What is factor of safety and how do you determine it?

Factor of safety (FoS) is the ratio of allowable stress (or yield/ultimate strength) to actual working stress. It accounts for uncertainties in loading, material properties, manufacturing variations, and environmental factors. FoS typically ranges from 1.5-3 for ductile materials under static loads, 3-5 for brittle materials, and higher for impact or fatigue loading. Selection depends on consequences of failure, load predictability, and industry standards.