Tipping in the US and UK

A tip is a small amount of money given as a gift, usually in addition to the official price, for a small service performed:
Shall I leave a tip for the waiter?
a tip of 10%
The action of giving a relatively small amount of money for services rendered (as by a waiter) is called tipping.

Cultural Note: Tipping in the US and UK

In the US and UK, people tend to give tips for particular services, although tipping is much more common and important in the US. In restaurants in the US, people usually give the waiter between 15 and 20% of the bill, but in the UK they leave about 10% of the bill. If the service is bad, however, people in both countries give a smaller tip or do not leave any tip. It is normal to give a tip to porters who carry your bags at the airport or in a hotel, and to give a 10% tip to taxi drivers and to people who cut your hair.