What is the definition of a cup of coffee

The most common global standard for recipe and marketing purposes** is 150 ml (5 oz), with a caffeine content around 100 mg. This commonly refers to instant, filter, or drip coffee, or a mixed espresso coffee like a latte

Coffee is traditionally served in a smaller cup than other hot beverages, mainly due to strength and expense. For example, an Italian espresso cup is around 30 to 60 ml (1 to 2 oz) in size, with a caffeine content a little more then 100 mg

Some references

1. http://www.ico.org/caffeine.asp
2. http://www.scaa.org/?page=resources&d=cupping-standards

** e.g. marketing description of a "6 cup coffee maker" refers to 6 x 150 ml cups

In the US, generally a "cup" of coffee is six ounces. Unfortunately, that still doesn't answer your concerns as the strength of coffee varies widely. Is a "cup" on a coffee maker always 6 oz? Is this a standard in the US?

A standard cup according to the Specialty Coffee Association of America is defined as 8.25 grams of roasted coffee beans in 150 ml of water.

It's not the size of the cup, it's the amount of coffee in it. But if you want a more objective answer, things start to get murky because different coffee experts define a cup different things depending on their agenda.

Most of the big-name percolator companies including KitchenAid, Bunn, Mr. Coffee, etc. advertise a "cup of coffee" as being 5-oz. because it makes their coffee pots seem larger.

Folgers says a "cup of coffee" is 6 oz. because they work with these percolators, but they want people to use more coffee grounds.

Starbucks sells cups of coffee at 8 oz. at the lowest because what do they care? They want to make customers happy.

It gets even more muddled when you start getting into commercial urns and percolators who consider a cup anywhere between the demitasse size of 2 ounces to the imperial size of 8 ounces.

It's actually a very controversial topic. There are some camps who believe that a cup is 5 ounces always and there are other camps who think we should only go by the 8-ounce cup for simplicity.

If you're interested, there's more information about the debate here: http://www.jesrestaurantequipment.com/jesrestaurantequipmentblog/coffee-carafe-sizing/

Drip percolators (the kind on many/most kitchen counters in the US) usually count a cup as between 4.5 - 6 oz. So, a 10-cup coffee machine brews 60 ounces (depending on the brand). If you pour those 60 ounces into the also very common 12-oz 'to-go cup' (or cofee mug, for that matter), you have 5 servings.

As you see in the other answers, there are different kinds of coffee and different kinds of cups (the ones I just mentioned are not the standard-measure 8 ounce cups).

I'm not sure what a health guru is, but I have seen claims of the reverse - that more coffee is healthier. I'm inclined to agree.

I did a small measurement on my Mr. Coffee pot and got between 5.2 - 5.5 U.S. fluid ounces per "cup", definitely not the six. I then looked at the instruction manual for a Mr. Coffee coffee maker, and it says one "cup" is equal to 5 ounces.

Short version:

Effective July 16, 2016 in the US, the official serving size of a cup of coffee is 12 ounces.

Long version:

For purposes of nutritional labeling, the US Food and Drug Administration has set "Reference amounts customarily consumed (RACC) per eating occasion" for common foods.

The official reference amount for a sweetened cup of coffee was 8 ounces (240 mL).

Interesting, that black coffee has no reference amount. I believe that black coffee is exempt from nutrition labeling (and reference amounts) because black coffee has scant nutritional value.

You notice I stated "was 8 ounces." Effective July 16, 2016, the official description of a cup of coffee will be 12 ounces (21 CFR 101.12 it's about a quarter way down the page).

The Federal Register entry announcing the change said:

Since that time, consumption patterns have changed so that the RACC for some beverages has increased from 8 oz to 12 oz. Because the consumption amount has increased for certain beverages, such products for which the RACC has increased may appropriately no longer be able to make ‘‘free’’ claims. As noted previously, we intend to consider in a future rulemaking issues such as whether any changes in eligibility for claims would assist consumers in constructing healthy diets and whether the criteria for claims remain appropriate.