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factor of safety (FoS)

Contributor(s): Corinne Bernstein

Factor of safety (FoS) is ability of a system's structural capacity to be viable beyond its expected or actual loads. An FoS may be expressed as a ratio that compares absolute strength to actual applied load, or it may be expressed as a constant value that a structure must meet or exceed according to law, specification, contract or standard. 

Design and engineering standards usually specify the allowable stress, or ultimate strength of a given material divided by the factor of safety, rather than use an arbitrary safety factor, because these factors can be misleading and have been known to imply greater safety than is the case. For example, a safety factor of 2 does not mean that a structure can carry twice as much load as it was designed for.

The safety factor depends on the materials and use of an item. Different industries have varying ideas on what FoS should be required. Although there is some ambiguity regarding safety factors, there are some general guidelines across multiple verticals. If the consequences of failure are significant, such as loss of life, personal harm or property loss, a higher FoS is likely to be required by design or by law. When a structure’s ability to carry load must be determined to a reasonable accuracy but comprehensive testing is impractical (such as with bridges and buildings), safety factors need to be calculated using detailed analysis.

Cost is also a consideration. As the FoS increases, the cost of the product also increases, so it may be necessary to determine how much extra it might cost per part to achieve a certain FoS, and whether that is a viable business model. Striking a balance between cost reduction and safety is essential.

This was last updated in May 2018

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