A safety factor (also called an uncertainty factor or assessment factor) is a number by which some variable such as load or dose is multiplied or divided in order to increase safety. Safety factors are used in engineering design, toxicology, and other disciplines to avoid various types of failure.

The sources of failure that safety factors are intended to protect against can be divided into two major categories: (a) the variability of conditions that influence the risk of failure, such as variations in the strength of steel and in the sensitivity of humans to toxic substances, and (b) the uncertainty of human knowledge, including the possibility that the models used for risk assessment may be inaccurate.

Safety factors are used to obtain a safety reserve, a margin between actual conditions and those that would lead to failure. Safety reserves can also be obtained without the use...