IStock Photo 305352 © Nathan Maxfield

Your dinner is over, the bill has been paid, and the only thing standing between you and the door is that pesky post-meal math problem: tipping. Questions over who, what, and when to tip often make the simplest excursions—a cab ride to the airport, or an afternoon at the salon—end on an uncomfortable note. Tipping, it seems, is confusing business.

Making heads or tails of tipping etiquette can be difficult. These days even stay-behind-the-counter types like baristas and shopkeepers keep a tip jar next to the cash register. Who should you tip, and when should you keep that wallet tucked away? Americans seem to agree on some basics: waiters, for starters. The odds an adult usually tips a waiter are 1 in 1.02 (98%). Ditto for food delivery people; 85% of adults (1 in 1.17) will offer a tip to friendly folks who show up bearing pizza or Chinese food.

In some other arenas, though, the rules are less clear. The usher who helps a theater-goer find the right seat. Is he waiting for a tip? If he’s not a volunteer, then maybe. While 1 in 4.71 adults usually tip an usher, 1 in 1.27(79%) don’t.

It’s not enough simply to figure out who to tip, you’ve also got to figure out how much. According to travel supplier Magellan’s worldwide tipping guidelines, how much to tip varies widely by country. In America, it’s customary to tip waiters between 15 - 20% of the total bill, while cab drivers receive from 10 - 15%. If a porter is toting your luggage, tipping one or two dollars per bag should do.

But what if that porter drops your suitcase, spilling your socks (and worse!) all over the hotel lobby? Or what if your waiter stands chatting with the bartender when you’re ready to order dessert, and is still there when you want to settle up? Is a tip still in order?

According to studies published by Professor Michael Lynn, an expert on tipping at the Cornell University School of Hotel Administration, tip percentages are only weakly related to customers’ ratings of the service quality they received. For restaurant diners in particular, other factors influencing tip percentage include the size of the dining party, whether or not the customer pays by credit card, and the sex of both the customer and the waiter. Lynn’s research found that men left larger tips than women in cases where the server was female, and women tipped more than men when served by a male.