Tipping norms vary enormously
Tipping is a cultural convention rather than a calculation, and the correct percentage depends entirely on where you are. In the United States, restaurant servers are frequently paid a sub-minimum tipped wage on the assumption that tips make up the difference, which makes tipping effectively part of the wage rather than a bonus — 18 to 20 percent is the common expectation for table service.
Much of Europe includes service in the price and pays full wages, so rounding up or leaving 5 to 10 percent is generous rather than expected. In Japan, tipping is not customary and can cause genuine awkwardness. Attempting to apply American norms universally is neither generous nor polite; it is simply misreading the context.
Within a single country, expectations differ by service type. Counter service, delivery, taxis, hairdressers and hotel staff all carry different conventions from table service, and the prompts on a payment terminal are not a reliable guide to any of them.
Before or after tax, and what service charges mean
Whether to tip on the pre-tax or post-tax total is genuinely contested. Tipping on the pre-tax subtotal is the more defensible position — sales tax is money going to the government, not a measure of the service provided — and etiquette guidance generally supports it. The difference is small in absolute terms, a few percent of the tip.
A service charge already added to the bill is a different matter, and worth reading for. Where a mandatory service charge or gratuity is included — common for large groups — an additional tip is not expected. Paying both is a frequent and expensive mistake on a large bill.
It is also worth knowing that a service charge does not necessarily reach staff. Practices vary by jurisdiction and establishment, and some legal regimes now require it to be distributed. Where you want to be certain the money reaches the person who served you, cash handed directly is the reliable route.
Splitting a bill fairly
Splitting evenly is simplest and is fine when everyone ordered comparably. It stops being fair when consumption differed sharply — someone drinking water and eating a salad subsidising a table sharing wine and steak is a common source of quiet resentment.
Splitting by item is fairer and more work. The reasonable middle ground is to divide shared items equally and assign individual ones, then apportion tax and tip proportionally to each person's subtotal rather than equally. Apportioning the tip equally while splitting the food by item reintroduces the unfairness in a smaller form.
Where rounding leaves the total slightly short, someone must absorb the difference. Rounding each share up to the nearest small unit and letting the surplus go to the tip is the tidiest resolution, and avoids the table hunting for coins.