IBU Calculator

In short Bitterness is milligrams of isomerised alpha acid per litre of beer, and the arithmetic that gets you there has one property worth understanding before any of the rest: THE BATCH SIZE CANCELS. IBU is hop mass over volume, so doubling the hops and the volume together does not move the number at all. That is why a recipe scales cleanly, why any hop charge is really a figure per litre, and why halving your volume while keeping the hops doubles the bitterness of what is left. The model here is Tinseth’s, which has two parts. A time term that rises towards a ceiling and never reaches it, and a gravity term that falls as the wort gets denser. Both have consequences people find surprising. BOILING LONGER STOPS HELPING, and sooner than expected: going from 60 minutes to 90 buys about 7% more bitterness, and 90 to 120 only about 2%. There is an absolute ceiling of a quarter of the alpha acid, roughly, however long you boil, because the time term approaches one over 4.15 and stops. A two-hour boil costs an hour of gas and a lot of evaporation for a handful of IBU. And stronger wort extracts less, which is most of the reason big beers need so much more hop to taste balanced. The number brewers actually judge balance by is not IBU but the BITTERNESS RATIO — IBU divided by gravity points. Around 0.5 is balanced, under 0.4 reads malty and over 0.8 is firmly bitter. Bitterness only means anything against the malt behind it, which is why 40 IBU in a light lager is aggressive and 40 IBU in a barley wine is barely noticeable. One caveat that matters more than any of the above: TINSETH’S CURVE IS A FIT, NOT A LAW. Rager and Garetz published different curves that give noticeably different answers for the same recipe, sometimes 20% apart. None is right. Use one consistently, calibrate it against beers you have actually tasted, and treat any recipe’s IBU figure as meaningless unless it says which formula produced it.