Rager, Garetz, and Tinseth Bitterness Formulas

Norm Pyle’s Hops FAQ: “An IBU is defined as 1 mg/l of iso-alpha-acid in a solution. By estimating IBUs rather than HBUs, the brewer can get a more accurate (though admittedly still rough) approximation of the bitterness imparted into the beer by the hops. It is independent of batch size so that a 5 gallon batch with 29 IBU’s has the same bitterness as a 50 barrel batch with 29 IBU’s. The equations are commonly quoted from Jackie Rager’s article in the “Zymurgy” Hops and Beer Special Edition published in 1990. Revised numbers and formulae have recently been presented by Glenn Tinseth and Mark Garetz, in separate works. Rager has been taken to task for not supplying enough background references, and not fully explaining how he got his numbers. In general, his utilization estimates are believed to be optimistic. Garetz has been accused of extrapolating scant laboratory information, and overgeneralizing because of it. His numbers have been labelled unrealistic on the pessimistic side. Tinseth has just presented a revised method and set of tables, and though they are thought to be quite accurate, they have not stood the test of time. The calculated numbers tend to fall in between Rager’s and Garetz’s. Note also that these are all estimates. Actual IBUs can be measured in a laboratory, but the average homebrewer has no access to such equipment. The Rager, Garetz, and Tinseth estimation methods follow.”