Unlike some other countries, tipping is never mandatory in France: by law, service must already be included in the price shown at restaurants, cafés and bars. That doesn't mean it never happens, plenty of customers leave a little something to acknowledge good service, but it is never expected or calculated as a percentage the way it is in the United States.
Service is already included, by law
The words "service compris" on a bill or menu (or simply their absence, which means the same thing) mean the price shown already includes service. Staff are paid a fixed wage, unlike some countries where tips make up for a very low base salary. Leaving nothing extra is never rude.
A small extra is common, with no fixed percentage
Many customers round up the bill or leave a few coins to acknowledge particularly attentive service. There is no percentage rule like the North American 15-20%: a couple of euros is plenty, even on a large bill.
Cafés and bars, even more informal
At the counter, leaving small change on your way out is a nice gesture but entirely optional. Many Parisians simply leave their change on the table rather than calculating anything.
Taxis and rideshares, rounding up is enough
There's no particular expectation for a taxi or rideshare driver: rounding the fare up to the nearest euro is common but rarely noticed if skipped. Paying the exact amount shown is completely fine.
Hotels and bellhops, a slightly different case
For a bellhop or room service, one or two euros per bag carried is a fairly common practice, closer to international norms than the rest of French tipping habits. Housekeeping staff generally don't expect anything in particular.
Paying by card doesn't always make tipping easy
Unlike in the US, French card payment terminals rarely offer a tip-percentage prompt on screen. To leave something extra, it's often easiest to do it in cash separately.
Tips
If a server pushes for a tip percentage or claims service isn't "included" at a typical Parisian restaurant, check for "service compris" on the bill or the menu posted in the window before giving in. Keeping a bit of change on hand is useful for informal tipping moments (cafés, taxis), since many places don't make it easy to add a tip by card.
