United States
Tipping is generally expected at 15-20% of the pre-tax bill at sit-down restaurants, with 20% increasingly common as a baseline. Many US service workers rely on tips as a significant part of their income, which is why tipping norms are higher than almost anywhere else.
United Kingdom and Ireland
A service charge of around 10-12.5% is often already added to restaurant bills, especially for larger groups — check before tipping again on top. If no service charge is included, rounding up or adding roughly 10% for good service is typical, though it's less rigidly expected than in the US.
Continental Europe
In much of Western Europe (France, Germany, Spain, Italy), a service charge is frequently built into the price already. Rounding up the bill or leaving small change is a common, appreciated gesture, but leaving nothing at all generally won't be seen as rude the way it would in the US.
Japan
Tipping is not customary and can occasionally be seen as confusing or even mildly offensive, since excellent service is considered the standard, not something requiring extra payment. Attempting to leave a tip in most restaurants may result in staff trying to return the money to you.
Australia and New Zealand
Service staff are paid a full minimum wage regardless of tips, so tipping isn't culturally expected. Rounding up or leaving a small amount for exceptional service is appreciated but entirely optional.
A simple rule of thumb
When traveling somewhere you're unfamiliar with, a quick search for "tipping etiquette in [country]" before you go can save an awkward moment — norms genuinely vary this much, and there's no single global standard.