How to work out a tip
A tip is a percentage of the bill: tip = bill × percentage ÷ 100, and the total is the bill plus the tip. For a bill of 85.50 at 18%, the tip is 85.50 × 0.18 = 15.39 and the total is 100.89.
For a quick sum without a calculator, find 10% by moving the decimal point one place to the left. Then 15% is 10% plus half of it, and 20% is 10% doubled. On a bill of 64, 10% is 6.40, 15% is 9.60 and 20% is 12.80.
Splitting the bill fairly
Divide the total by the number of people. Shared between three, the 100.89 above is 33.63 each. Sometimes the total does not divide into whole cents. A bill of 100 with a 10% tip, 110.00 in all, between three people is 36.666... each, which cannot be paid exactly.
The calculator handles this the way people do at the table: two people pay 36.67 and one pays 36.66, so the shares add up to exactly 110.00 and nobody has to guess who covers the missing cent.
Rounding up to a whole number
Many people round the amount they pay up to a whole number so there is no change to count. Rounding raises the tip, and the calculator shows by how much. Rounding each of the three shares above up from 33.63 to 34.00 makes the total 102.00, so the tip becomes 16.50, which is 19.3% instead of 18%. Rounding only the total up to 101.00 makes the tip 15.50, or 18.13%.
Choose Round the total up when one person pays, and Round each share up when everyone pays their own part in cash.
Before or after tax, and local custom
There is no single rule about whether to tip on the amount before tax or on the total. Enter whichever amount you want the percentage applied to. Tipping customs also differ widely between countries and between kinds of places. In some places a tip is expected, in others it is a small extra, and some bills already include a service charge. Check the bill so you do not tip twice, and use the percentage that suits where you are.