How much to tip
In the United States, 15% is the low end for adequate table service, 18% to 20% is standard, and above 20% signals genuinely good service. Counter service, takeaway and coffee typically attract less or nothing. Elsewhere the norms are entirely different: much of continental Europe rounds up or leaves 5% to 10%, Japan does not tip at all and attempting to can cause awkwardness, and in Australia and New Zealand it is appreciated but not expected.
Tip on the pre-tax amount
The convention is to tip on the food and drink total before sales tax. On a large bill in a high-tax state the difference is real — 20% on a $200 bill with 9% tax is $40 pre-tax and $43.60 post-tax. Enter the pre-tax subtotal in the bill field to follow the convention.
Watch for service already added
Large groups and many restaurants outside the US have a service charge of 12% to 20% already on the bill. Adding a tip on top duplicates it. Check the itemised lines before you calculate — anything labelled service, gratuity or couvert is already a tip.
Splitting without arguments
An even split is fastest and fairest when everyone ate roughly the same. When one person had a starter and three drinks, split the tip evenly by headcount and the food by what each person ordered. Rounding each share up is a graceful way to close the gap that the arithmetic leaves behind. The percentage calculator handles any other percentage you need on the fly.