For anyone who’s already taken a look at our IBU calculator (if you haven’t, do so, we’re very proud of it), you may already be familiar with what an International Bittering Unit (IBU) is, or you may be vaguely familiar, but don’t know the full story.

Firstly, before the haters start hating, we’re going to use IBU and IBUs interchangeably. The term stands for International Bittering Units and some people’s style with pluralising acronyms is to not have an s, for some it is. Do we say “this beer is 30 IBU,” “this beer is 30 IBUs,” “this beer has 30 IBU,” etc. It really doesn’t matter, and, for the sake of making coherent sentences, we’re going to use whatever feels right for that sentence in particular. So consistency be damned! Although feel free to comment with your preferred term.

With that aside, as mentioned, IBU stands for International Bitter Unit. Obviously this is some sort of measurement of the bitterness of a beer. Along with calculating a beer’s alcohol, and its turbidity, the bitterness of a beer goes a long way towards what it is; that is, there are guidelines on these factors that will categorise a beer style. Get too bitter and that easy pale ale you’re home brewing is getting mighty close to becoming and India Pale Ale.

On that though, it’s important to balance maltiness, alcohol, and bitterness when making a beer. While bitterness can be a good thing, if it’s all by itself without some sort of body in the backbone to back it up, it can come across as harsh and unpleasant. That’s what the IBU calculator is for. Combining this with proper mash temperatures will ensure the body is right for the amount of bitterness in a new home brew recipe.

So Why is it Called IBU?

It’s a strange and arbitrary name. Similar to the unit of heat called “British Thermal Units.” Usually units are named after someone or something, or have a name of some sort to make them unique. Bittering Units just sounds like whoever was studying how bitter something is wasn’t creative at all. Why not just call it “Bitters” as in “this beer has 50 Bitters.” Or “Puckers,” or “Aceribics,” or “Hopsies.” Anything better than generic units!

Well, the International Bittering Units scaled was created purely for measuring the bitterness of beer, and it seems the American Society of Brewing Chemists, who came up with the system, were more concerned with describing beer than they were about naming their new scale.

The IBU scale, which was developed by the American Society of Brewing Chemists, looked to get a good, quantitative number on how bitter a beer was. Without a definitive number, all brewers have to go on is the perceived bitterness of a beer. Perception isn’t very scientific, plus what would person may perceive as extremely bitter, the next may only perceive as mildly bitter. So a good, scientific way of working out how much bitterness is in a beer is needed so that consistent numbers that we can all agree on can be used.

Enter the IBU scale!

The IBU scale measures the amount of iso-alpha acids in a beer or wort through spectrophotometry. Iso-alpha acids are produced when hops are boiled and are what give beer its bitterness. Because of this, any scale that wants to measure bitterness (in beer anyway) needs to look at how much iso-alpha acid is in that beer. Iso-alpha acids absorb and reflect different wavelengths of light, and , when a spectrophotometer is tuned to the right wavelengths, it’s easy for chemists to measure exactly how much of the acid is in a solution.

Adding to this, as mentioned, the International Bittering Unit scaled has nothing to do with what is perceived. Often bitterness can be masked by other flavours such as roasted or smoky flavours. Or if a beer is particularly sweet it may balance well with the bitterness and give more neutral flavours. However masked, a beer can still have high IBUs because of the amount of iso-alpha acids in it.

This is why brewing is such a balancing act. It’s the balance of flavours that make a beer great. Flavours push and pull each other to hang in the perfect unison of one great beer. Get one a little off, and the whole beer is out of harmony. An amazing beer turns into an OK beer.

And that’s about it really. All that is happening is a measurement of some simple acids. No witchcraft or (too) fancy science.

Calculating IBU for Home Brew

Unless you have a spectrophotometer at home, any calculation of IBU is going to be an estimate only. But with simple tools, it’s easy to calculate the IBU of home brew. What tools like this do is estimate how much alpha acid has been converted to iso-alpha acid through boiling. This is done by knowing how much alpha acid is in a given hop, how long it’s been boiled, and what the specific gravity of the boil is.

Of course, there’s no way to be 100% accurate through this method. The hops used at home may be stale, the exact alpha acids percentage may be slightly off, and the gravity of the boil changes over time. But at home calculations are good enough for home brewers to know where they’re aiming within a few percent of the real number.

So mathematically deriving the IBU of a beer is OK. Even if it’s not 100% doesn’t mean it shouldn’t be done. It gives a very close approximation and lets brewers know they’re in the right ball park.

But at least we all know exactly what’s being measured.