>>213056983You tip service staff.
Both
>>213057027 and
>>213057135 got it right.
In case you actually aren't from the States, aren't trolling, and legitimately don't know:
20% is customary. Anything less is either making a statement about your dissatisfaction or flat out rude. Move the decimal point over to the left one place and multiply by 2. You can round the number to the nearest dollar if you want. Rounding up is polite. While rounding down isn't really that big a deal, when we're talking a few cents, it's kind of a cheap move.
If the total bill is $74.60, to calculate the tip you move the decimal point over so its $7.46, then multiply by two. You get $14.92. I'd just round up to $15. More likely, would leave $15.40 just so the total paid is an even $90.00. You don't have to do that, but you should leave a minimum of $14.92 so the total will be $89.52. If the experience was really good, or you felt staff went above and beyond, or you need to apologize for some sort of faux pas, or you feel like flashing cash, feel free to tip more. If you're a regular at a place, being known to be a good tipper is a smart idea (though don't tip extra expecting a quid pro quo).
Some people dislike how tipping increases the cost of a bill, but it's just part of the culture in the States. Build it into your costs. If 20% is going to break the bank, then don't get the service.