Work out hop utilisation ibu instantly with clear inputs, formula shown and shareable results.
Tinseth's model computes utilisation as the product of a bigness factor, which falls with wort gravity, and a time factor of (1 - e^(-0.04t))/4.15. IBU is then hop mass times alpha acid fraction times utilisation, scaled to volume. Utilisation peaks around 30% at long boil times in low-gravity wort.
Tinseth utilisation
Utilisation = 1.65 x 0.000125^(SG-1) x (1 - e^(-0.04t)) / 4.15
IBU
IBU = hop mass (g) x alpha acid fraction x utilisation x 1000 / volume (L)
Isomerised alpha acids are less soluble and are also adsorbed onto trub and yeast, both of which increase in dense wort. Strong beers need proportionally more hops for the same bitterness.
Some, but far less. Below about 80 C isomerisation slows sharply, so whirlpool hops contribute aroma and modest bitterness that Tinseth's model does not capture well.