This chapter provides a practical guide to applying Rules of Thumb (ROT) for quick and effective problem-solving in various scenarios. Through a series of solved examples, it demonstrates how to use these simplified guidelines to estimate, design, and analyse common challenges encountered in engineering, construction, and building services. Each example is structured to highlight the assumptions, methodologies, and step-by-step calculations involved, ensuring clarity and ease of application. The goal is to equip readers with a hands-on understanding of how rules of thumb can serve as reliable tools for preliminary assessments and decision-making, saving time while maintaining reasonable accuracy.