The utilization of agricultural waste for biomass briquette production offers a sustainable approach to renewable energy development. This study investigated the effect of mixture ratio between candlenut shell charcoal (Aleurites moluccanus) and tamarind peel (Tamarindus indica L.) on the physical and mechanical properties of biomass briquettes, with particular emphasis on density and compressive strength. Briquettes were produced using five mixture ratios (1:3, 1:2, 1:1, 2:1, and 3:1, w/w) and evaluated for moisture content, density, and compressive strength. The results showed that moisture content remained low and relatively constant across all treatments (≈6%), while density and compressive strength exhibited significant dependence on mixture ratio. Density displayed a non-linear trend with a clear maximum near the 1:1 ratio, whereas compressive strength increased linearly with density. Based on these relationships, a semi-empirical predictive model was developed by coupling a quadratic density–ratio model with a linear density–strength model. The integrated model successfully captured the observed experimental trends and identified an optimal mixture region consistent with the highest mechanical performance. The proposed model provides a simple and physically meaningful framework for predicting briquette strength from mixture composition, enabling preliminary formulation optimization with reduced experimental effort. This approach supports efficient utilization of agricultural residues for sustainable solid biofuel production.