Coastal forest rehabilitation increasingly relies on Casuarina equisetifolia because of its capacity to establish under challenging coastal conditions and contribute to biomass accumulation. However, different allometric equations may produce substantially different biomass estimates, and comparative evidence for rehabilitated coastal forests in Indonesia remains limited. This study evaluated the relative agreement of three candidate allometric equations—Chave et al., Brown et al., and Ketterings et al.—with the Dudley and Fownes species-specific equation used as a reference for C. equisetifolia in a rehabilitated coastal forest in Kulon Progo, Indonesia. Field measurements of diameter at breast height and tree height were obtained from 20 sampling points, and aboveground biomass was estimated using the four equations. Agreement was assessed using standard deviation, coefficient of variation, mean absolute error (MAE), root mean square error (RMSE), mean bias error (MBE), and Pearson correlation coefficient. Among all statistical tests performed, the EWM results ranked Ketterings first (0.66), followed by Chave (0.57) and Brown (0.26). The principal limitation is the absence of independent or destructive biomass measurements; therefore, the findings demonstrate relative agreement with the reference model rather than absolute model accuracy. Practically, the results provide a transparent framework for comparative screening of allometric equations when independent biomass validation is unavailable. The study contributes comparative evidence for rehabilitated coastal C. equisetifolia in Indonesia and identifies Ketterings et al. as the equation showing the strongest agreement with the selected reference under the tested conditions. Independent biomass validation across additional sites and tree-size classes is needed to determine whether this agreement reflects greater absolute accuracy.