Work out the tip, then divide
Multiply the bill by the tip percentage, then divide the bill plus tax plus tip by the number of people. A calculator that only gives you the tip leaves you doing the second step in your head at the table, so this one gives the per-person figure.
Whether to tip on the pre-tax amount or the post-tax total is genuinely contested. Etiquette guides say pre-tax; the receipt prints the post-tax total in large type. The gap is small — on a $100 bill with 8% tax, an 18% tip is $18 versus $19.44.
There is an option to round the total up to the next dollar, which is what most people do when writing a tip on a card slip. The rounding difference is added to the tip.
Splitting the bill
Many restaurants add an automatic 18-20% gratuity for parties of six or more. Tipping again on top means paying twice. Look for a 'service charge' or 'gratuity' line on the receipt first.
An even split becomes unfair when people ordered very differently — particularly when some drank and others did not. In that case it is better for each person to apply the same tip and tax percentage to their own items.
Do not carry US norms abroad. Japan and Korea have no tipping culture, and most of Europe includes service, where 5-10% is plenty. The 18-20% figure is a US number.
A map of the customs
Countries fall into three groups. First, those that do not tip at all: Japan, Korea, China, Taiwan and Singapore. Leave money on a Japanese table and a server will follow you outside to return it; in China the practice long carried associations with bribery. Singapore and Taiwan substitute a 10% service charge on the bill.
Second, those that round up or add a little: most of western Europe. France includes service (service compris), so a couple of euros is enough; Switzerland has had service in the price since 1974; Sweden expects no more than rounding to the nearest krona. Germany runs 5-10%, handed over as you pay rather than left on the table. Australia and New Zealand do not expect one at all, because their minimum wages do not depend on tips.
Third, those where a tip is clearly expected: the United States at 18-20%, Canada at 15-20%, and India, the UAE, Egypt, Saudi Arabia, Israel and South Africa at 10-15%. Tipping feels obligatory in the US because tipped workers have a separate minimum wage — a federal cash wage of $2.13 an hour, with tips expected to bring earnings up to the minimum.
What is printed on the bill also differs. Italy's coperto and Argentina's cubierto are cover charges, not tips, so treating them as one means the server gets nothing. A British 12.5% service charge or a Brazilian taxa de serviço is the opposite — it stands in for the tip and needs nothing on top. India and the UAE are the awkward case: the charge is there, but it often does not reach the staff, so tipping separately has become the custom.