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IBU

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International Bittering Units attempt to quantify the bitterness of beer. IBUs have units of mg/L and 1 IBU is 1 mg of isomerized alpha acid per liter of beer. As an example, lite American lagers have 8-12 IBUs, and imperial IPAs typically have 60-120 IBUs.

Short of sending your beer off for analysis, you may only estimate the bitterness using one of a number of formulae. [1]

[edit] Tinseth's Approximation

IBUs = 1.65 * 0.000125^(SG - 1) * ((1 - e^(-0.04 * t)) / 4.15) * ((AA * m * 1000) / V)

...where SG is specific gravity, t is the time of the hops in the boil, AA is the alpha acid rating of the hops (i.e. 7.0% alpha acid means AA = 0.07), m is the mass of the hops in grams, and V is the final volume of the wort in liters.

[edit] Rager's Approximation

U = 0.1811 + 0.1386 * tanh[(t - 31.32) / 18.27]

G = (SG - 1.050)/0.2 if SG > 1.050, otherwise 0

IBUs = (U * AA * m * 1000) / (V * (1+G))

...where tanh() is hyperbolic tangent, SG is specific gravity, t is the time of the hops in the boil, AA is the alpha acid rating of the hops (i.e. 7.0% alpha acid means AA = 0.07), m is the mass of the hops in grams, and V is the final volume of the wort in liters.