The arithmetic of a tip
A tip is a percentage of a base amount, and the only real decision is which base. Multiply the base by the percentage and divide by 100. Twenty percent is the easiest to do in your head: move the decimal one place left to get 10 percent, then double it. For 15 percent, take 10 percent and add half of it. For 18 percent, take 20 percent and subtract a tenth of that.
Worked example with the defaults: a bill printed at $64.80 that includes 8 percent sales tax has a pre-tax subtotal of 64.80 / 1.08 = $60.00, so the tax was $4.80. A 20 percent tip on the pre-tax subtotal is $12.00, making the grand total $76.80, or $38.40 each for two people. Tipping 20 percent on the full $64.80 instead would be $12.96, and the total $77.76. The difference, 96 cents, is what the tax-base argument is worth on a bill this size.
US tipping norms
In American restaurants with table service, 18 to 20 percent is the current standard for good service, 15 percent reads as adequate or slightly disappointing, and 25 percent or more signals that something was excellent. The floor has drifted up over the past two decades: 15 percent was standard in the 1990s. Etiquette guides accept tipping on the pre-tax subtotal, and that is what this calculator does by default, though many people simply use the printed total because it is faster.
Other common rates: 15 to 20 percent for a bartender or at least $1 to $2 per drink, 15 to 20 percent for a taxi or rideshare, 20 percent at a hair salon, $2 to $5 per day for hotel housekeeping left daily, $1 to $2 per bag for a bellhop, and 10 to 15 percent for food delivery with a $3 to $5 minimum in bad weather. Counter service with a tablet prompt is genuinely optional; nothing is expected for simply handing you a coffee, though a dollar or 10 percent is a kindness. Tipped servers can legally be paid a federal cash wage of $2.13 an hour as long as tips bring them to the full minimum, which is why table-service tipping is not treated as discretionary in practice.
Splitting the bill
The simple split divides the grand total by the number of people. It is fair when everyone ordered roughly the same thing and it avoids twenty minutes of arithmetic at the table. When one person had the steak and another had a salad, split by item instead: each person's share of the tip and tax should be proportional to what they ordered, which you get by multiplying their item total by the grand total divided by the subtotal.
Rounding up is the friendliest way to handle the odd cents. Rounding each person's share to the next whole dollar collects a little more than the target and the surplus goes to the server, which is the option most groups prefer. If the party was six or more, look for an automatic gratuity already printed on the check, usually 18 to 20 percent; adding a second tip on top of it is a common and expensive mistake. Service charges that go to the house are not tips, and in several cities the check will say so explicitly.