Factor of safety
Definition
Definition
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There are two definitions for the factor of safety (FoS):
* The ratio of a structure's absolute strength (structural capability) to actual applied load; this is a measure of the [reliability](/wiki/Reliability_engineering "Reliability engineering") of a particular design. This is a calculated value, and is sometimes referred to, for the sake of clarity, as a *realized factor of safety*.
* A constant required value, imposed by law, [standard](/wiki/Technical_standard "Technical standard"), [specification](/wiki/Specification "Specification"), contract or [custom](/wiki/Convention_%28norm%29 "Convention (norm)"), to which a structure must conform or exceed. This can be referred to as a *design factor*, *design factor of safety* or *required factor of safety*.

The realized factor of safety must be greater than the required design factor of safety. However, between various industries and engineering groups usage is inconsistent and confusing; there are several definitions used. The cause of much confusion is that various reference books and standards agencies use the factor of safety definitions and terms differently. [Building codes](/wiki/Building_code "Building code"), [structural](/wiki/Structural_engineering "Structural engineering") and [mechanical engineering](/wiki/Mechanical_engineering "Mechanical engineering") textbooks often refer to the "factor of safety" as the fraction of total structural capability over what is needed. Those are realized factors of safetyYoung, W.: *Roark's Formulas for Stress and Strain*, 6th edition. McGraw\-Hill, 1989\.Shigley, J and Mischke, C: *Standard Handbook of Machine Design*, page 2\-15\. McGraw\-Hill, 1986\.ASME BTH\-1: *Design of Below\-the\-Hook Lifting Devices*, Section 1\-5, ASME, 2005\. (first use). Many undergraduate [strength of materials](/wiki/Strength_of_materials "Strength of materials") books use "Factor of Safety" as a constant value intended as a minimum target for designBeer, F and Johnson, R: *Mechanics of Materials*, second edition. McGraw\-Hill,1992\.[Timoshenko, S](/wiki/Stephen_Timoshenko "Stephen Timoshenko"): *Strength of Materials*, Volume 1\. Krieger, 1958\.Buchanan, G: *Mechanics of Materials*, Page 55\. Holt, Reinhart, and Watson,1988\. (second use).


Why might different industries or engineering groups require different factors of safety?