Stirrups of reinforced concrete members are very prone to corrosion compared with longitudinal reinforcements, resulting from their small concrete covers, which lead to concrete cracking and spalling. Due to the adverse effects of corrosion, this article aims to investigate the amount of reduction in the capacity of reinforced concrete specimens in different corrosion degrees. For this purpose, an experimental investigation is carried out on 22 reinforced and non-reinforced rectangular prism specimens, of which 12 reinforced specimens are corroded. The test variables contain the corrosion percentage, and the stirrup diameter and spacing. Eventually, all specimens are tested for compressive strength for 90 days. The experimental results show that the reduction of compressive strength depends on the corrosion percentage and stirrup diameter. According to this conclusion, a new formulation is proposed to express the relationship between compressive strength reduction and its effects.