V = W / SG

V is the volume in liters
W is the weight of the beer in kilos
SG is the specific gravity, e.g. 1.040

(lb in grams)/(UsGal in ml) => scaling facator

454 / 3785 => 0.11995 => 0.12 (2 d.p.)

V(USgal) = (0.12) * W(lb) / SG

= 0.12 * 1 / 1.002
= +/- 0.12 USgallons

SG_1_vol_CO2 = 1.82/1000
             = 0.0018 SG

= 1.82 * 2.4 / 1000
= 0.004 SG