The crack susceptibility curves (crack susceptibility vs. solute content Co) of binary Al alloy systems were calculated by plotting the maximum │dT/d(fS)1/2│ vs. Co , where T is temperature, fS fraction solid, and the maximum │dT/d(fS)1/2│ a crack susceptibility index proposed and verified recently. Two simple equations were derived for binary alloys to calculate the location (Co)peak and height │dT/d(fS)1/2│peak of the peak of the crack susceptibility curve without having to calculate the curve first. The effect of the eutectic line, liquidus line and solidus line of a eutectic phase diagram, which is typical in Al alloys, on the crack susceptibility was demonstrated. First, │dT/d(fS)1/2│peak tends to decrease when the eutectic line is raised, the liquidus line steepened, or the solidus line made shallower. Second, back diffusion tends to increase (Co)peak and decrease │dT/d(fS)1/2│peak. These tendencies agreed with the two equations and with the reported crack susceptibility of Al-Si and Al-Mg alloy systems.