Elastomeric proteins made of tandemly arranged individually folded globular domains have been used as building blocks to construct protein hydrogels with tailored mechanical properties. However, classical rubber elasticity theory cannot be directly used to describe the mechanical properties of such protein hydrogels, due to the fact that only part of the structure of such folded globular domains contributes to the elastic properties of the hydrogel network. Here, we present a length equivalent molecular weight approach to adapt the classic rubber elasticity theory to such elastomeric protein-based hydrogels, fulfilling the practical purpose of predicting the mechanical properties of protein hydrogels semi-quantitatively. The validity of this approach was tested using different elastomeric protein-based hydrogels. This new approach provides a practical tool to quantitatively predict/explain the mechanical properties of such elastomeric protein-based hydrogels and helps in the rational design of novel protein-based dynamic hydrogels for various applications.