The present paper focus on the residual strength of pitted mild steel rectangular plate under biaxial compression. This paper aims to propose a general and practical formula to predict the residual strength of pitted rectangular plates under biaxial compression starting from the classic formula for intact rectangular plates and assessing whether it can be applicable to pitted plates, where the degree of pitting corrosion is modelled as one key parameter. Firstly, the numerical model is verified with an existing case study. Afterwards, a series of nonlinear FEM analysis are performed, changing geometrical attributes of both pits and plates, i.e., the radius and location of pits and the slenderness of plates. Based on those simulation results, it is found that the classic formula for intact rectangular plates can be applied reasonably well for pitting corroded plates. A unique parameter DOP (degree of pitting), which is easily determined, is employed to evaluate the effect of pitting corrosion with adequately accuracy and without bias to either longitudinal or transverse compressive stress. The proposed formula can provide guidance during the process of ship structural maintenance decision-making and strength reassessment conveniently.