<p>Accurate estimation of initial prestress is essential for ensuring the structural integrity, durability, and quality management of prestressed concrete (PSC) girders. However, conventional estimation methods that rely on design provisions or experimental data often fail to reflect real field conditions due to variations in material properties, construction methods, and environmental factors. This study presents a novel and rational method that performs the initial calibration of PSC-I girders during prestressing using on-site measurement data. By analyzing tendon elongation and camber measured during prestressing, the proposed method estimates the wobble and curvature friction coefficients and the concrete elastic modulus, thereby improving the accuracy of evaluating initial prestress and deformation behavior. The analytical formulation incorporates equilibrium equations that account for self-weight, prestressing forces, and prestress losses due to friction, anchorage slip, and elastic shortening of concrete. Validation using field data from multiple PSC girders demonstrated strong agreement between analytical predictions and measurements, confirming the reliability of the proposed method. The novelty of this study lies in integrating field-based calibration into the prestress estimation process bridging the gap between design assumptions and actual construction performance. The proposed approach enables quality and performance management of PSC girders through statistical analysis of initial prestressing data, provides reliable initial conditions for time-dependent analysis, and improves the accuracy of long-term prestress loss prediction, contributing to safer and more efficient PSC bridge construction.</p>