobiotic approach to diet, tomatoes are actually considered as the opposite of acidic.
In these traditions tomatoes are considered to be an alkalizing vegetable that will help lower the body's acidity when digested and metabolized. For example, it is the ability of tomatoes to help alkalize the blood that makes them recommended as a food that might help improve certain health problems that are understood as problems of excessive acidity. Because a slightly alkaline whole body balance is viewed as desirable in these traditions, tomatoes are including along with other alkalizing foods as those that are recommended to help restore this balance.