Recently perovskite/crystalline Si (cSi) tandem cells draw considerable attention because their high efficiency can reduce the levelized cost of electricity and increase the power density of photovoltaics (PVs) to accelerate the energy transition. While the theoretical limits for tandem cells are well known, the practical limits are less clear. Herein, a new method is presented to calculate the efficiency of a four‐terminal (4T) tandem based on the performance of single‐junction perovskite and cSi cells, using their detailed‐balance efficiency fraction. This calculation method is validated with experiments on 4T perovskite/cSi tandem cells that provide a maximum efficiency of 28.0% and with the literature data available for similar configurations. A maximum efficiency of about 36% is estimated for 4T perovskite/cSi tandem cells that would use present record perovskite and cSi PV cells. This can be regarded as the practical efficiency limit for 4T perovskite/cSi tandem devices.