Updated: Mar 10, 2021
What is an IBU?
When I was a teenager there was only one number that mattered in the beer world: ABV. No matter the brand, style or serving method, as long as you knew the ABV of the beer in your glass, you were safe. Nowadays, though there’s a new ‘number’ in town, everyone’s favourite beer metric: the IBU. Whilst IBU doesn’t tell you anything at all about the strength of your beer, it does tell you the bitterness of your beer and can give you an approximation of how hoppy the beer will be. But what actually IS an IBU and does it affect my brewing?
If you have half a day to spare and a more in-depth discussion and my complete ramblings on the IBU head over to the Dr. TankNStein blog post.
What is an IBU?
IBU stands for International Bitterness Unit and, as the name suggests, is a way of approximating how bitter your beer will be. The actual numerical value is a measure of the amount of hop-derived iso-alpha acids are present in a sample of beer. 1 IBU indicated a concentration of 1 part per million (ppm) of iso-alpha acid or in other words: 1 mg of iso-alpha acid in every litre of beer. The specifics don’t really matter, what is important is that the iso-alpha acids are responsible for beer bitterness and therefore: the higher the IBU, the higher the bitterness.
Why do I care?
You may not care. You may not like bitter beers. BUT, like it or not, 99.99% of beers out there do have an IBU rating so might as well take at least SOME note of it. This is especially true for brewers. Having an IBU target for a specific style gives a really good indication of how many hops to add to a beer and therefore a really good idea of how to balance your beer. This is because the iso-alpha acids that the IBU measures come from hops and so having an IBU target is basically the same as having a hop quantity target.
What should I do about it?
In a lab, IBU is measured using a UV-Vis Spectrophotometer which analyses the beer sample after a somewhat lengthy sample preparation procedure. Unless you have one of these at home, this probably doesn’t concern you. Thanks to the great work of a man named Glenn Tinseth, though, you don’t need any of that to make a good approximation of the IBU of your homebrewed beer. Tinseth devised a formula for estimating the IBU of a beer that can be seen below. So, once you have your style and you’ve designed your recipe, plug your hop additions into the good old Tinseth equation to see if you’re over or under on your IBUs.
IBU = Hop Weight (g) * Hop Alpha Acid (%) * Utilisation Factor * 1000 / Brew Length (L)
NOTE: Alpha Acid must be included as a decimal percentage e.g. 5% = 0.05. Hop utilisation can be calculated using a lengthy calculation (found on the TankNStein post) but there are plenty of really good correlation tables out there such as this one.