安全系数（Factor of safety或是safety factor，简称SF）是指一个结构或是机械所能负荷的负载可以超过预期负载的程度。

安全系数为大于等于1的数值，安全系数越高，表示结构或机械的安全度越高，但也意味着其成本也相对提高。

一般而言，结构所能负荷的程度需比预期的负载要大。在许多专案中（如桥梁及建筑物），不可能进行完整的测试，但先确定结构允许负荷的程度，而且需要到一定的准确度。

许多系统在设计时会加以考量，不止在正常条件下可以使用，在一些较特殊的严酷条件下（如紧急情形、较预期要大的负载、异常使用或系统退化）仍能正常使用。

Factor of safety (FoS), also known as (and used interchangeably with) safety factor (SF), is a term describing the capacity of a system beyond the expected loads or actual loads. Essentially, the factor of safety is how much stronger the system is than it usually 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 load must be determined to a reasonable accuracy.