The single most common overpayment by American visitors to France is a tip calculated as a share of the bill. Service is already accounted for in the price shown.
The price on the menu is the price due
French menus and cafe tariffs are required to show prices with service included, and the bill reflects that same figure without an added service line.
Staff are paid a salary rather than a reduced tipped wage topped up by customers, which removes the structural reason American tipping exists.
An American applying a percentage on top is therefore adding a bonus rather than paying a shortfall, and the amounts add up quickly across a trip.
Small change is customary, percentages are not
Leaving a small amount after good service is normal and appreciated, typically coins after a drink or a modest rounding up after a meal.
The scale is different by an order of magnitude from American practice, and a large tip reads as unfamiliarity rather than generosity.
Nothing is owed if the service was ordinary, and leaving nothing carries no social penalty at all.
Card terminals do not prompt
French card machines generally do not offer a tip screen, and the amount charged is the amount on the bill.
Where a tip is left, it is usually cash on the table, which is a habit worth keeping in a country where cash use has otherwise declined.
Asking to add a tip to a card payment is sometimes possible but often not, so a traveler intending to leave something should carry coins.
Other services follow their own conventions
Taxi fares are typically rounded up rather than tipped by percentage, and hotel porters and housekeeping receive small fixed amounts if anything.
Tour guides and drivers on private arrangements are the main exception, where a larger gesture at the end is more common.
Hairdressers, delivery and counter service sit closer to the coins-if-anything end of the scale.
What to watch on the bill instead
The line worth checking is not service but the cover and drink items, since bottled water and side dishes are charged separately and add up.
Tap water in a carafe is free on request in restaurants, and asking for it is completely routine rather than a request for a favor.
A bill that does show a service charge separately is unusual and worth querying, since the standard practice is inclusion rather than a line item. It appears most often in places used mainly by visitors, which is itself informative.
An itemized bill is normal and arrives only when requested, because a table is not cleared or turned until the diners ask, and waiting for the check to appear on its own can take a long time.