<human>:  How do I calculate a tip in a restaurant in the US?
<bot>: In the United States, it is customary to leave a tip of 15-20% of the total cost of the meal, depending on the service you received. To calculate a tip, simply take the total cost of the meal and multiply it by the percentage of your desired tip. For example, if the total cost of your meal is $30 and you would like to leave a 20% tip, multiply $30 by 0.20 to get the tip amount of $6.