Factor of Safety

Ultimate Tensile Stress (5)

  • The highest amount of stress a material can withstand.
  • This is usually during the Necking Down process.

Factor of Safety

  • Since a material should be working at stresses under its breaking point, the Factor of Safety is how much tolerance is given to a material.
  • This can be found by:
\begin{align} FoS = \frac{UTS}{\sigma_{max}} \end{align}


  • FoS is the Factor of Safety (ratio)
  • UTS is the material's Ultimate Tensile Stress (Pa)
  • $\sigma_{max}$ is the maximum stress the material is likely to encounter (Pa)


  • Within a system, there needs to be a compromise between a high Factor of Safety and the Efficiency of the system.
  • Any system definitely should have a FoS above 1.