This varies in two ways: state to state and the person.
Wait staff can be paid as little as half the applicable minimum wage in some states. And they have to declare all their tips. one can make as little as minimum wage.
Along with those rules are other factors. Table turns, night of the week, speed of the kitchen, etc etc.
20% is considered customary for "good service" in the restaurant industry.
(15% is more of a base amount for services rendered, and is considered an average tip.)
The minimum compensation is no less that $2.13 per hour. Pay is at the employers discretion. However, if gratuity plus "regular" pay equals less than minimum wage, federal law requires employers to compensate the employee. You'll rarely see a "regular" wage of any more than the federally required amount.
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