[en] The relationship between target gain (thermonuclear energy-released/ion energy incident) and such factors as the total ion energy, the ion beam radius, the ion range, the ion power, the beam geometry, and the released target debris is critical in assessing the feasibility of a particular heavy ion inertial confinement fusion reactor concept. The length of the heavy ion accelerator will depend strongly on the ion particle energy - the ion range. The beam focus and transport both depend on the ion particle energy as does the efficiency of coupling ion energy to the target fuel. Target symmetry requirements determine whether the ion beams are clustered in two arrays for two-sided target illumination or in a symmetric configuration for multisided uniform illumination. The amount of debris energy in charged particles and x-rays impacts chamber design and determines wall loading constraints. The purpose of this paper is to present gain scaling relations that incorporate most of the critical factors