3 Answers

If you are traveling 10% is reasonable, unless the service is poor, do not leave any tip..

If you are somewhere that you patronize OFTEN, 20% will generally get you better service every time you return

If there is a TIP JAR, where the served item is FREE, like the drink server at an affair, hand the person a generous tip PERSONALLY.

When that person serves you again they will be more attentive, expecting subsequent generous tips which may or may not come, however   ;-)

Multiply the bill times the standard gratuity rate of .18.  ( or .15 or .20)  Ex.  $42.78 x .18 = $7.70. 

Tip Calculators

Source

A tip (also called a gratuity) is a voluntary extra payment made to certain service sector workers in addition to the advertised price of the transaction. Such payments and their size are a matter of social custom

(If eating out, 20% is standard)

T.I.P. comes from To Insure Prompt Service..........