Factor of safety (FoS) can mean either the fraction of structural capability over that required, or a multiplier applied to the maximum expected load (force, torque, bending moment or a combination) to which a component or assembly will be subjected. The two senses of the term are completely different in that the first is a measure of the reliability of a particular design, while the second is a requirement imposed by law, standard, contract or custom. Careful engineers refer to the first sense as a factor of safety, or, to be explicit, a realized factor of safety, and the second sense as a design factor, but usage is inconsistent and confusing, so engineers need to be aware of both senses. There are several things called a Multiplier. ... In physics, force is an influence that may cause a body to accelerate. ... It has been suggested that this article or section be merged with Moment (physics). ...

The realized factor of safety is just a defnition and needs no elaboration.

Appropriate design factors are based on several considerations. Prime considerations are the accuracy of load and wear estimates, the consequences of failure, and the cost of overengineering the component to achieve that factor of safety. For example, components whose failure could result in substantial financial loss, serious injury or death usually use a safety factor of four or higher (often ten). Non-critical components generally have a design factor of two. An interesting exception is in the field of aerospace engineering, where design factors are kept low (about 1.15–1.25) because the costs associated with structural weight are so high. This low design factor is why aerospace parts and materials are subject to more stringent quality control. Look up failure in Wiktionary, the free dictionary. ... Aerospace engineering is the branch of engineering that concerns aircraft, spacecraft, and related topics. ... In engineering and manufacturing, quality control and quality engineering are involved in developing systems to ensure products or services are designed and produced to meet or exceed customer requirements. ...

A factor of safety of 1 implies no "overengineering" at all. Hence some engineers prefer to use a related term, Margin of Safety (MoS) to describe the design parameters. The relation between MoS and FoS is MoS = FoS − 1. Margin of Safety is often described in percentage, i.e., a 50% Margin of Safety is equivalent to a factor of safety of 1.5.

See also

• Limit state design
• Redundancy (total quality management)
• Probabilistic design