Indoor organic and perovskite photovoltaics (PVs) have been attracting great interest in recent years. The theoretical limit of indoor PVs has been calculated based on the detailed balance method developed by Shockley–Queisser. However, realistic losses of the organic and perovskite PVs under indoor illumination are to be understood for further efficiency improvement. In this work, the efficiency limit of indoor PVs is calculated to 55.33% under indoor illumination (2700 K, 1000 lux) when the bandgap (E_g) of the semiconductor is 1.77 eV. The efficiency limit was obtained on the basis of assuming 100% photovoltaic external quantum efficiency (EQE_PV) when E ≥ E_g, there was no nonradiative recombination, and there were no resistance losses. In reality, the maximum EQE_PV reported in the literature is 0.80–0.90. The proportion of radiative recombination in realistic devices is only 10^-5–10^-2, which causes the open-circuit voltage loss (ΔV_loss) of 0.12–0.3 V. The fill factor (FF) of the indoor PVs is sensitive to the shunt resistance (R_sh). The realistic losses of EQE_PV, nonradiative recombination, and resistance cause the large efficiency gap between the realistic values (excellent perovskite indoor PV, 32.4%; superior organic indoor PV, 30.2%) and the theoretical limit of 55.33%. In reality, it is feasible to reach the efficiency of 47.4% at 1.77 eV for organic and perovskite photovoltaics under indoor light (1000 lux, 2700 K) with V_OC = 1.299 V, J_SC = 125.33 µA/cm², and FF = 0.903 when EQE_PV = 0.9, EQE_EL = 10^-1, R_s = 0.5 Ω cm², and R_sh = 10⁴ kΩ cm².