How much should you tip?
The generally accepted value is 15% to 20%, though outside North America it may be different.

If service was horrible, tip nothing and notify management. If the service was slow, tip 10%. If service was ok, tip 15%. If service was great, tip 20%.

Keep in mind if the service is slow it is not always the servers fault. If you ever plan to return then the service was good enough that you should tip something. If the service was so poor that you plan not to tip, then it was also poor enough that you should notify management.

How do you figure the tip without a calculator?
So, you're at your favorite restaurant without an Internet connection and you left your trusty calculator in your other pocket protector? All is not lost, there is a somewhat easy way to calculate the tip.

15% = 10% + 5%. So what you need to do is figure out 10% then add half (5%).

10% is very easy to calculate, you just move the decimal place one to the left. For example, if the bill is $25.00 then 10% would be $2.50.

Then to figure out 5%, just take half of that, which would be $1.25.

Now you have 10% which is $2.50 and 5% which is $1.25, add them together for a total of $3.75.

Often, the bill ends up at some obscure value such as $17.73, which may be a bit difficult to calculate using the above method. In cases like that, just round it to the nearest dollar. It would be much easier to calculate the tip for $18.00.