I have a new calculation that I've been using when determining the expected balance of my beer recipes. It is a child of the commonly-used Bitterness Ratio (BU:GU), and the numbers output can be read in the same way as BU:GU. However, the one thing that is taken into account with the Relative Bitterness Ratio (RBR) that BU:GU does not account for is Apparent Attenuation (ADF).
The higher the degree of Apparent Attenuation (ADF), the more fermentable sugars are consumed and the less residual sweetness is left behind. That means that as ADF gets higher, beer balance tends more toward the bitter end of the scale. As ADF gets lower, beer balance tends more toward the sweet end of the scale.
For example: A beer that starts out at an OG of 1.050 at 25 IBU would be said to have a Bitterness Ratio of 0.5. If it were split into two batches and one had an apparent attenuation of 80% (Beer A), while another had an apparent attenuation of 60% (Beer B), Beer A would be perceived to be more bitter than Beer B, as the latter has considerably more residual sweetness.
Beer A would have an RBR of 0.517 while Beer B would have an RBR of 0.417--a fairly big gap in perceived balance.
In the pages linked below, you can find a plethora of information explaining details about the Relative Bitterness Ratio (RBR). If you aren't interested in the details, I've included the formula for figuring out RBR as well as a simple RBR calculator
if you just want to input some numbers and get the results.
To quickly explain the formula:
RBR = Relative Bitterness Ratio. ADF = Apparent Attenuation. 0.7655 is the average ADF of all beer styles (according to the BJCP style guidelines). Since the Relative Bitterness Ratio takes into account balance relative to all beer styles, it uses this as a constant. You are comparing your beer's ADF against the average ADF (0.7655), then adjusting the standard Bitterness Ratio accordingly (it goes up if your ADF is higher than average, down if your ADF is lower than average). Just like BU:GU, higher numbers mean more bitter, lower numbers mean less bitter, and 0.5 is roughly average balance.
RBR = (BU:GU) x (1 + (ADF - 0.7655))
You can see the full original post (basically the same as this one) over at Mad Alchemist
, but I'll be happy to read and respond to posts here.