Compare a material's strength (yield or ultimate) against the actual working stress to get the factor of safety, then size the allowable stress for a target factor. The verdict is colour-coded so you can see at a glance whether a design is unsafe, marginal, or adequate.
The factor of safety (FoS) is the single most important number in mechanical design: it expresses how much stronger a part is than it strictly needs to be to carry its load. This calculator divides the material strength by the actual working stress to give the realised factor of safety, then works backward from a target factor to the allowable stress you should design to. It also reports the margin of safety, FoS − 1, the figure aerospace and structural codes prefer.
The factor of safety is the ratio of a structure's capacity to the demand placed on it: FoS = strength / applied stress = S / σ.
An FoS of 1 means the part is loaded exactly to its strength — no reserve, and certain to fail if anything is slightly off. An FoS of 2 means the material is twice as strong as the working stress requires. Because real loads, material scatter, and stress concentrations are never known perfectly, a value comfortably above 1 is always used.
You must be explicit about which strength you put in. Designing to the yield strength keeps the part fully elastic, with no permanent deformation — the right basis for structural members and machine parts that must stay dimensionally stable. Designing to the ultimate (tensile) strength only guards against fracture and allows yielding first; it is used where some plastic deformation is acceptable but separation is not.
A part may have FoS = 2 on yield but only 1.3 on ultimate, or vice versa. Always state the basis alongside the number.
Rearranging the definition gives the allowable (design) stress for a chosen target factor: σ_allow = S / FoS_target. Keeping the working stress below this value guarantees the desired margin.
The closely related margin of safety is MoS = FoS − 1. A positive margin means reserve capacity; a margin of 0 means the part is exactly at its limit; a negative margin means it is overstressed. Aerospace structural reports almost always quote MoS rather than FoS for this reason.
There is no universal "correct" factor — it depends on consequences, load certainty, and material behaviour. Common guidance: 1.5–2 for ductile materials with well-known, steady loads and good data; 2–3 for ordinary materials under average conditions with some load uncertainty; 3–4 (or more) for brittle materials, shock or impact loads, poorly known service conditions, or where failure is catastrophic. Codes such as ASME, AISC, and Eurocode embed these factors through design stresses or partial safety factors.
They describe the same reserve in two forms. Factor of safety is a ratio, FoS = strength/stress, so FoS = 1.5 means 50% reserve. Margin of safety is MoS = FoS − 1, expressed as a fraction or percentage, so the same case gives MoS = 0.5 (50%). A positive margin is safe; a margin of zero or below is not.
Use yield strength when permanent deformation is unacceptable and the part must stay elastic — most machine and structural members. Use ultimate strength when only fracture (separation) must be prevented and limited yielding is tolerable. Many designs check both and take the smaller resulting factor as the governing one.
No. Excess safety factor means more material, weight, and cost, and can mask a poor understanding of the loads. The goal is an appropriate factor — large enough to cover real uncertainties and consequences of failure, but not so large that the design becomes heavy and uneconomical. Critical, hard-to-inspect, or life-safety parts justify higher factors.
A factor below 1 means the applied working stress exceeds the material strength you entered — the part is predicted to fail. Reduce the load or stress, increase the cross-section, or choose a stronger material until the ratio rises above your target. Treat any value under 1 as an immediate red flag.
Not directly. This tool compares a static stress to a static strength. Fatigue (cyclic loading), buckling (slender compression members), creep, and fracture toughness each need their own dedicated check with their own factors. A static FoS above 1.5 does not guarantee a part survives millions of load cycles — use a fatigue analysis for that.