Factor of Safety Calculator
Calculate the factor of safety from material strength (or failure load) and working stress, the margin of safety, utilization, the allowable stress for a target factor and typical design factors.
How it works
Factor of safety FoS = strength ÷ working stress (or failure load ÷ working load) · margin of safety = FoS − 1 · utilization = working ÷ strength · allowable stress = strength ÷ required FoS
The factor of safety (FoS, or safety factor) says how many times stronger a part is than it needs to be for the expected load. It covers uncertainty in loads, material properties, manufacturing and analysis. Ductile metals are usually checked against yield strength and brittle materials against ultimate strength; design codes such as ASME, AISC or Eurocode prescribe the factors (or partial factors) you must use. A FoS below 1 means the part is predicted to fail; the margin of safety is simply how much of the strength is left in reserve.
Worked example
A steel part with a yield strength of 250 MPa working at 100 MPa has a factor of safety of 250 ÷ 100 = 2.50 and a margin of safety of 1.50. It uses 40% of its strength. Against a required factor of 2, the allowable working stress is 250 ÷ 2 = 125 MPa, so the design meets the requirement with a 25% margin.
Sources: Engineering ToolBox — Factors of safety · Wikipedia — Factor of safety
Frequently asked questions
What is a good factor of safety?
It depends on the application and on how well loads and materials are known: about 1.5 for well-understood aerospace parts, 2–4 for general machine design against yield, and 5–10 for lifting gear, wire ropes and parts where failure endangers people.
What is the difference between factor of safety and margin of safety?
The factor of safety is the ratio strength ÷ load. The margin of safety is FoS − 1, the fraction of extra capacity. A FoS of 2.5 is a margin of 1.5 (150%). In aerospace the margin is often measured against the required factor instead.
Should I use yield or ultimate strength?
Use yield strength for ductile materials (steel, aluminum) where permanent deformation counts as failure, and ultimate strength for brittle materials (cast iron, glass, concrete in tension) or when the code’s factors are defined against ultimate.
Can the factor of safety be calculated with loads instead of stresses?
Yes. Failure load ÷ working load gives the same ratio as long as stress is proportional to load — enter both in the same unit.
Related calculators
Engineering estimate for preliminary sizing and education. Verify final designs against the applicable codes, manufacturer data and a qualified engineer.