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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.

The realized factor of safety is just a definition 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 can 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 1.50 – 3.00 because the costs associated with structural weight are high. This low design factor is why aerospace parts and materials are subject to more stringent quality control.

A factor of safety of 1.0 implies no "overengineering" (not exceeding design requirements). Many government agencies and companies require the use of a Margin of Safety (M.S.) to describe the ratio of the strength of the structure to the requirements. The relationship between M.S. and FoS is M.S. = FoS − 1. Margin of Safety is sometimes, but infrequently, used as a percentage, i.e., a 0.50 M.S vs. a 50% M.S. The equivalent factor of safety would be 1.5.

The factor of safety used in steam boilers is usually between 8 and 10. For example, if the required working pressure is 250 pounds per square inch (psi), and the safety factor is 10, the bursting point must be not less than 2,500 psi. This allows a large margin of safety in a new boiler but, as the boiler ages and is affected by corrosion, the margin of safety will diminish. For this reason, steam boilers are hydraulically tested, at regular intervals, to 1.5 times or 2 times the working pressure.

Even the most gentle of drugs, such as penicillin, can cause death when administered at excessively large quantities. The farther away the effective dose (ED) is from the lethal dose (LD) defines the margin of safety, or the natural, built-in safety factor, for that particular drug. For example, the effective dose of penicillin, i.e. the quantity that will be effective in treating an infection, is so vastly minute in relation to the multiplicity of doses necessary to prove fatal that penicillin is considered an extremely safe drug.

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