structural capacity of a system beyond the expected normal or actual loads. how much stronger the system is than it usually needs to be for an intended load. For example. a system with safe working load of5.000 lbs. and a safety factor of 2.0 would have a rated load of 10.000 lbs. (compare to design factor).
In engineering, a factor of safety (FoS), also known as (and used interchangeably with) safety factor (SF), expresses how much stronger a system is than it needs to be for an intended load. Safety factors are often calculated using detailed analysis because comprehensive testing is impractical on many projects, such as bridges and buildings, but the structure's ability to carry a load must be determined to a reasonable accuracy.
Many systems are intentionally built much stronger than needed for normal usage to allow for emergency situations, unexpected loads, misuse, or degradation (reliability).