<p>Accurate determination of real-time crack growth length constitutes the fundamental requirement in metallic materials’ fatigue crack growth (FCG) testing. To develop a more reliable FCG rate measurement method, establishing a new real-time crack length monitoring technique for pre-cracked specimens becomes imperative. Consequently, semi-analytical models characterizing both the load–displacement relationship and stress intensity factor (<i>K</i>)-load relationship for mode I pre-cracked specimens were developed based on the equivalent energy principle. The validities of these models were verified through finite element analyses (FEA). Subsequently, a new real-time crack length quantification methodology was established, utilizing exclusively the peak load and corresponding displacement data per fatigue cycle. Furthermore, a semi-analytical equivalent energy method was successfully developed to characterize the FCG behavior in mode I pre-cracked specimens. To validate the efficacy of the proposed methodology, fatigue crack growth experiments were performed on compact tension (CT) specimens fabricated from two different materials. The equivalent energy method successfully generated FCG curves, demonstrating good correlation with results obtained through the traditional unloading compliance method. Consequently, the proposed equivalent energy method necessitates only peak load and corresponding displacement measurements to determine real-time crack growth length. Comparative analysis reveals this method exhibits superior stability to the traditional unloading compliance method, particularly demonstrating enhanced reliability for FCG rate characterization under extreme conditions such as elevated temperatures or high pressures.</p>