Calculate the factor of safety from a material’s failure or yield strength and the actual working stress, with a plain reading of the result.
Use yield strength for designs that must not deform permanently, or ultimate strength where fracture is the limit. Both values must be in the same units.
Factor of safety
How to use this tool
Type the material’s yield strength (for no permanent deformation) or ultimate strength (for no fracture), in MPa.
Type the maximum stress the part actually sees in service, in the same units.
The factor is strength divided by working stress; 2 means the part can carry twice its working load before the chosen limit.
Required factors vary by industry and failure mode — lifting gear and pressure vessels demand more than a static bracket.
Formula & worked example
A steel with 500 MPa ultimate strength carrying 125 MPa working stress has a factor of safety of 500 / 125 = 4.0, a 300% margin.
Limitations — read this before you rely on it
A single factor hides the failure mode: fatigue, buckling, corrosion and impact can all fail a part well below its static strength. Always apply the relevant design code, not just this ratio.
Frequently asked questions
It depends on the application: around 1.5–2 for well-understood static loads, 3–4 or more for lifting, pressure equipment or uncertain loads.
Failure by the chosen criterion — the working stress already exceeds the strength, so the design is unsafe.
Use yield strength when permanent deformation counts as failure, and ultimate strength when fracture is the limit. State which you used.
No — beyond the code requirement it adds weight, cost and material with no safety benefit, and can hide poor analysis.
No. Repeated loading needs a fatigue (S–N) analysis; static factor of safety alone does not predict fatigue life.