Determining the capacity of a modulation scheme is a fundamental topic of interest. Index Modulations (IM), such as Spatial Modulation (SMod), Polarized Modulation (PMod) or Frequency Index Modulation (FMod), are widely studied in the literature. However, finding a closed-form analytical expression for their capacity still remains an open topic. In this paper, we formulate closed-form expressions for the instantaneous capacity of IM, together with its $2$nd and $4$th order approximations. We show that, in average, the $2$nd approximation error tends to zero for low Signal to Noise Ratio (SNR) and is $o\left(\textrm{SNR}\right)$. Also, a detailed analysis of the ergodic capacity over Rayleigh, Rice and Nakagami-$m$ channel distributions is provided. As application of the capacity analysis, we leverage the proposed expressions to compute the ergodic capacities of SMod for different antenna configuration and correlations, PMod for different channel components and conditions, and FMod for different frequency separations.