This book demystifies fluid phase equilibrium calculations, guiding readers through equations of state, phase boundaries, and flash algorithms with clarity and precision.Designed for students and professionals in science and engineering, it bridges the gap between thermodynamic theory and real-world computation. Starting from core concepts such as energy and entropy, it builds step by step toward algorithms used in computational thermodynamics. By the end,readers will not only understand the theory but also how to implement it in practice.