Background: Alginate impression materials are widely used in dentistry due to their ease of manipulation and cost-effectiveness; however, their dimensional stability is highly susceptible to water diffusion, evaporation, and chemical interactions within the material matrix. Variations in thickness and storage time can significantly influence weight loss and, consequently, impression accuracy. This study evaluated the dimensional stability and weight changes of two commercially available alginates Tropicalgin and Blueprint Cremix and one experimentally formulated alginate when stored in air over different time intervals. Methods: Three alginate materials (two commercial and one experimental) were tested using standardized metal molds of 1 mm and 3 mm thickness. A total of 36 samples were prepared following manufacturers’ recommendations for mixing and setting. After initial weighing, samples were stored in air at 23 °C ± 1 and reweighed at 10 minutes, 30 minutes, and 1 hour. Weight loss (%) was calculated to assess dimensional change. Data were analyzed using one-way ANOVA with a significance level of p < 0.05. Results: All three materials exhibited progressive weight loss with increased storage time. Across all intervals, 1 mm samples demonstrated approximately double the weight loss observed in 3 mm samples, indicating a strong influence of material thickness on water retention. At 1 hour, weight loss in 1 mm samples ranged from 14.47% to 18.26%, whereas 3 mm samples showed an average reduction of around 8%. Among the materials tested, the experimentally formulated alginate consistently demonstrated the least dimensional change at all time points. Conclusion: Storage time and material thickness significantly influence the dimensional stability of alginate impression materials. The experimental alginate formulation exhibited superior stability, with the lowest weight loss across all conditions. Although all alginates showed measurable contraction over time, thicker samples retained moisture more effectively, underscoring the importance of minimizing delays between impression making and model pouring in clinical settings.