Let's look at a well-known, yet little understood way of measuring how bitter a beer is. IBUs are often thrown around as a trendy stat on product labels and beer descriptions, and while they're a very relevant way of measuring certain aspects of a beer's flavor and aroma, they do not (in any way) make a beer better or worse. It's all about matching your preferences to the measurable facts about a beer to find that perfect sweet spot for you.
What's an IBU?
International Bitterness (or bittering) Unit
IBUs were invented because it was hard to measure how "bitter" a beer was, just like it's hard to measure how "comfortable" your favorite sweater is...it was all about the perception. Since the early 20th century, the IBU scale was introduced (and has evolved) as a way to put a number to, or quantify, this perception and assess just how bitter a beer turned out to be when it was ready to drink.
The strict definition is simple: An IBU is an empirical measurement, measured in parts-per-million (ppm) of isohumulone (which is the main chemical compound derived from hops that makes your beer taste bitter) in a given volume of beer. Isohumulone is created when the alpha acids in hops isomerize (basically dissolve or breakdown) in the boil. That's it. That's literally what an IBU is.
We want to be clear on something though. Beer is about the balance of ingredients and taste. Just because a beer has a higher IBU doesn't necessarily mean it is perceived (or tastes) to be as bitter as something with a lower IBU. You can drink a strong Amber ale rated to 60 IBU that doesn't taste nearly as bitter as a 55 IBU Pale Ale. The stronger malt flavor of the Amber ale matches the IBU's of the beer and balances them accordingly. The IBU scale simply measures the amount of the chemicals in a beer that make it taste bitter. Make sense?
Now...that being said, IBU's are generally indicative of how bitter a beer will taste. Generally speaking, the more IBU's, the more bitter it will taste.
THE IBU SCALE
International Bitterness Units are measured on a scale from 0 to....infinity basically. There's no ceiling on the IBU scale because you could make a beer more and more bitter (with more and more of the chemicals found in beer's ingredients, namely hops and malt) without an end in site. Technically speaking, there's probably a limit on how many IBU's a beer can have, simply because there is a physical limit on how many of these bittering compounds you can shove into a glass of beer. There are documented beers that have rated to over a 1000 IBU's, but it's rare to see anything nearly that high.
Almost all the beer you're ever going to drink will have an IBU rating between 5 and 120.
5 having a very low measured bitterness and 120 having a very high measured bitterness.
We're not going to get into the science of how this is actually measured, because frankly it's way above our heads and interest levels, not to mention that accurate IBU measurement involves spectrometers, isooctanes, industrial grade acids, and complicated/expensive machinery like centrifuges.
Basically, commercial breweries have this equipment, and measure IBUs, as a quality control tool. They make a large quantity of the same beer, and they want to make sure the beer tastes the same every time. Quality control is a huge part of brewing, and the IBU measurement quantifies the bitterness of the beer their brewing, allowing them to put out a consistent product.
Taste and IBUs
Here are some well known beers and their associated IBUs:
As you can see, the perceived bitterness goes up along with the IBU count on each of these beers. Heady Topper is noticeably more bitter than Sierra Nevada Pale Ale.
In conclusion...IBU's are a great way to measure a very specific aspect of your beer. They do not indicate flavor, aroma, perceived bitterness, or really any other factor that allows you to enjoy the beer you drinking, but IBU's are part of the industry, and it's worth knowing a little more about them. As you drink more beer, and hear their IBU counts, you can start to find the zone you generally prefer, and help build your own beer palate.