It is customary in the United States for patrons to tip their servers 15 to 20 percent of the amount of their bill before taxes. Patrons can give more or less than is customary depending on the quality of the service they receive. In most places, tipping is voluntary.
Some restaurants make tipping mandatory, including the tip amount on the final bill. Restaurants that have a mandatory gratuity policy are supposed to disclose it to patrons on the menu. If people receive exceptionally poor service at a restaurant that practices mandatory tipping, they can ask the manager of the restaurant to at least reduce the tip percentage.