The concept of depth of closure (DoC) plays a pivotal role in coastal morphodynamics and engineering, serving as a key parameter for estimating sediment budgets, designing beach nourishment projects, and modeling shoreline changes. Despite its significance, field measurements of DoC remain limited due to the logistical, operational, and financial challenges of systematic topo-bathymetric data collection. This study provides a field-based evaluation of DoC along the mainland Portuguese coast, encompassing diverse wave energy conditions, geomorphological characteristics and exposure. 64 nearshore-beach profile surveys conducted across 14 locations between 2018 and 2021, under the Portuguese Coastal Monitoring Program (COSMO), were analyzed. To define DoC, the study introduces the Relative Morphological Variability (RMV) criterion, setting a critical threshold of 1.2 local measurement uncertainty. DoC results indicate significant regional variability: on the high-energy western coast, ranges from 7.8 m to 14.1 m (MSL); on the lower-energy southern coast, values are shallower, from 4.1 m to 7.2 m (MSL), reflecting not only the lower wave energy environment but also seasonal data acquisition limitations. Empirical estimations using Hallermeier’s and Birkemeier’s formulas were compared with field data. While Hallermeier’s method consistently overestimates the DoC, Birkemeier’s approach aligns more closely with observed values, particularly along the western coast. These findings highlight Birkemeier’s formula as the preferred empirical method for estimating DoC along the Portuguese coast when field data is scarce or unavailable. This work underscores the importance of precise field measurements and validated empirical approaches, providing new insights for coastal management and engineering applications.