Here in North Carolina, wait staff at restaurants get only $2 per hour, unless the owner feels more generous, which, as a waitress related to me, goes entirely to taxes. So what is a tip?
Where personnel of a restaurant are concerned, there are two different types. There are the cooks / chefs, then there are the wait staff. Cooks / chefs get a living wage. Wait staff rely upon tips for their entire income.
When considering a tip, do not consider the quality of the food. The wait staff has absolutely nothing to do with that. For example, I took myself out to have a steak on Sunday at the local Outback. The steak left a lot to be desired. I told the waitress of my dissatisfaction with the quality of the steak. She took that too her manager, which is where it belongs. However, I gave the waitress a full 20% tip, my usual. She had done a very good job and that is why we tip.
Earlier that same day, I went out for breakfast at a local restaurant. The food was good, the waitress was good. But I got to talking to the waitress and I happened to mention how I saw a group of four people leave only a $5 tip. Now this is not an inexpensive breakfast place. The average meal will cost $20 or more. That meant the table I saw spent about $80, probably more. That meant the tip was less than 4% of the bill. My waitress related that it had been a bad tip day with some people leaving nothing. I find that to be inexcusable.
Another thing to consider is how busy the restaurant is. Did you have to wait to get a seat? Then you must take into account a harried wait staff. Meals come more slowly at such times. That they come more slowly had little to do with the wait staff.
So what should you tip? A 10% tip is low average, 15%, average, and 20% above average. I almost always give the 20% tip because I can afford it. But many years ago I was told by the head waitress at a family restaurant their take on tips. If a person leaves nothing, the think the person simply forgot but if a person leaves some pocket change, that is a signal that the waitress/waiter had done a poor job. Personally, I find it rare that a person does a poor job. To my waitress that morning, I gave her a tip which exceeded the price of my breakfast by a good amount. I stared her in the eyes, said nothing, and finally she took the money after protesting that it was too much. I told her that I was simply doing the right thing.
When you go out for a meal, always consider the tip into the total cost of the meal and prepare for that. I recommend that everyone give the wait staff a 20% tip unless they truly have done less than expected. But remember at the same time, this is what they live on.