What is an IBU?

I recently had a friend ask me what an IBU was? It stands for International Bitterness Unit. In short, it is a rating for the bitterness associated with a beer. Me being a hophead, I enjoy a higher IBU in my beer. I can't think of anything better than a Double IPA which has an even higher IBU rating on the IBU scale.

And every beer that you will drink will have a rating on the IBU scale. For instance, your lambic beers and blonde ales have a very low rating on the scale, where an irish stout has a more significant IBU rating than other beers.

It is my opinion that this has a lot to do with what beers most people will drink. If someone does not like a bitter beer, they are not going to like the IPA's or stouts, but will drink your golden ales or lagers.