Short Answer: The widely circulated industry standard says you should use about two tablespoons of ground coffee per six ounces of water. Do this, and you will live.
More Complicated Answer: For most of us brewing at home, the above guideline will suit us just fine. For the crazies out there, or for people that want to brew a significantly more amount of coffee than, say, 6 cups or so, it starts to get a little more complicated. I'll show you a couple of reasons why: Basically the two tablespoons per six ounces model (hereafter the 2t6) is basing its assessment off of the volume of the bean and not it's ground weight. So lets say you get a very dense bean, you'll get a stronger cup vs. a weaker cup with a less dense bean... "Ok then," you say, "how many grams of coffee per ounces of water?" HA! You think you've got me. Well my friend, further complicating this process is that the coffee to water ratio, no matter if you're basing it off of volume or weight, is not necessarily "scalable." Some brilliant math on someone else's part basically says that while 2t6 works for a small cup of coffee, if you want to brew a larger amount you end up using a whole pound of coffee for only three quarts of coffee. Too much.
A good cup of coffee, no matter how you brew it relies on two things:
1. Extraction Percentage
2. Brew Strength
Let's tackle the first one... 1. Extraction Percentage This is the amount of stuff you're actually pulling out of the coffee bean and putting in your cup using the lovely medium of water. 28% of the roasted coffee bean is water soluble. This means that if you brew it long enough, 28% of that coffee bean is going into your coffee cup. The problem is (there are many problems apparently) is that you only want, at maximum, 22% of that bean. Why you ask? Because the most advanced science known to coffee says that last 6% is just plain gross. It's the bitter, tanic, crappy parts of coffee that only pigs and maybe Pig-Pen enjoy. To get that magic percentage out of the bean is a blog for another day but suffice it to say that it has to do with how fine you grind your coffee and how long it's exposed to the brewing water.
So now you've extracted just the right amount out of the coffee bean we move on to our next topic: 2. Brew Strength You need to know how much of the above magic extraction percentage goes into the proper amount of water which brings us to a short but necessary explanation: The amount of coffee extraction to the rest of the beverage should equal somewhere around 1.3%. Here's a little graphic shamelessly stolen from SCAEurope to give you a better idea of how this relationship works:
So this is the goal: 18-22% extraction rate for 1.3% of the finished beverage. Everything else starts tasting either bitter, weak, too strong, underdeveloped or a combination of some sort.
"So how do I do this???" you may ask with your eyes long-glazed over but still anxiety-ridden after reading this verbacious blog. Outside of using a refractometer I would say one of my favorite phrases in coffee land- use your tongue. Seriously after all that I am going to tell you to just taste your coffee. If your coffee tastes a little overextracted and bitter, try making the grind a little courser or don't let it steep for too long. If seems a little weak or underdeveloped, try adding a little more coffee or making the grind a little finer.
So start with the two tablespoons method if you want to get it in the ball-park, then taste from there. Coffee is one of those things that allows for endless experimentation and you can get as technical or free-flow as you want. It's about what you like.
Here's a nifty little chart if you want to see someone's take on brewing ratios (I haven't experimented with these ratios myself so be forewarned...)
If you've an iPhone, Intelligentsia has one of the best apps out there that I've seen for brewing...
Rose Park Roasters
Long Beach, CA