The external confinement of existing concrete columns using Fiber Reinforced Polymer (FRP) and Steel Reinforced Polymer (SRP) composite materials has become a widely adopted method for structural retrofitting. However, the design equations in current guidelines are often based on limited experimental data, necessitating more robust validation. This study focuses on SRP confinement, proposing a new design equation to predict the compressive strength of confined concrete with improved accuracy. Developed using the “design assisted by testing” methodology outlined in Annex D of EN-1990, the model is based on an extensive experimental database comprising compression tests on concrete cylinders confined with unidirectional steel cord textiles. The probabilistic framework used ensures reliability calibration for design purposes. The proposed equation’s performance is compared with the design formula from the recently revised Italian Guidelines CNR-DT 200 R2 (2024), initially developed for FRP confinement. The results demonstrate the accuracy and practicality of the proposed model, illustrating that a linear relationship more effectively captures the actual behavior.